An Investigation of the Bragg-Gray Principle with ...
Transcript of An Investigation of the Bragg-Gray Principle with ...
AN IIM3~GAltON aF mE JUAGG-GBAY PRINCIPLE WI~ PLlJORESCENJ X-RAYS
by
JIAROU) VINCmJ IARJ()lf
A~IS
subnitted to
ill partfal tultillment ot tbe requL~ts tor tbe
degree or
MAS~ OF SCIENCE
June 1957
lillWt
Redacted for Privacy
Redacted for PrivacyIllrrcL farfrmr eil$elrm
Io ercilr of hrl
ampt d lnrrtxt of Wor
Redacted for Privacy
Redacted for Privacy
ampim of fahf ampret fFililtttr
Redacted for PrivacySrm d iBratrr lampa[
ffr thair ir lmntri T nutAr(1
nn tr lurlry frilfir
lbe author desires to ~retts hie gratitude to Dr w c Boesch
tor euggestilla this problem enc1 to I ~ M1ers for advice on ~bnt
ques
AN DM$~00 OF 1HE BRAGltJ-GRAX PRINCJPLE tttZFLtJORESCENlr xRAYS
s-~ ~ 4t Introcluct1on l
Obleetive 3
Method middot5
~tion Cllambers 9
ValiUty of the ~G~ Pz1nciple 20
B1bl1~ 26
table No atle I l rheorettw Values of JAJJair 10
2 ~rttical VaJue of JcuJatr 10
~~ ot ~tical tm4 ExPerlDiental3 Velues o~ JAliJIJLr middot leu~aJr 23
4 Comparison ot lheoretical 8lld Bl(perimental Value ot J auJAl 23
amp1tle
1
2
3
~son vttb Attix s
Extrepolation CUrves
Extrapolation CUrves
lata 4
2l
22
a=Jtle
~
2
IOllizatton Chamber AssemblY
~ Vaeuurn Chamber and tbe IomzatLQl ~ber 1u Relation to the Fluolfscent XmiddotRau Source 15
1ftle
1
2
13
17
AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS
IN)RODlTC)ION
-he primary standard tor low energy X-ray dosimetry measurements
is the tree a1r ionization chamber For some measurements such as
that o~ surface dOse the free air ionization cbamber is difficult or
iDgossible to use For these measurements the Bragg-Gray chamber
(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu
able instrument A comparLscn ot tree air ionization chamber
measurements with Bragg-Gray chamber data using lov energy xrays
is needed to vertiy the BraggbullGray principle and establis the limitbull
ations on a Bragg-Gray cbamber in this energy region
~e Bragg-Gray principle has been the subject of numerous
1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle
are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and
(24 pp581-589) A rigorous dermiddotivation ot the principle was made
by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the
mediUirl soft xray energy region artnelli (20 p251) expresses the
Bragg-Gray principle in tbe tolloWing torm
l) =S W r h lrP11 q (l)
D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second
3 a ion current per gram ot gas in the cavity
W bull energy in ergs necessa17 to form an ion pair in the cas
2
S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation
11 number of incident photons of energy h-v middot per em- per seconcl
hv bull eJgtergy of the photon in ergs
11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam
the mass energr absorption coetncient ts detined by
~ltpoundgt=It-s (2)
tN bull mass absorption coefficient in em2 per gram
ltE) = the average energy- ot the electrons at the tJme of production in erss
At low photon energies the mass energy absorption coefflctent ba$
two camponen~s
MIt = poundG r (1- ~ )Jll (3)-middot ltV tl
ltPibull Calppton absorption ooefficient per electron in em2
7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2
f- bull fluorescent yield tor the K level
~ =btMInamp energy ot tbe K level
~=yen= nUDber of electrons per gram of wall materieJ
~ middot Avogadros number
A bull atomic weight
Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and
(17 pp 57-68) tried to establish tbe validity of equation 1) tor C
Al end CU walled ionization chambers end for Xrays with e~ective11
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
lillWt
Redacted for Privacy
Redacted for PrivacyIllrrcL farfrmr eil$elrm
Io ercilr of hrl
ampt d lnrrtxt of Wor
Redacted for Privacy
Redacted for Privacy
ampim of fahf ampret fFililtttr
Redacted for PrivacySrm d iBratrr lampa[
ffr thair ir lmntri T nutAr(1
nn tr lurlry frilfir
lbe author desires to ~retts hie gratitude to Dr w c Boesch
tor euggestilla this problem enc1 to I ~ M1ers for advice on ~bnt
ques
AN DM$~00 OF 1HE BRAGltJ-GRAX PRINCJPLE tttZFLtJORESCENlr xRAYS
s-~ ~ 4t Introcluct1on l
Obleetive 3
Method middot5
~tion Cllambers 9
ValiUty of the ~G~ Pz1nciple 20
B1bl1~ 26
table No atle I l rheorettw Values of JAJJair 10
2 ~rttical VaJue of JcuJatr 10
~~ ot ~tical tm4 ExPerlDiental3 Velues o~ JAliJIJLr middot leu~aJr 23
4 Comparison ot lheoretical 8lld Bl(perimental Value ot J auJAl 23
amp1tle
1
2
3
~son vttb Attix s
Extrepolation CUrves
Extrapolation CUrves
lata 4
2l
22
a=Jtle
~
2
IOllizatton Chamber AssemblY
~ Vaeuurn Chamber and tbe IomzatLQl ~ber 1u Relation to the Fluolfscent XmiddotRau Source 15
1ftle
1
2
13
17
AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS
IN)RODlTC)ION
-he primary standard tor low energy X-ray dosimetry measurements
is the tree a1r ionization chamber For some measurements such as
that o~ surface dOse the free air ionization cbamber is difficult or
iDgossible to use For these measurements the Bragg-Gray chamber
(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu
able instrument A comparLscn ot tree air ionization chamber
measurements with Bragg-Gray chamber data using lov energy xrays
is needed to vertiy the BraggbullGray principle and establis the limitbull
ations on a Bragg-Gray cbamber in this energy region
~e Bragg-Gray principle has been the subject of numerous
