880.P20 Winter 2006 Richard Kass 1
Scintillation Devices
As a charged particle traverses a medium it excites the atoms (or molecules)in the the medium. In certain materials called scintillators a small fraction ofthe energy released when the atoms or molecules de-excite goes into light. ENERGY IN LIGHT OUT
The use of materials that scintillate is one of the most common experimentaltechniques in physics.
Used by Rutherford in his scattering experiments
Scintillation light can be used to: Signal the presence of a charged particle Measure the time it takes for a charged particle to travel a known distance
(“time of flight technique”) Measure energy since the amount of light is proportional to energy deposition
There are lots of different types of materials that scintillate:non-organic crystals (NaI, CsI, BGO)organic crystals (Anthracene)Organic plastics (see table on next page)Organic liquids (toluene, xylene)Our atmosphere (nitrogen)
880.P20 Winter 2006 Richard Kass 2
Scintillators
A typical plasticScintillator system
violet blue
Emission spectrum of NE102APlastic scintillator
Properties of common plastic scintillators
Typical cost 1$/in2
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Photomultiplier Tubes
We need a way to convert the scintillation photons into an electrical signal.Photons photoelectric effect electrons
Use a photomultiplier tube to convert scintillation light into electrical currentProperties of phototubes: very high gain, low noise current amplifier
gains 106 possiblepossible to count single photons
Off the shelf item, buy from a companywide variety to choose from (size, gain, sensitivity)tube costs range from $102-$103
Sensitive to magnetic fields (shield against earth’s): use “mu-metal”
In situations where a lot of lightis produced (>103 photons) aphotodiode can be used in place of a phototube, e.g. BaBar’s calorimeter
Quantum efficiency of bialkali cathode vs wavelength
violet blue green
Electric field accelerates electronsElectrons crash into dynodes create more electrons
light
e’s
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Scintillation Counter ExampleSome typical parameters for a plastic scintillation counter are: energy loss in plastic scintillator: 2MeV/cm scintillation efficiency of plastic: 1 photon/100 eV collection efficiency (# photons reaching PMT): 0.1 quantum efficiency of PMT 0.25
What size electrical signal can we get from a plastic scintillator 1 cm thick? A charged particle passing perpendicular through this counter:
deposits 2MeV which produces 2x104’s of which 2x103’s reach PMT which produce 500 photo-electrons
Assume the PMT and related electronics have the following properties:PMT gain=106 so 500 photo-electrons produces 5x108 electrons =8x10-11CAssume charge is collected in 50nsec (5x10-8s)
current=dq/dt=(8x10-11 coulombs)/(5x10-8s)=1.6x10-3AAssume this current goes through a 50 resistor
V=IR=(50 )(1.6x10-3A)=80mV (big enough to see with O’scope)
So a minimum ionizing particle produces an 80mV signal.What is the efficiency of the counter? How often do we get no signal (zero PE’s)?The prob. of getting n PE’s when on average expect <n> is a Poisson process:
!)(
n
ennP
nn
The prob. of getting 0 photons is e-<n> =e-500 0. So this counter is 100% efficient.Note: a counter that is 90% efficient has <n>=2.3 PE’s
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Time of flight with ScintillatorsTime of Flight (TOF) is a particle identification technique. measure particle speed and momentum determine mass
t=x/v=x/(c) with =pc/E=pc/[(mc2)2+(pc)2]1/2
pc
pmcxt
2/122 ))((
Consider two particles with different masses but same momentum:
2
22
21
2
2
222
2
2
221
222
21
)(
)(
))((
)(
))((
p
mmx
pc
pcmx
pc
pcmxtt
))(( 212122
21 tttttt
221
22
21
2
21 )(
)(
ptt
mmxtt
For high momentum (e.g. p>1 GeV/c for ’s):t1+t2=2t and x/tc
psec/meter)(
16672
)(2
22
21
2
22
21
21 p
mmx
cp
mmxtt
Actually, we measurethe time it takes for theparticle to travel a known distance.
