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Comparison of the standards for absorbed dose to water

of the ARPANSA and the BIPM for 60

Co gamma radiation

C. Kessler1, D.T. Burns

1, P.J. Allisy-Roberts

1, D Butler

2, J. Lye

2, D. Webb

2

1 Bureau International des Poids et Mesures, F-92312 Sèvres Cedex, France

2 Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Australia

Abstract

A comparison of the standards for absorbed dose to water of the Australian

Radiation Protection and Nuclear Safety (ARPANSA), Australia, and of the

Bureau International des Poids et Mesures (BIPM) was carried out in the 60

Co

radiation beam of the BIPM in June 2010 under the auspices of the key

comparison BIPM.RI(I)-K4. The comparison result, based on the calibration

coefficients measured for two transfer standards and expressed as a ratio of

the ARPANSA and the BIPM standards for absorbed dose to water, is

0.9973 (53). This result replaces the 1997 ARPANSA value of 1.0024 (30) in

this key comparison. The degrees of equivalence for the ARPANSA and the

other participants in this comparison have been calculated and the results are

given in the form of a table for the national metrology institutes (NMIs) that

have results published in this ongoing comparison for absorbed dose to water.

A graphical presentation is also given.

1. Introduction

An indirect comparison of the standards for absorbed dose to water of the Australian

Radiation Protection and Nuclear Safety Agency (ARPANSA), Australia and of the Bureau

International des Poids et Mesures (BIPM) has been carried out in 60

Co radiation. The

measurements at the BIPM took place in June 2010. This absorbed dose to water comparison

replaces the indirect comparison made between the two laboratories in 1997 [1] that was

previously registered in the BIPM.RI(I)-K4 key comparison [2].

The absorbed dose to water is determined at the ARPANSA using a graphite calorimeter with

a calculated dose conversion factor from graphite to water using Monte Carlo methods. The

BIPM primary standard is a parallel-plate graphite cavity ionization chamber [3].

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The comparison was undertaken using two ionization chambers of the ARPANSA as transfer

standards. The result of the comparison is given in terms of the mean ratio of the calibration

coefficients of the transfer chambers determined at the two laboratories under the same

reference conditions.

The comparison result has been approved by the Consultative Committee for Ionizing

Radiation (CCRI) and the degrees of equivalence for the ARPANSA and the other participants

in this ongoing comparison for absorbed dose to water are presented in the form of a table in

Section 5. A graphical presentation is also given.

2. Determination of the absorbed dose to water

At the BIPM, the absorbed dose rate to water is determined from

ikseWmID Π))(( ac,BIPM w, , (1)

where

I is the ionization current measured by the standard,

m is the mass of air in the ionization chamber,

W is the mean energy expended in dry air per ion pair formed,

e is the electronic charge,

is the ratio of the mean mass stopping powers of graphite and air, and

ki is the product of the correction factors applied to the standard.

This product of correction factors ki includes the fluence perturbation factor kp, which

itself is the product of ( /en

)w,c, the ratio of the mean mass energy-absorption

coefficients, w,c, the ratio of the photon energy fluences, and (1 + )w,c, the ratio of the

absorbed dose to collision kerma ratios.

The values of the physical constants and the correction factors entering in (1) are given in [4]

together with their uncertainties, the combined relative standard uncertainty being

2.9 10–3

. The uncertainty budget is reproduced in Table 1.

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Table 1. Physical constants, correction factors and relative standard

uncertainties for the BIPM ionometric standard for absorbed dose to water

Symbol Parameter / unit Value Relative standard uncertainty

(1)

si ui

Physical constants

a dry air density (0°C, 101.325 kPa) / kg m–3

1.2930 – 0.01

( en/ )w,c ratio of mass energy-absorption coefficients 1.1125 (2)

0.01 (2)

0.14 (2)

sc,a ratio of mass stopping powers 1.0030 } 0.11

(3)

W/e mean energy per charge / J C–1

33.97

Correction factors

kp fluence perturbation 1.1107 0.05 0.17

kps polythene envelope of the chamber 0.9994 0.01 0.01

kpf front face of the phantom 0.9996 – 0.01

krn radial non-uniformity (4)

1.0056 0.01 0.03

ks saturation (4)

1.0017 0.01 0.01

kh humidity 0.9970 – 0.03

Measurement of I /

effective volume / cm3 6.8810

(5) 0.19 0.03

I ionization current (T, P, air compressibility) – – 0.02

short-term reproducibility (including

positioning and current measurement) (6)

0.02 –

Combined uncertainty of the BIPM determination of absorbed dose to water rate

quadratic summation 0.20 0.21

combined relative standard uncertainty 0.29

(1) expressed as one standard deviation.

si represents the relative uncertainty estimated by statistical methods, type A

ui represents the relative uncertainty estimated by other methods, type B.

