Compton Photon Calorimeter

1
Compton Photon Calorimeter Gregg Franklin, B.
Quinn Carnegie Mellon

• Design Considerations
• Light Yield and Photoelectrons
• Detector Geometry, EGS Simulations, Linearity
• Decay time
• Crystal Properties

2

• Light yield and Photoelectrons

Calculate contribution of finite photoelectrons
per MeV energy deposited
First, write mean total photoelectrons as
(integrated flux) x (Compton cross section d?/dE)
x (bin size)
3
Probability of getting npe photoelectrons from
Compton Photons of energy Ei
? photons giving npe photoelectrons
? photons
Convolution of two gaussians gives variance for
npe,i
If ? energy independent, error on summed energy
is
Finite photoelectron term small if ?Emax large
4
Photoelectrons not a big issue for integrated
energy BUT Electron tagged data may be easier
to analyze with more photoelectrons Other
calibration issues?
1MeV
20 MeV
5 MeV
Measured energy deposited for 1 Mev, 5 MeV, and
20 MeV energy deposions
Simulation includes only photoelectron statistics
and PMT gain variance
Measured Energy Deposited (MeV)
5

• Detector Geometry, EGS Simulations, Linearity

EGS simulation by Brian Quinn
12.75 MeV photons ISaint-Gobain BrilLanCe
380 LaBr3(Cd)
1 inch diam. 4 inch thick ( 5.3 rad lengths)
511 keV escape peaks
Density 5.29 g/cm3
Energy Deposited
6
Infinite slab still looses energy due to
backscattering
Finite slab energy loss goes up with photon energy
7
Linearity improves with thickness, but is it
important?
4 inches
8
3.0
Analyzing Power of summed Deposited Energy as
function of Deposited Energy Threshold
1.5
EDep Thresh.
25 MeV
change in Analyzing Power
1 change in analyzing power
1 MeV
EDep Thresh.
5 MeV
9

• 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?)

10

• Crystal Properties

PbWO4 BGO GSO CeF3 BriLanCe 380 PreLude 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 56600 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
11
This summer

• 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)