1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle
are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and
(24 pp581-589) A rigorous dermiddotivation ot the principle was made
by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the
mediUirl soft xray energy region artnelli (20 p251) expresses the
Bragg-Gray principle in tbe tolloWing torm
l) =S W r h lrP11 q (l)
D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second
3 a ion current per gram ot gas in the cavity
W bull energy in ergs necessa17 to form an ion pair in the cas
2
S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation
11 number of incident photons of energy h-v middot per em- per seconcl
hv bull eJgtergy of the photon in ergs
11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam
the mass energr absorption coetncient ts detined by
~ltpoundgt=It-s (2)
tN bull mass absorption coefficient in em2 per gram
ltE) = the average energy- ot the electrons at the tJme of production in erss
At low photon energies the mass energy absorption coefflctent ba$
two camponen~s
MIt = poundG r (1- ~ )Jll (3)-middot ltV tl
ltPibull Calppton absorption ooefficient per electron in em2
7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2
f- bull fluorescent yield tor the K level
~ =btMInamp energy ot tbe K level
~=yen= nUDber of electrons per gram of wall materieJ
~ middot Avogadros number
A bull atomic weight
Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and
(17 pp 57-68) tried to establish tbe validity of equation 1) tor C
Al end CU walled ionization chambers end for Xrays with e~ective11
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
lbe author desires to ~retts hie gratitude to Dr w c Boesch
tor euggestilla this problem enc1 to I ~ M1ers for advice on ~bnt
ques
AN DM$~00 OF 1HE BRAGltJ-GRAX PRINCJPLE tttZFLtJORESCENlr xRAYS
s-~ ~ 4t Introcluct1on l
Obleetive 3
Method middot5
~tion Cllambers 9
ValiUty of the ~G~ Pz1nciple 20
B1bl1~ 26
table No atle I l rheorettw Values of JAJJair 10
2 ~rttical VaJue of JcuJatr 10
~~ ot ~tical tm4 ExPerlDiental3 Velues o~ JAliJIJLr middot leu~aJr 23
4 Comparison ot lheoretical 8lld Bl(perimental Value ot J auJAl 23
amp1tle
1
2
3
~son vttb Attix s
Extrepolation CUrves
Extrapolation CUrves
lata 4
2l
22
a=Jtle
~
2
IOllizatton Chamber AssemblY
~ Vaeuurn Chamber and tbe IomzatLQl ~ber 1u Relation to the Fluolfscent XmiddotRau Source 15
1ftle
1
2
13
17
AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS
IN)RODlTC)ION
-he primary standard tor low energy X-ray dosimetry measurements
is the tree a1r ionization chamber For some measurements such as
that o~ surface dOse the free air ionization cbamber is difficult or
iDgossible to use For these measurements the Bragg-Gray chamber
(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu
able instrument A comparLscn ot tree air ionization chamber
measurements with Bragg-Gray chamber data using lov energy xrays
is needed to vertiy the BraggbullGray principle and establis the limitbull
ations on a Bragg-Gray cbamber in this energy region
~e Bragg-Gray principle has been the subject of numerous
1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle
are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and
(24 pp581-589) A rigorous dermiddotivation ot the principle was made
by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the
mediUirl soft xray energy region artnelli (20 p251) expresses the
Bragg-Gray principle in tbe tolloWing torm
l) =S W r h lrP11 q (l)
D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second
3 a ion current per gram ot gas in the cavity
W bull energy in ergs necessa17 to form an ion pair in the cas
2
S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation
11 number of incident photons of energy h-v middot per em- per seconcl
hv bull eJgtergy of the photon in ergs
11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam
the mass energr absorption coetncient ts detined by
~ltpoundgt=It-s (2)
tN bull mass absorption coefficient in em2 per gram
ltE) = the average energy- ot the electrons at the tJme of production in erss
At low photon energies the mass energy absorption coefflctent ba$
two camponen~s
MIt = poundG r (1- ~ )Jll (3)-middot ltV tl
ltPibull Calppton absorption ooefficient per electron in em2
7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2
f- bull fluorescent yield tor the K level
~ =btMInamp energy ot tbe K level
~=yen= nUDber of electrons per gram of wall materieJ
~ middot Avogadros number
A bull atomic weight
Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and
(17 pp 57-68) tried to establish tbe validity of equation 1) tor C
Al end CU walled ionization chambers end for Xrays with e~ective11
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
AN DM$~00 OF 1HE BRAGltJ-GRAX PRINCJPLE tttZFLtJORESCENlr xRAYS
s-~ ~ 4t Introcluct1on l
Obleetive 3
Method middot5
~tion Cllambers 9
ValiUty of the ~G~ Pz1nciple 20
B1bl1~ 26
table No atle I l rheorettw Values of JAJJair 10
2 ~rttical VaJue of JcuJatr 10
~~ ot ~tical tm4 ExPerlDiental3 Velues o~ JAliJIJLr middot leu~aJr 23
4 Comparison ot lheoretical 8lld Bl(perimental Value ot J auJAl 23
amp1tle
1
2
3
~son vttb Attix s
Extrepolation CUrves
Extrapolation CUrves
lata 4
2l
22
a=Jtle
~
2
IOllizatton Chamber AssemblY
~ Vaeuurn Chamber and tbe IomzatLQl ~ber 1u Relation to the Fluolfscent XmiddotRau Source 15
1ftle
1
2
13
17
AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS
IN)RODlTC)ION
-he primary standard tor low energy X-ray dosimetry measurements
is the tree a1r ionization chamber For some measurements such as
that o~ surface dOse the free air ionization cbamber is difficult or
iDgossible to use For these measurements the Bragg-Gray chamber
(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu
able instrument A comparLscn ot tree air ionization chamber
measurements with Bragg-Gray chamber data using lov energy xrays
is needed to vertiy the BraggbullGray principle and establis the limitbull
ations on a Bragg-Gray cbamber in this energy region
~e Bragg-Gray principle has been the subject of numerous
1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle
are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and
(24 pp581-589) A rigorous dermiddotivation ot the principle was made
by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the
mediUirl soft xray energy region artnelli (20 p251) expresses the
Bragg-Gray principle in tbe tolloWing torm
l) =S W r h lrP11 q (l)
D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second
3 a ion current per gram ot gas in the cavity
W bull energy in ergs necessa17 to form an ion pair in the cas
2
S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation
11 number of incident photons of energy h-v middot per em- per seconcl
hv bull eJgtergy of the photon in ergs
11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam
the mass energr absorption coetncient ts detined by
~ltpoundgt=It-s (2)
tN bull mass absorption coefficient in em2 per gram
ltE) = the average energy- ot the electrons at the tJme of production in erss
At low photon energies the mass energy absorption coefflctent ba$
two camponen~s
MIt = poundG r (1- ~ )Jll (3)-middot ltV tl
ltPibull Calppton absorption ooefficient per electron in em2
7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2
f- bull fluorescent yield tor the K level
~ =btMInamp energy ot tbe K level
~=yen= nUDber of electrons per gram of wall materieJ
~ middot Avogadros number
A bull atomic weight
Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and
(17 pp 57-68) tried to establish tbe validity of equation 1) tor C
Al end CU walled ionization chambers end for Xrays with e~ective11
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
table No atle I l rheorettw Values of JAJJair 10
2 ~rttical VaJue of JcuJatr 10
~~ ot ~tical tm4 ExPerlDiental3 Velues o~ JAliJIJLr middot leu~aJr 23
4 Comparison ot lheoretical 8lld Bl(perimental Value ot J auJAl 23
amp1tle
1
2
3
~son vttb Attix s
Extrepolation CUrves
Extrapolation CUrves
lata 4
2l
22
a=Jtle
~
2
IOllizatton Chamber AssemblY
~ Vaeuurn Chamber and tbe IomzatLQl ~ber 1u Relation to the Fluolfscent XmiddotRau Source 15
1ftle
1
2
13
17
AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS
IN)RODlTC)ION
-he primary standard tor low energy X-ray dosimetry measurements
is the tree a1r ionization chamber For some measurements such as
that o~ surface dOse the free air ionization cbamber is difficult or
iDgossible to use For these measurements the Bragg-Gray chamber
(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu
able instrument A comparLscn ot tree air ionization chamber
measurements with Bragg-Gray chamber data using lov energy xrays
is needed to vertiy the BraggbullGray principle and establis the limitbull
ations on a Bragg-Gray cbamber in this energy region
~e Bragg-Gray principle has been the subject of numerous
1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle
are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and
(24 pp581-589) A rigorous dermiddotivation ot the principle was made
by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the
mediUirl soft xray energy region artnelli (20 p251) expresses the
Bragg-Gray principle in tbe tolloWing torm
l) =S W r h lrP11 q (l)
D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second
3 a ion current per gram ot gas in the cavity
W bull energy in ergs necessa17 to form an ion pair in the cas
2
S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation
11 number of incident photons of energy h-v middot per em- per seconcl
hv bull eJgtergy of the photon in ergs
11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam
the mass energr absorption coetncient ts detined by
~ltpoundgt=It-s (2)
tN bull mass absorption coefficient in em2 per gram
ltE) = the average energy- ot the electrons at the tJme of production in erss
At low photon energies the mass energy absorption coefflctent ba$
two camponen~s
MIt = poundG r (1- ~ )Jll (3)-middot ltV tl
ltPibull Calppton absorption ooefficient per electron in em2
7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2
f- bull fluorescent yield tor the K level
~ =btMInamp energy ot tbe K level
~=yen= nUDber of electrons per gram of wall materieJ
~ middot Avogadros number
A bull atomic weight
Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and
(17 pp 57-68) tried to establish tbe validity of equation 1) tor C
Al end CU walled ionization chambers end for Xrays with e~ective11
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
amp1tle
1
2
3
~son vttb Attix s
Extrepolation CUrves
Extrapolation CUrves
lata 4
2l
22
a=Jtle
~
2
IOllizatton Chamber AssemblY
~ Vaeuurn Chamber and tbe IomzatLQl ~ber 1u Relation to the Fluolfscent XmiddotRau Source 15
1ftle
1
2
13
17
AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS
IN)RODlTC)ION
-he primary standard tor low energy X-ray dosimetry measurements
