Compton Photon Calorimeter Gregg Franklin, B. Quinn Carnegie Mellon
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Transcript of Compton Photon Calorimeter Gregg Franklin, B. Quinn Carnegie Mellon
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Compton Photon CalorimeterGregg Franklin, B. QuinnCarnegie Mellon
Design Considerations• Light Yield and Photoelectrons• Detector Geometry, EGS Simulations, Linearity• Decay time• Crystal Properties
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,
,
energy of bin i
mean photoelectrons per photon of energy
=mean number of photons of energy
pe i i iphotonenergybin
i
i i
i i
n N E
where
E
E
N E
First, write mean total photoelectrons as:
Calculate contribution of finite photoelectrons per MeV energy deposited
(integrated flux) x (Compton cross section d/dE) x (bin size)
• Light yield and Photoelectrons
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22i ii
22i ii
-( ) / 2-(N ) / 2pe i
-( / ) /(2 / N )-(N ) / 2
Prob(n , E ) e e
e e (using N )
i pe ii
pe i i ii
E n EN
n E ENi
2 2 2,
1= (1+ ) n i i i ii
E N E
max 2
0,
max
0
11
E
n sumEsum
Esum pe
dNdE E
dE EdNE n dE EdE
Probability of getting npe photoelectrons from Compton Photons of energy Ei
photonsphotonsgiving npe photoelectrons
Convolution of two gaussians gives variance for npe,i:
If energy independent, error on summed energy is: Finite photoelectronterm small ifEmax large
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Measured Energy Deposited (MeV)
20 MeV
5 MeV
1MeV
Measured energy deposited for1 Mev, 5 MeV, and 20 MeV energy deposions
Photoelectrons not a big issue for integrated energy
BUT: Electron tagged data may be easier to analyze with more photoelectrons
+Other calibration issues?
Simulation includes onlyphotoelectron statistics andPMT gain variance
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• Detector Geometry, EGS Simulations, Linearity
EGS simulation by Brian Quinn
12.75 MeV photons
ISaint-Gobain“BrilLanCe 380”LaBr3(Cd)
Density: 5.29 g/cm3
1 inch diam.4 inch thick(~ 5.3 rad lengths)
Energy Deposited
511 keVescape peaks
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Infinite slab still looses energy due to backscattering
Finite slab energy loss goes up with photon energy
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Linearity improves with thickness,but is it important? 4 inches
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5 MeV
25 MeV
1% change inanalyzing power
1 MeV
Analyzing Power of summed Deposited Energy as function of Deposited Energy Threshold
% change in Analyzing Power
1.5%
3.0%
EDep Thresh.
EDep Thresh.
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• Decay Time Consideration
Why not use BGO (decay time ~300 nS)?• Bremstrahlung
• If ~10 kHz and “deadtime” 3* 300 ns, get 1% deadtime• Other
• Coincidence and singles data• Electronics set up for ~100 nS gate• Larger background from tails
Prefer faster decay time (50 ns?)
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PbWO4 BGO GSO CeF3BriLanCe
380PreLude
420
Density
(6/cm3)8.30 7.13 6.70 6.16 5.29 7.1
Rad Length
(cm)0.90 1.12 1.39 1.68 ~1.9 1.2
Moliere Radius
(cm)2.0 2.3 2.4 2.6 ? ?
Decay time
(ns)50 300 56:600 30 16 41
Light output
(% NaI)0.4% 9% 45% 6.6% 165% 84%
photoelectrons
(# / MeV)8 170 850 125 3150 1600
$$$
4 in max
Natural
decay
• Crystal Properties
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Need to settle on crystal (at least for test)
Test FADC algorithm at CMU this summer• Gated and integrating modes (simulate summing algorithm)• Does ADC sum represent #photoelectrons?
• Test resolution on sources• Need to slow down signal?• Possibly clip large pulses?
Better linearity simulations• GEANT4 (Optimization by Guido, some work at CMU)
This summer