(2) included in the uncertainties for kp.

(3) uncertainty value for the product sc,a W/e.

(4) values for the CISBio beam adopted in November 2007.

(5) standard CH4-1.

(6) over a period of 3 months. The long-term reproducibility over a period of 15 years, uR is 0.0006.

At the ARPANSA, the absorbed dose rate to water is determined by means of a graphite

calorimeter operated in quasi-isothermal mode and the absorbed dose conversion from

graphite to water is made by calculation using the Monte Carlo method. The absorbed dose to

graphite is determined from

)(ARPANSA core,

mPD , (2)

where:

P is the change in electrical power required to exactly compensate the radiation

heating (calculated from voltages measured across the core thermistor and a

known resistor in the same circuit), and;

m is the mass of the core.

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The absorbed dose to water is determined from

MCcorewARPANSA core,ARPANSA w,DDDD , (3)

where (Dw/Dcore)MC is the ratio of absorbed dose to water to absorbed dose to the calorimeter

core, calculated using the Monte Carlo method (see Figure 1). The correction for vacuum

gaps usually required in graphite calorimetry is included in this ratio.

The design and operation of the calorimeter is described in [5] and some pertinent details are

given in the following paragraphs. A summary of the components of uncertainty is indicated

in Table 2, giving a combined relative standard uncertainty of 3.1 10–3

.

The Australian absorbed dose standard is a graphite calorimeter of the Domen design [6]. The

electrical calibration factor of the calorimeter and the digital voltmeter calibration factor are

evaluated before starting the calorimetry runs. Calorimetry runs are first made in a quasi-

adiabatic mode and the mean dose-rate is determined. Using this dose-rate, appropriate

heating powers for the core, jacket and shield of the calorimeter are calculated based on their

respective masses. The calorimeter is then operated in a quasi-isothermal mode [7], in which

the core, jacket and shield are heated with the selected heating rates. After a 150 second

electrical heating run, the electrical power is stopped and the 60

Co beam is opened. The

irradiation time is 300 seconds and at the end of the radiation heating the electrical heating is

started for another 150 seconds. The runs are recorded using a Labview program and analysed

using Matlab scripts.

The EGSnrc user code BEAMnrc was used to model the Eldorado 78 Cobalt 60 source and

housing. A phase-space file output at 100 cm from the centre of the source was then used as

the source for subsequent BEAMnrc models of the calorimeter and water phantom. Figure 1

shows the geometry of the calorimeter model on the left hand side, and the right hand side

shows the geometry used to model the water phantom. The calorimeter and water phantom are

set up so that the core is at the same distance (105 cm) from the 60

Co source for the effective

point of measurement in the water phantom at 5 cm depth. The dose-scoring voxel in the

water phantom is the same dimension as the calorimeter core, ensuring that the absorbed dose

to water rate, as described in Equation 3, is the absorbed dose-rate averaged over the central

2 cm diameter core size.

Figure 1 Schematic of geometry used in BEAMnrc for modelling the ARPANSA calorimeter (left) and the

water phantom (right).

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Some 5 × 108 primary histories were used in the model of the

60Co source, combined with the

directional bremsstrahlung splitting variance reduction technique. The model of the cobalt

source was validated against water tank measurements of radial beam profiles and the

percentage depth dose curve of a 10 cm × 10 cm field. In the model of the calorimeter and

water phantom, no variance reduction was employed, and 9 × 1010

primary histories were

used. Each model was run four times with different seeds, and the average dose and standard

uncertainty of the mean were calculated from the runs.

The uncertainty in the ratio (Dw/Dcore)MC has a type A component of 0.21 % from the standard

uncertainty in multiple MC runs, and a type B uncertainty of 0.32 % due to the uncertainties

in the Monte Carlo model. The Type B uncertainty was estimated from uncertainties in the

modelled geometry (such as the graphite density and calorimeter dimensions) and by

estimating an upper limit to the error introduced by the Monte Carlo transport and interaction

coefficients, assessed by comparing measured depth-dose curves in graphite and water with

their modelled counterparts.