is the tree a1r ionization chamber For some measurements such as
that o~ surface dOse the free air ionization cbamber is difficult or
iDgossible to use For these measurements the Bragg-Gray chamber
(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu
able instrument A comparLscn ot tree air ionization chamber
measurements with Bragg-Gray chamber data using lov energy xrays
is needed to vertiy the BraggbullGray principle and establis the limitbull
ations on a Bragg-Gray cbamber in this energy region
~e Bragg-Gray principle has been the subject of numerous
1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle
are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and
(24 pp581-589) A rigorous dermiddotivation ot the principle was made
by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the
mediUirl soft xray energy region artnelli (20 p251) expresses the
Bragg-Gray principle in tbe tolloWing torm
l) =S W r h lrP11 q (l)
D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second
3 a ion current per gram ot gas in the cavity
W bull energy in ergs necessa17 to form an ion pair in the cas
2
S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation
11 number of incident photons of energy h-v middot per em- per seconcl
hv bull eJgtergy of the photon in ergs
11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam
the mass energr absorption coetncient ts detined by
~ltpoundgt=It-s (2)
tN bull mass absorption coefficient in em2 per gram
ltE) = the average energy- ot the electrons at the tJme of production in erss
At low photon energies the mass energy absorption coefflctent ba$
two camponen~s
MIt = poundG r (1- ~ )Jll (3)-middot ltV tl
ltPibull Calppton absorption ooefficient per electron in em2
7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2
f- bull fluorescent yield tor the K level
~ =btMInamp energy ot tbe K level
~=yen= nUDber of electrons per gram of wall materieJ
~ middot Avogadros number
A bull atomic weight
Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and
(17 pp 57-68) tried to establish tbe validity of equation 1) tor C
Al end CU walled ionization chambers end for Xrays with e~ective11
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS
IN)RODlTC)ION
-he primary standard tor low energy X-ray dosimetry measurements
is the tree a1r ionization chamber For some measurements such as
that o~ surface dOse the free air ionization cbamber is difficult or
iDgossible to use For these measurements the Bragg-Gray chamber
(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu
able instrument A comparLscn ot tree air ionization chamber
measurements with Bragg-Gray chamber data using lov energy xrays
is needed to vertiy the BraggbullGray principle and establis the limitbull
ations on a Bragg-Gray cbamber in this energy region
~e Bragg-Gray principle has been the subject of numerous
1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle
are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and
(24 pp581-589) A rigorous dermiddotivation ot the principle was made
by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the
mediUirl soft xray energy region artnelli (20 p251) expresses the
Bragg-Gray principle in tbe tolloWing torm
l) =S W r h lrP11 q (l)
D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second
3 a ion current per gram ot gas in the cavity
W bull energy in ergs necessa17 to form an ion pair in the cas
2
S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation
11 number of incident photons of energy h-v middot per em- per seconcl
hv bull eJgtergy of the photon in ergs
11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam
the mass energr absorption coetncient ts detined by
~ltpoundgt=It-s (2)
tN bull mass absorption coefficient in em2 per gram
ltE) = the average energy- ot the electrons at the tJme of production in erss
At low photon energies the mass energy absorption coefflctent ba$
two camponen~s
MIt = poundG r (1- ~ )Jll (3)-middot ltV tl
ltPibull Calppton absorption ooefficient per electron in em2
7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2
f- bull fluorescent yield tor the K level
~ =btMInamp energy ot tbe K level
~=yen= nUDber of electrons per gram of wall materieJ
~ middot Avogadros number
A bull atomic weight
Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and
(17 pp 57-68) tried to establish tbe validity of equation 1) tor C
Al end CU walled ionization chambers end for Xrays with e~ective11
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
2
S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation
11 number of incident photons of energy h-v middot per em- per seconcl
hv bull eJgtergy of the photon in ergs
11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam
the mass energr absorption coetncient ts detined by
~ltpoundgt=It-s (2)
tN bull mass absorption coefficient in em2 per gram
ltE) = the average energy- ot the electrons at the tJme of production in erss
At low photon energies the mass energy absorption coefflctent ba$
two camponen~s
MIt = poundG r (1- ~ )Jll (3)-middot ltV tl
ltPibull Calppton absorption ooefficient per electron in em2
7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2
f- bull fluorescent yield tor the K level
~ =btMInamp energy ot tbe K level
~=yen= nUDber of electrons per gram of wall materieJ
~ middot Avogadros number
A bull atomic weight
Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and