x
880.P20 Winter 2006 Richard Kass 6
Time of Flight with Scintillators
psec/meter)(
16672
)(2
22
21
2
22
21
21 p
mmx
cp
mmxttt
As an example, assume m1=m (140MeV) , m2=mk (494MeV), and x=10mt=3.8 nsec for p=1 GeVt=0.95 nsec for p=2GeV
Time resolution of a “good” TOF system is 150ps (0.15 ns)
Scintillator+phototubes are capable of measuring such small time differences
In colliding beam experiments, 0.5 <x< 1 m small x puts a limit of t.
x =1 meter
For x=1 m, p=1 GeV K/ separation t psec < 3 separation
1.4 GeV/c ’s and K’s
No pulse height correction
with pulse height correction
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Photoelectric Effect absorbed by material, electron ejected
Compton Scatteringe-→e- “elastic scattering”
Pair Production→e+e- creates anti-matter
Basic Physics Processes in a Sodium Iodide (NaI) Calorimeter
e- e- e-
e+
hv < 0.05 MeV 0.05 < hv < 10 MeV hv > 10 MeV -ray must have E>2me
The amount of light given off by NaI is proportional to the amount energy absorbed. The light yield is ~ 1 photon per 25 eV deposited in NaI, max=415 nm, decay time ~250nsec
NaI
radiation length of NaI ~2.5 cmbut only useful for E > few MeV
NaI is often used to measure the energy low gamma rays
Attenuation of the gamma rays is energy dependent
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NaI & Homeland Security
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Example: Cs137 -ray Spectrum in NaI
E=662keVphotopeak
1800 backscatterE=184keV
K-shell x-raysE~35 keV
Compton scatterings
Compton Edge
ene
rgy
decay
decay E=662keV
decay gives off electrons with a range of energiesEmax = 514 keV, 1170 keV
decay gives off a monchromatic photonE = 662 keV
decay
Cs137
e-
1800
backscatter
forwardscatteredelectron
energy resolution:E/E~2.5%@ 662KeV
NaI crystal ~ 5cm X 5cm
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NaI is a Dirty Bomb Detector
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What’s in Your Air?
I set up a NaI counter in PRB3153 and took data for 24 hours.Find lots of -ray peaks Use ROOT to fit the -ray peaks to a Gaussian (signal) + linear background Pb214, Bi214 are Radon (Rn222) by-products (~1pc/L in PRB3153) K40 is common in many building materials (and bananas) TL208 (Thallium 208) is from Rn220
Energy (keV) Energy (keV)
Pb214
Bi214
Bi214 K40
Bi214
TL208Bi214
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Cerenkov Light
The Cerenkov effect occurs when the velocity of a charged particle traveling througha dielectric medium exceeds the speed of light in the medium. Index of refraction (n) = (speed of light in vacuum)/(speed of light in medium)Will get Cerenkov light when:
Angle of Cerenkov Radiation:
nc
v 1
For water n=1.33, will get Cerenkov light if v > 2.25x1010 cm/s
n
1cos
No radiationradiation
ct(c/n)t
In a time t wavefront moves (c/n)tbut particle moves ct.
Huyghen’s wavefronts
speed of particle > speed of light in medium
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Threshold Momentum for Cerenkov
Radiation Example: Threshold momentum for Cerenkov light:
nt
1
1
1
11
1222
nn
n
tt
t
)1)(1(
1
1
12
nnn
tt
For gases it is convenient to let =n-1. Then we have:
)2(
1
tt
The momentum (pt) at which we get Cerenkov radiation is:
)2(
m
mp ttt
For a gas +2 so the threshold momentum can be approximated by:
2
mmp ttt
For helium =3.3x10-5 so we find the following thresholds:electrons 63 MeV/c kaons 61 GeV/cpions 17 GeV/c protons 115GeV/c
Medium =n-1 t
helium 3.3x10-5 123CO2 4.3x10-4 34H2O 0.33 1.52glass 0.46-0.75 1.37-1.22
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Number of photons from Cerenkov RadiationFrom classical electrodynamics (Frank&Tamm 1937, Nobel Prize 1958) wefind the following for the energy loss per wavelength () per dx for charge=1, n>1:
])(
11[
2222
n
E
dxd
dE
With =fine structure constant, n() the index of refraction which in general depends on the wavelength () of light.We can re-write the above in terms of the number of photons (N) using: dN=dE/E
For example see Jackson section 13.5
])(
11[
2]
)(
11[
2222222
ndxd
dN
n
E
dxd
dE
We can simplify the above by considering a region were n() is a constant=n:
2222
sincos1)(
11
n
2
2222sin
2]
)(
11[
2
dxd
dN
ndxd
dN
We can calculate the number of photons/dx by integrating over the wavelengths thatcan be detected by our phototube (1, 2):
]11
[sin2sin221
22
22
1
d
dx
dNNote: if we are using a phototubewith a photocathode efficiency thatvaries as a function of then we have:
2
1
22 )(
sin2
df
dx
dN
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Number of photons from Cerenkov RadiationFor a typical phototube the range of wavelengths (1, 2) is (350nm, 500nm).