Table 2. Physical constants and correction factors entering in the ARPANSA

determination of the absorbed dose rate to water at 5 g cm–2

,

and estimated relative standard uncertainties

Source of uncertainty Value Relative standard uncertainty

100 si 100 uj

Determination of ARPANSAcore

)( D

P electrical power 0.08

m mass of the core / g 1.5622 0.01

Repeatability 0.07

krn radial non-uniformity 1.0000 0.05

Conversion to absorbed dose rate to water by calculation

(Dw/Dcore)MC conversion from graphite to water 0.9308 0.21 0.32

Uncertainty in ARPANSAw

)( D

quadratic summation 0.24 0.32

combined relative standard uncertainty 0.40

Reference conditions

Absorbed dose to water is determined at the BIPM under reference conditions defined by the

CCRI, previously known as the CCEMRI [8]:

the distance from the source to the reference plane (centre of the detector) is 1 m;

the field size in air at the reference plane is 10 cm 10 cm, the photon fluence rate at the

centre of each side of the square being 50 % of the photon fluence rate at the centre of the

square; and

the reference depth in the water phantom is 5 g cm–2

.

At the ARPANSA:

the distance from the source to the water surface is 1 m;

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the field size in air at the water surface is 10 cm 10 cm, the photon fluence rate at the

centre of each side of the square being approximately 50 % of the photon fluence rate at

the centre of the square; and

the reference depth in the water phantom is 5 cm, leading to a source-detector distance of

1.05 m.

Reference values

The BIPMw,

D value is taken from the mean of the four measurements made around the period

of the comparison. The value is given at the reference date of 2010-01-01, 0 h UTC as is the

ionization current of the transfer chambers. The half-life of 60

Co was taken as 1925.21 days

(u = 0.29 days) [9].

The value of ARPANSAw,

D used for the comparison is the mean of 32 calorimetry measurements

made before and immediately after the period of the comparison. The ARPANSA reference

value of 9.0144 mGy/s is normalized to the laboratory reference date 2010-05-07, 10 h, UTC

using the same 60

Co half-life [9].

3. The transfer chambers and their calibration

The comparison of the ARPANSA and BIPM standards was made indirectly using the

calibration coefficients w,DN for the two transfer chambers given by

lablabw,labw,,IDN

D , (4)

where lab w,D is the water absorbed dose rate at each lab and Ilab is the ionization current of a

transfer chamber measured at the ARPANSA or the BIPM. The current is corrected for the

effects and influences described in this section.

The ionization chambers NE 2571 serial number 3075 and NE 2561 serial number 328, both

belonging to the ARPANSA, were used as transfer chambers for this comparison. Their main

characteristics are listed in Table 3. These chambers were calibrated over several months at

the ARPANSA before and after the measurements at the BIPM.

The experimental method for calibrations at the ARPANSA is described in [10] and that for

the BIPM in [4]. The essential details are reproduced here.

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Table 3. Characteristics of the ARPANSA transfer chambers

Characteristic/Nominal values NE 2571 NE 2561

Dimensions Inner diameter 6.4 mm 7.4 mm

Wall thickness 0.065 g cm–2

0.090 g cm–2

Cavity length 24.0 mm 9.2 mm

Tip to reference point 13 mm 5 mm

Electrode Length 21.0 mm 6.4 mm

Diameter 1.0 mm 1.7 mm (hollow)

Volume Air cavity 0.60 cm3 0.30 cm

3

Wall Material graphite graphite

Density 1.7 g cm–3

1.7 g cm–3

Voltage applied to

outer electrode Negative polarity 250 V 200 V

Positioning

At each laboratory the chambers were positioned with the stem perpendicular to the beam

direction and with the same orientation (text or line on the stem of the chambers facing the

source and text on the sleeves facing the source).

Applied voltage and polarity

A collecting voltage as indicated in Table 3 was applied to the outer electrode of each chamber

at least 30 min before measurements were made. No polarity correction was applied as both

laboratories apply the same polarity.

Volume recombination

Volume recombination is negligible at a dose rate of less than 15 mGy s–1

for these chambers

at these polarizing voltages, and the initial recombination loss will be the same in the two

laboratories. Consequently, no correction for recombination was applied.

Charge and leakage measurements

The charge Q collected by the transfer chamber was measured using a Keithley electrometer,

model 642 at the BIPM. The source is operational during the entire exposure series and the

charge is collected for the appropriate, electronically controlled, time interval. At the

ARPANSA, the charge collected was measured with the ARPANSA Current Integrator, a

system for measuring small currents constructed by ARPANSA in 1993 using a published

design [11]. At both the BIPM and the ARPANSA, pre-irradiation was for at least 30 min

before any measurements were made.