(17 pp 57-68) tried to establish tbe validity of equation 1) tor C
Al end CU walled ionization chambers end for Xrays with e~ective11
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
3
energies fran 25 to 125 Kev by ustns an extrapolation chamber
8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since
heterogeneous X-rays were used as the source ot radiation Siuce
these X-~s were JlOt monoenergetic the average mass energy absorp
tian eoettioient used in the ealeulations does not necessarily
con-espond to the average elierSf in the X-rar spectrum lbe extra
polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode
spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a
good extrapoletioli eJ(l)erlmenal data should have been obtained for
eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was
ignored in their cowputations
Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot
Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al
cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to
vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current
per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current
per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au
chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe
Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull
polatiou the chamber spacing shoUld have been reduced another order
of JDqllituae Also~ the radiation Wed in tlUs ~~t wa
fllterecl x~ Deepite the heavy fUtrat1ou there rematne a
oonsiderable spread 1n energy ot these x~s
~e present WOJk was undertaken to establish tl1e WUdity or laek
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
4
---shy----
cu
0 LARSON 343 KEV
ATTIX 38 KEV
AL
-----shy
FIGURE I
COMPARISON WITH ATTIXS OATA
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
5
ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl
816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers
MEtlHOD bull
For low eDerQ work the above method ot extrapolatinamp is
~tiw NOt anJy would the plate sepention measurements be
lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae
electrical ditficulties aDd talst17 volUme ~ts Another
method ot extrapolating is to hold the volume ot the chaaber constant
and vary the pressure ~ a curve ot the current per sram of air
in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot
measured by a tree air ion cbember versus pressure could be extra
polated to z~o pressure ~s latter e~rapolation technique
allows the pressure of the gas between tlle electrodes and thus the
distance between the electrodes to be reduced by at least another
order of m88Jlitude as shown in Figure 1 Equation (4) expresses the
relationslUp betveen the current (in amperes) per unit pressure (J)
and the ion current per gram of the detecting gas
(4) V =~22 X0
8J (~) ( z)~-it
J = 76o mm-bs
Itt bull volume of the Braeg-G~ chamber in cm3
r bull 273 20Jc
~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3
r bull absolute tellperature of the air in 01(
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
6
Equation (4) is a conversion of oxperlmentaJ data according to the
pertect gas lavs For a tree air ion chamber
J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)
~~~ 70 pd
~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber
r =current in amperes as read by tbe vibrating reed electroshymeter
Vir eollectiDg volume of the tree air ion chamber in cm3
7J ~ absolute temperature of the alr in the free air ion chamber in OK
P bull atmospheric pressure in Dlll-hg
D1vi41ng equation (4) by equation (5) results in I
r7 - J ( tl gt( 7 ) (6) -rj Vc r
Also for chambers With different vall materials but the ssme
detecting gas
(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its
tbetretical counterpart equation (10~ in the energy region from
816 to 34middot3 Kev
A nearly monoenergetic photon source (18 ppl00-102 proViding
energies han 816 to 109 Kev was developed by Larson et eJ wbich
uses the K fluorescence ra41ation trom targets of different atanic
numbers ~ deVice provides a near~ uniform six-inch beam with
max1mum intensity depending on ~ energJ from 0055 to 141 rhr
~s instrument eHmnated the need for usina heterogenous X-rqs
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
7
~e radiators used tor tbe experiment were copper zirconium~ cadmium
and lanthanum which correspond to energies of 816 161 237 and 34middot3
Kev respectvely ~ above energies wre verified b1 absorption
measurements in an aluminum medium
VaJues tor the factor nlrv (equatton 1) ean be found by tree air
ion chamber measurements ~e equation relating errergy flux and
measured current per Ullit mass 1st
(8)
~= maoc energy absorption coefficieut for air in cm2 per gram
~ above method of obtainitl8 hliV is impractical for high energy gamma
rays Since the range of the ionizing electrons would exceed a praotishy
cal cheimber electrode spacing middot A substitution of equation (8) into
e uation (1) recults in
(9)
or
(10)TJ =~ middot~ s
W cancels and does not eppear tn equation 10) beeause 1t de~
owyl onmiddot the gas used which ic ascumed to be air tor both the tree
oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett
1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
8
1n equation (10) are the quantities measured in the present eJCperishy
ment ~ quantities on tbe right ere reasonably well known ~
right side ot tbe equation V1ll be called the theoretical value