photons/cmsin39010
]5
1
5.3
1[sin2]
11[sin2 2
52
21
2
cmdx
dN
We can simplify using:
21222222 )1)(1()1)(1(1
1cos1sinn
nn
n
nn
n
For a highly relativistic particle going through a gas the above reduces to:
)1(2)1)(1()1)(1(
singas,12122
2
nn
nn
n
nn
photons/cm)1(780 ndx
dN
For He we find: 2-3 photons/meter (not a lot!)For CO2 we find: ~33 photons/meterFor H2O we find: ~34000 photons/meter
GAS
Photons are preferentially emittedat small ’s (blue)
For most Cerenkov counters the photon yield is limited (small) due to space limitations, the index of refraction of the medium, and the phototube quantum efficiency.
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Types of Cerenkov CountersThere are three different types of Cerenkov counters used to identify particles.Listed in order of their sophistication they are:
Threshold counter (on/off device)Differential counter (makes use of the angle of the Cerenkov radiation)Ring imaging counter (makes use of the “cone” of light)
Each of the above counter is designed to work in a certain momentum range.Threshold counter: Identify the particle(s) which give off light. Can use to separate electrons from heavier particles (, K, p) since electrons will give off light at a much lower momentum (e.g. 68 MeV/c vs 17 GeV/c for He)Problems with device: above a certain momentum several particles will give light. usually threshold counters use gas which implies low light levels (n-1 small) low light levels leads to inefficiency, e.g. <n>=3, the prob. of zero photons: P(0)=e-3=5%!Phototubes must be shielded from magnetic fields above a few tenths of a gauss.
880.P20 Winter 2006 Richard Kass 17
Types of Cerenkov Counters
Differential Cerenkov Counter:Makes use of the angle of Cerenkov radiation and only samples light at certain angles.For fixed momentum cos is a function of mass:
Not all light will make it to phototube
np
pm
Epnn
22
)/(
11cos
Differential cerenkov counters typically on work over a fixed momentum range (good for beam monitors, e.g. measure or K content of beam).Problems with differential Cerenkov counters: Optics are usually complicated. Have problems in magnetic fields since phototubes must be shielded from B-fields above a few tenths of a gauss.
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Ring Imaging Cerenkov Counters (RICH)RICH counters use the cone of the Cerenkov light.The ½ angle () of the cone is given by:
np
pm
n
2211 cos
1cos
The radius of the cone is: r=Ltan, with L the distance to the where the ring is imaged.
L
r
For a particle with p=1GeV/c, L=1 m, and LiF as the medium (n=1.392) we find:
deg r(m) 43.5 0.95
K 36.7 0.75P 9.95 0.18Thus by measuring p and r we can identify what type of particle we have.
Problems with RICH: optics very complicated (projections are not usually circles) readout system very complicated (e.g. wire chamber readout, 105-106 channels) elaborate gas system photon yield usually small (10-20), only a few points on “circle”
Great /K/p separation!
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CLEO’s Ring imaging Cerenkov Counter
The figures below show the CLEO III RICH structure. The radiator is LiF, 1 cm thick, followed by a 15.7 cm expansion volume and photon detector consisting of a wire chamber filled with a mixture of TEA and CH4 gas. TEA is photosensitive. The resulting photoelectrons are multiplied by the HV on the wires and the resulting signals are sensed by a rectangular array of pads coupled with highly sensitive electronics.
Challenge is to separate ’s from K’s in the range 1.5 <p < 3GeV (B Vs BK)
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CLEO’s Ring imaging Cerenkov Counter
Lithium Floride (LiF) radiator
Assembled radiators. They are guarded by Ray Mountain. WithoutRay “living”at the factory that produced the LiF radiators we would stillbe waiting for the orderto be completed.