The ionization current measured from each transfer standard was corrected for the leakage

current. For both chambers at both laboratories this correction was less than 1 10–4

in

relative terms, except for the NE 2561 at the BIPM where the value was 3 10–4

.

Ambient conditions

During a series of measurements, the water temperature is measured for each current

measurement and was stable to better than 0.01 °C at the BIPM and 0.2 °C at the ARPANSA.

The ionization current is normalized to 293.15 K and 101.325 kPa at both laboratories.

Relative humidity is controlled at (50 5) % at the BIPM. At the ARPANSA the humidity is

not controlled but was stable at (40 5) % during chamber measurements. Consequently, no

correction for humidity is applied to the ionization current measured.

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Radial non-uniformity correction

At the ARPANSA, no correction is applied to the ionization current for the radial non-

uniformity of the beam over the cross-section of the transfer chambers. Instead, an uncertainty

of 5 10–4

is applied. At the BIPM, the corrections applied to the ionization current would

only be 1.0002 for the NE 2561 and 1.0008 for the NE 2571, each with an uncertainty of

2 10–4

. Consequently, no non-uniformity correction is made.

PMMA phantom window and sleeve

Both laboratories use a horizontal radiation beam and, at the BIPM, the thickness of the

PMMA front window of the phantom is included as a water-equivalent thickness in g cm–2

when positioning the chamber. In addition, the BIPM applies a correction factor kpf (0.9996)

that accounts for the non-equivalence to water of the PMMA in terms of interaction

coefficients. At the ARPANSA, the window density is not taken into account when setting the

chamber position, but rather the water tank window correction kwin (1.0012) corrects for the

electron density difference between water and PMMA over the window thickness (1.9 mm).

Individual waterproof sleeves of PMMA were supplied by the ARPANSA for each NE

chamber. The same sleeves were used at both laboratories and, consequently, no correction

for the influence of each sleeve was necessary at either laboratory.

Uncertainties

Contributions to the relative standard uncertainty of lab w,,D

N are listed in Table 4. The two

laboratories determine absorbed dose by methods that are quite different. Although the

ARPANSA makes a global Monte Carlo calculation for the dose conversion, some correlation

exists between the BIPM and the ARPANSA in the values used forcw,en

. In accordance

with the analysis used for the existing data in the BIPM.RI(I)-K4 comparison [2], this

uncertainty component is taken to have a correlation coefficient fk = 0.95 and an attempt to

take this into account for RDw has been included in the Table 4. Consequently, the combined

uncertainty of the result of the comparison is obtained by summing in quadrature the

uncertainties of BIPMw,

D andARPANSAw,

D , taking correlation into account and including the

contributions arising from the use of transfer chambers. These latter terms include the

uncertainties of the ionization currents measured, the distance to the reference plane and the

depth positioning.

The relative standard uncertainty of the mean ionization current measured with each transfer

chamber over the short period of calibration was estimated to be 1 10–4

(two calibrations

with repositioning, in series of 30 measurements for each chamber) at the BIPM. At the

ARPANSA the calibration of each chamber was repeated twice with repositioning before and

after the measurements at the BIPM. The relative standard uncertainty of the mean ionization

current measured at the ARPANSA with a given transfer chamber over the several months

required for this comparison was 7 10–4

.

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Table 4. Estimated relative standard uncertainties of the calibration

coefficient,lab w,,D

N , of the transfer chambers and of the comparison result,w,D

R

BIPM ARPANSA

Relative standard uncertainty of 100 si 100 ui 100 si 100 ui

Absorbed dose rate to water (tables 1 and 2), uDw 0.20 0.21 0.24 0.32

Ionization current of the transfer chambers 0.01 0.02 0.07 –

Distance 0.02 – – 0.10

Depth in water 0.02 0.06 – 0.11

Normalization T, P – – – 0.05

Electrometer calibration factor – – – 0.05

Ion recombination – – – 0.01

Polarity – – – 0.01

Source decay – – – 0.02

Radial non-uniformity – 0.02 – 0.05

Phantom window – – – 0.02

Relative standard uncertainties of lab w,,D

N

quadratic summation 0.20 0.22 0.25 0.36

combined uncertainty 0.30 0.44

Relative standard uncertainties of w,D

R 100 si 100 ui

quadratic summation 0.32 0.43

combined uncertainty, uR 0.53

The relative standard uncertainty of the mean ionization current measured with each transfer

chamber over the short period of calibration was estimated to be 1 10–4

(two calibrations

with repositioning, in series of 30 measurements for each chamber) at the BIPM. At the

ARPANSA the calibration of each chamber was repeated twice with repositioning before and

after the measurements at the BIPM. The relative standard uncertainty of the mean ionization

current measured at the ARPANSA with a given transfer chamber over the several months

required for this comparison was 7 10–4

.