ot JJeJrbull It the theoretical and e~rimental value are found
to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple
Ccmpton bsorpt1on coetticients per electron fG were obtaiDed
frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy
efiicients per tan (~z) were interpolated trail tables canpiled
by White (26 pp48-58) ~ average K shell bind1og energies
(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission
lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned
fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe
fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld
o46 respective~ 1lree correctiollS vere made to the absorption
coetfic1ents 7he first was a correction for tbe small amount ot
high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe
second vas a correction tor tbe ~ti sin the waU ~J~aterials ~
1mpurities in tM aluminum were determined by a quazrt1tetive spectro
chemical analysis and the copper impurities were determined by tbe
matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh
values ot tbe copper absorption coetficients tbe 1DJpurities in
copper were les important thau the impurities in aluminum lbe
third correction vas tor the contribution of tbe L level photoshy
electric effect to the mass energy bsorption coeftieients
stowiJlg power ratios were cazwuted tran the non-relativLstic
BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
co ot
lt gtw (11)
s--~-- ltgt
(pound)~otw
lt2 ~~
N
lt Q
t ( bull )f
hc gtgt ~ (12)
t1 bull JIIIEUdr 4Uienaion ot
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
10
Energy SAl e (cm2jg) JAJfJair
Kev Al Air
816 0835 433 7-84 661pound 026
161 0851 5~75 0942 7middot17 l 030
237 o86o 181 0283 741t- t 032
34-3 o864 0-556 oo9lf4 682f 029
ergy seu ea (em2fg) JeuJaJr
cv Cu r
1 1
237
34-3
o683
0 700
0709
46
rr o
621
942
0 283
0944
72 l l
85 8f 36
28 l 3middot9
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
11
However in determining the amount opound energy per gram tbat 1s absorbed
trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat
the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy
where in tbe absorbing medium For this to be true the xponential
X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity
must be at least nne electron range thick
(13)
(l4)
bull linear absorption coetficient of the I8Jl material
t bull extrapolation chamber wall thickneGs
and
(15)
R range of the secondary electrons in the wall material
-he assembly of the ionization cllembers that vere used 1n this f
experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot
m1ze absorption of the xray$ in the chamber mlls thin foils Of
Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls
were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al
se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al
foil and we1gbiDS a known area of CU foil Duco ccment was used to
bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy
l8 long) Side walls were crewed onto the MFPlo insulating
rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode
vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
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14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
r - WALL MATERIAL
tgggSI - AL ELECTRICAL SHIELDING
1~middot~~middot~1 - LUCITE RINGS~ tbullgt
-----1 -MFP-10 RINGS
PLATE I IONIZATION CHAMBER ASSEMBLY
I- )
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
13
0
1amp1 CD 2
~
~X
~0
0
C)Z
o
oshy0~
ZN
~z
0
UC
bullGl IC
HL
UID
WH
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
14
1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~
the cunent leakaampe to a neeligible value A ~ eleetlOde
ebamber vas used ampO that the coUectina electrode vould be eleetroshy
stati~ shielded Another advantaae o~ the three electrode
chamber was that 1t helped to elbdnate transfer current By transfer
ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which
cross the cbamber 1-lle etteet ot ttellSfer current vas complete~
elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad
averaatns the two r~s 1-le cbamber lengths were measured w1th
a micraneter caliper and the diameters of the lucite and MFP-10
~s were determined by a metal ruler Hw~r1 tJle largest error
in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin
metal foUs At ~ssures above two mbullhs the condftion o~ voltage
satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot
zatton cllambers bull
rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD
standard steel pipe A steel plate l4 thick was welded over the
tront of tM plpe In the center ot the steel plate a hole SfJt
inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was
mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound
a l4n JU Plate Eoth tbe front and the rev ~s were vacuum
sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere
chosen so that the Umbra ot the beam from tlle fluorescentent X-ray
sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum
cbambel vas Qheeked by measurinS the current collected by the
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
15
bull ~
tTTI
~ ~ ~ ~
11J~ ~ ~
middot -
~ middot STEEL
~ - LEAD
~-BRASS 13 -ALUMIMJM
middot ~ middot bull ~ ~ II o
a ---RADIATOR
__ ~ bull ~ bull
tTTI
~ ~ ~
1- SHIEI0pound0 CA81E
I_ I~~~ I I
ELECT_~ I
FOilpound bull -PLASTICPUIII
IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~
-~~In ~ ~
0 1 I I 4
ICALl UICMI S
PLATE 2
THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
16
extrapolatton chambers 1llth tbe vacuum chamber present and WithOut
the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was
tound to be negllgible A l4 lucite rod se~ted tbe grounded
vacuum middotchamber tran the extrapolation chambers he expertmental
system 1G picture4 in Pbotogra_ph 2
~ ion current was measllled wtth a Vibrating reed eJeotrcmeter
~ Ballle glasabullseeled resistor was used in the head or the Vibretine
reed eleetraueter tor both the tree atr ion chamber current ~shy
ments and tbe extrapolation cbamber current measurements A value
tor tbe resistance (R need not be known since the factor R SJpears
on both sides of equation (9) as a tactor in eaeb of the J terms
~ teurperature coeifici~t (25 p2 oi the resietor wed was
l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se
low coett1e1ents pemitted neglect ot cbanSes in resistance due to
temperature and vtiltase
A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice
~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat
of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube
vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1
suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer
Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil
was used middotas the working fluid in the manometer ~ density ot tbe
oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd
tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS
the weight and the volume of tbe oi~ the density opound the oil ws
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
18
C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five
microns Wtth an oil rnauaneter in COlljunction with a cathetometer1
e pressure of one mm-hg could be dete cd to ttGr than one ])er eent
aocuracy
In equation (5) the factor lairVaJr was determined by the use of
a tree air ionization cballber in eonjunct1on Yith a vtbrating reed
electrometer ~e tree atr ionization chamber consisted ot two parshy
allel oluminum plates that were separated by a distance o~ nine inchesmiddot
~ proper plate separation vas detemtned by plotttns a curve ot
current ve-sun plate srncins and ttnding that spacing beyond Yhich
the current no longer depeoded on the plate spacing rhe dimen
sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull
OOlllleCted to ooo of the electrodes the proper voltaee tor saturation
was detenninad by plotting a curve ot ionization current versus voltshy
age fbe other eJectrode was cut tnto three sections ~ center
section (1 x 16 5n) WaS used as the collecting electrode aJld the
two outer sections (1011 x 165 ) were grounded and used as suard
rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber
through a ttro~inch diameter hole that was drUled in a l4 thick
lead sbeGt The etfective volume at the chamber was determined fran
tbe area opound this aperture1 the Width o-r the collecting cleetrode
alld the average length of ~P between the BUSrd plates and the colshy
le~tor ~s was not the eollectins volUllle11 but tllls volume is
equivalent to the coUeet1ng volUllle for any point source or element
of a finite source ~e ionization current vas 11eSsured by a
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
19
VibratitJg ~ electrometer
Lhe xray tube was alloved to warm up for a period of epproX
mateU twenty minutes before bull~ents -weremiddot taken A radiator
middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml
ber was centered on the ~ter line ot the side part ot the f~uores
eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~
voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro
111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes
ln this time interval tbe middottQQtn ~ture and p~sslle were also
recorded 1be free air 1oniut1on cluwber was removed and the aluminun
lWagg-Gr~ chamber was located so that tbe center ot the coUeot~
volume was iu tbe position that had pleviously been occupied by the
free air ionization chamber Jhe press~ inaide the aluminum
BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage
preampsure temperature and proper saturation voltage vere recorded by
be same procedure as~ U$ed tor the tree air ionization cbamber
middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull
ber vas reversed end the voltllge etc were recorded ~se two
voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot
tDeasurements were repeated ~or a series of lower pressures P21 P3 ete
Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy
tion cbamber was moved into position Another set of tree atr
ionization chamber measurements vasmiddot middotrecorded ~us the tree air
ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue
or Ja1r but also as a measurement of the stability ot the ~luoJescent
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
x-~ bullmiddot ~above expertmental steps were repeated tor the
cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping
the values of the X-ray tube voltege atJl cun-eat the same tor all
tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were
plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~
couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent
detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the
electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent
uncertatnty lbe above expe~tal steps were repeated tor aU ot
the tl~scent ~ators
~ ~tion curves tor the Al aDA middotCU BraagGrat chambers