A photodetector:CaF2 window+cathode pads
Assembledphotodetectors
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Performance of CLEO’s RICH
Number of detectedphotons on 5 GeV electrons
A track in theRICH
D*’s without/withRICH information
Preliminary dataon /K separation
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The BaBar DIRC
Here the challenge is to separate ’s and K’s in the range: 1.7<p< 4.2 GeV Detector of Internally Reflected Cerenkov light
DIRC uses quartz bars (490x1.7x3.5cm3) as radiator (n=1.473) and light guide The cerenkov light is internally reflected to the end of a bar bar must be very flat <5ÅDIRC is a 3D device, measures x, y, and time of Cerenkov photonsDetect the photons with an array of phototubes “Typical” photon has: =400 nm 200 bounces 5m path in quartz bar 10-60 ns propagation time
laser light propagating in a quartz bar
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The BaBar DIRC
1.5 T Solenoid Electromagnetic Calorimeter
(EMC)Detector of Internally
Recflected Cherenkov
Light (DIRC)
Instrumented Flux Return
(IFR) Silicon Vertex Tracker (SVT)
Drift Chamber (DCH)e- (9 GeV)
e+ (3.1 GeV)
phototube array
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Performance of the BaBar DIRCTiming information very useful to eliminate photons not associated with a track
300 nsec window500-1300 background hits
8 nsec window1-2 background hits
Note: the pattern of phototubes withsignals is very complicated. Thedetection surface is toroidal and thereforethe cerenkov rings are disjoint and distorted.
Use a maximum likelihood analysis to separate /K/p: L=L(c, t, n)
DIRC works very well!
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SuperK
481 MeV muon neutrino produces 394 MeV muon which later decays at rest into 52 MeV electron. The ring fit to the muon is outlined. Electron ring is seen in yellow-green in lower right corner. This is perspective projection with 110 degrees opening angle, looking from a corner of the Super-Kdetector (not from the event vertex). Color corresponds to time PMT was hit by Cerenkov photon from the ring. Color scale is time from 830 to 1816 ns with 15.9 ns step. In the charge weighted time histogram to the right two peaks are clearly seen, one from the muon, and second one from the delayed electron from the muon decay. Size of PMT corresponds to amount of light seen by the PMT. From: http://www.ps.uci.edu/~tomba/sk/tscan/pictures.html
SuperK is a water RICH. It uses phototubes to measure the Cerenkov ring.Phototubes give time and pulse height information
From SuperK site
SuperK has: 50 ktons of H2OInner PMTS: 1748 (top and bottom) and 7650 (barrel)outer PMTs: 302 (top), 308 (bottom) and 1275(barrel)
For water n=1.33For =1 particle cos=1/1.33, =41o
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Askaryan Effect Radio Frequency Cerenkov RadiationAskaryan Effect: EM showers in a dielectric medium generate coherent radio cerenkov emission
From: D. Saltzberg, Orion Workshop
An EM shower propagating in air+pb
In EM shower there will be more e-’s than e+’s (~20%), a net current which can radiate. No radiation if exactly same amount of + and - chargesExcess charge moving faster than speed of light will emit cerenkov radiation. In ice the peak frequency of radiation ~ 2 GHz (~15 cm).The radiation is coherent (rad lateral shower size) and power ~ E2
Possible to observe very high interactions in ice (or salt) Radiation is linearly polarized
predicted 1962, observed 2000
Saltzberg, et al, Phys.Rev.Lett. 86 (2001) 2802-2805
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Radio Frequency Cerenkov Radiation from IceFrom: Andrea Silvestri, UCI, International School in Cosmic Ray Astrophysics, July 2004, Erice-Sicily
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ANITA ExperimentAntarctic Impulsive Transient Antenna
From: Andrea Silvestri, UCI, presented at International School in Cosmic Ray Astrophysics, July 2004, Erice-Sicily
ANITA is an experiment designed to detector ultra high energy neutrino interactions 1017<E<1020 eVIt relies on detecting Askaryan Cerenkov radiation from very high energy neutrino interactions in ice.
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