4. Results of the comparison

The result of the comparison, w,DR , is expressed in the form

, BIPM w,,ARPANSA w,,w, DDD

NNR (5)

in which the average value of measurements made at the ARPANSA prior to those made at the

BIPM (pre-BIPM) and those made afterwards (post-BIPM) for each chamber is compared

with the mean of the measurements made at the BIPM. Table 5 lists the relevant values of

w,DN for each chamber at the stated reference conditions.

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Table 5. Results of the comparison

Transfer

Chamber

ARPANSAw,,DN

/ Gy µC−1

pre-BIPM *

BIPM w,,DN

/ Gy µC−1

ARPANSAw,,DN

/ Gy µC−1

post-BIPM *

ARPANSAw,,DN

/ Gy µC−1

overall mean w,DR uR,NMI

NE 2561-328 103.061 103.282 102.971 103.016 0.9974 0.0053

NE 2571-3075 45.174 45.274 45.134 45.154 0.9973 0.0053

Mean values 0.9973 0.0053

* Correction for ion recombination normally applied at the ARPANSA was not included here, as the

correction is the same at both laboratories.

The comparison result is taken as the unweighted mean value for both transfer chambers,

w,DR = 0.9973, with a combined standard uncertainty for the comparison of 0.0053,

demonstrating the agreement between the two absorbed dose to water standards.

5. Comparison with other National Metrology Institutes

Comparison of a given NMI with the key comparison reference value

Following a decision of the CCRI, the BIPM determination of the dosimetric quantity, here

Dw,BIPM, is taken as the key comparison reference value (KCRV) [12]. It follows that for each

NMI i having a BIPM comparison result RD,w,i that is denoted xi in the key comparison

database (KCDB), with combined standard uncertainty ui, the degree of equivalence with

respect to the reference value is given by a pair of terms:

the relative difference: Di = (Dwi – Dw,BIPMi)/ Dw,BIPMi = RD,w,i – 1 (6)

and

the expanded uncertainty (k = 2) of this difference,

Ui = 2 ui. (7)

The results for Di and Ui, are expressed in mGy/Gy. Table 6 presents the degrees of

equivalence Di for each of the NMI participants, using the acronyms given in the KCDB. The

results are presented graphically in Figure 2. It should be noted that these data, while correct

at the time of publication of the present report, become out-of-date as NMIs make new

comparisons. The formal results under the CIPM MRA [13] are those available in the KCDB.

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Table 6. Evaluation of degrees of equivalence as presented in the KCDB

The key comparison reference value is the BIPM evaluation of absorbed dose to water.

The degree of equivalence of each laboratory i with respect to the key comparison reference value is given by a pair of terms, both expressed in mGy/Gy:

the relative difference Di = (xi -1) and Ui, its expanded uncertainty (k = 2), with Ui = 2ui.

When required, the degree of equivalence between two laboratories is given by a pair of terms

both expressed in mGy/Gy: Dij = Di - Dj and Uij, its expanded uncertainty (k = 2). In evaluating Uij = 2 uij for these degrees of equivalence, account should be taken of correlation between ui

and uj.

Linking regional comparisons SIM.RI(I)-K4 or EUROMET.RI(I)-K4 to BIPM.RI(I)-K4

The value xi is the comparison result for laboratory i participant in SIM.RI(I)-K4 having been

normalized to the value of the NRC as the linking laboratory (see SIM.RI(I)-K4 Final Report), or in EUROMET.RI(I)-K4 having been normalized to the mean value of the linking laboratories (see EUROMET.RI(I)-K4 Final Report)

The degree of equivalence of each laboratory i participant in SIM.RI(I)-K4 or EUROMET.RI(I)-K4 with respect to the key comparison reference value is given by a pair of terms, both expressed in mGy/Gy: the relative difference Di = (xi - 1) and Ui, its expanded uncertainty (k = 2).

See the relevant RMO Final Report in the KCDB for the approximation used for Ui in the Matrix of equivalence.