are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the
curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent
per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be
Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese
valUea ere llsted in ~ble 3 along Vlth the theoretical velues A
ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the
ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4
shoal4 beve a zero slope However1- at preseltea below two -h6
voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers
uaeltl in tb1s eXPertment J]leretore1 this tact could not be
verified
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
21
---~~ ~ cu
c 816 KEV
6 161 KEV
0 237 KEV
0 0
ts
0 0
6shy
0 0
Ashy ~
PRESSURE IIIII - HGI
FIGURE 2
fXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAlA)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
--------
22
100
I I
f f
0
10 shy1shy
I I I I I I
cu - -
343 KEV
-------
-
-AL -
~
~
N
~-----f
f
1shy
f
0 o----o 0 0 0 0 0 -----
- -
-
I I I I I I I I I I I I I
0 2 4 6 8 10 12
PRESSURE (liM - H6)
FIGURE 3
EXTRAPOLATION CURVES
THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS
MEASURED BY A FREE AIR ION CHAMBER (JAIR)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
23
Energy JAJJaJr JeuJatr
Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal
816 641 l ozr 661 i o2S
161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36
3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9
COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ
JouJAJ
Kov 1heoretical r
61 9middot43l 34 100 l 042
middot1 ll l l 4 ll5 t 0 5
34middot3 13middot9 i middot5 136 1 6
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
24
A~ absorption correction wa-s aPPlied to the Cu and AJ
extra)Olation chamber data L10 air ebsorption eorrections were
mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between
the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot
tetion Qhamber and the seeolld -a tor the lack ot air absorption
in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs
A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt
X-~ beam ~e correction was detetmined from the density on
exposed photographic plates Both the free air toni zat1on ehamber
and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy
ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot
correction was_~ to the JAJ valW to campensate tor this tUttra -
ebsorbing leyer ot cellulose acetate the e1lr on tbe current
ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A
the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error
on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy
on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls
anA 1J ~or the cu vallE
abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the
lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or
ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm
and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and
e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the
JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S
enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to
bull
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
25
be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant
in this energy rall8e For ~le the Ct1 mass energy- ebsorptton
coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn
~ble 2 woula be 299 em2e it the fluoreseent neld factor were
lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh
t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d
be in error by au appreciable Mlmlnt but the en-ors eould ean0$1
out in the stopping paVer ratio Jhe ourves in ~ l extend over
a lar e r~ of pressure ~ese data ~ihov that at high poessureamp
the DraeemiddotG~ chamber response epprooches that of an air reJ1ed
chamber (9~PPmiddot231middot240)~
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
BIBLIOGRAPHY 26
1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949
2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)
3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)
4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953
$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p
6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4
7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954
8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937
9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954
10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$
11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
27
12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937
13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37
14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936
15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p
16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t
17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952
18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955
19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937
20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953
21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)
22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940
23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)
28
24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952
25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)
26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)