Lab i Di Ui

Lab i Di Ui

/ (mGy/Gy) / (mGy/Gy)

METAS -0.1 10.8 CIEMAT -4.9 7.3

MKEH -1.7 9.6 CMI -4.0 23.6

LNE-LNHB -3.0 10.6 RMTC -5.3 12.0

PTB -3.9 7.4 SSM -1.4 10.0

VSL -7.4 9.8 STUK -3.9 8.5

ENEA -0.1 8.8 NRPA 3.2 8.8

NPL -2.0 12.8 SMU -4.7 24.7

BEV -0.4 8.8 IAEA -0.4 10.0

VNIIFTRI -2.4 8.6 HIRCL 3.0 12.4

NRC -2.0 10.4 ITN -7.1 13.0

NMIJ -4.0 9.2 NIST -0.6 11.1

ARPANSA -2.7 10.6 LNMRI 1.0 15.0

CNEA 12.0 17.9

ININ 3.9 23

NMI acronyms are given in the KCDB

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Comparison of any two NMIs with each other

When required, the degree of equivalence between any pair of national measurement

standards can be expressed in terms of the difference between the two comparison results

jijiij RRDDD (8)

and the expanded uncertainty (k = 2) of this difference, Uij = 2 uij, where

k

jkk

k

ikkjcicijufufuuu

2

corr,

2

corr,

2

,

2

,

2 (9)

of which the final two terms take into account the correlation between the primary standard

methods. The relevant uncertainty components uk,corr and correlation coefficients fk for the

graphite calorimeters of the NMIs and the ARPANSA are given in Table 7.

Table 7. Correlated uncertainty components for graphite calorimeter standards

for absorbed dose to water, relative standard uncertainties per 103

Country NMIa Ref. kgap

cw,en

( β )w,cb

Hungary MKEH [2] 0.8 3.0 0.6

France LNE-LNHB [14] 1.5 1.5 0.5

Netherlands VSL [15] 0.7 3.0 0.6

Italy ENEA [16] 0.8 1.4 1.0

UK NPL [17] 1.3 1.8 0.5

Austria BEV [18] 1.5 1.0 1.0

Russia VNIIFTRI [19] 1.0 2.9 0.5

Japan NMIJ [20] 1 1.4 0.5

Australia ARPANSA – – 2.0 –

fk,BIPM – 0.95 0.7

fk,NMI 0.5 0.95 0.7

a Acronyms are given in the KCDB

b βw,c = (1 + )w,c, the ratio of the absorbed dose to collision kerma ratios in water (w) and

graphite (c).

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Figure 2 Graph of the degrees of equivalence with the KCRV

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

ME

TA

S

MK

EH

LN

E-L

NH

B

PT

B

VS

L

EN

EA

NP

L

BE

V

VN

IIF

TR

I

NR

C

NM

IJ

AR

PA

NS

A

ININ

CIE

MA

T

CM

I

RM

TC

SS

M

ST

UK

NR

PA

SM

U

IAE

A

HIR

CL

ITN

NIS

T

LN

MR

I

CN

EA

DN

MI /

(m

Gy/G

y)

BIPM.RI(I)-K4, 2002 SIM.RI(I)-K4 and 2005 to 2008 EUROMET.RI(I)-K4

Degrees of equivalence for absorbed dose to water

N.B. A black square indicates a result that is more than 10 years old.

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6. Conclusions

A new key comparison of primary standards for absorbed dose to water in 60

Co gamma rays

has been carried out between the ARPANSA (Australia) and the BIPM, using two ionization

chambers as transfer standards. The comparison result, based on the calibration coefficients

measured for the transfer chambers and expressed as a ratio of the ARPANSA and the BIPM

standards for absorbed dose to water, is 0.9973 (53).

When compared with the results of the other national metrology institutes

that have carried out comparisons in terms of absorbed dose to water at the BIPM, the

ARPANSA standard for absorbed dose to water is in satisfactory agreement.

References

[1] Allisy-Roberts P.J., Burns D.T., Boas J.F., Huntley R.B., Wise K.N., 1999, Comparison

of the standards of absorbed dose to water of the ARPANSA and the BIPM for 60

Co

gamma radiation, Rapport BIPM-1999/17.

[2] Allisy-Roberts P.J., Burns D.T., Summary of the BIPM.RI(I)-K4 comparison for

absorbed dose to water in 60

Co gamma radiation, Metrologia, 2005, 42, Tech. Suppl.,

06002

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