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Compton Photon Calorimeter Gregg Franklin, B. Quinn Carnegie Mellon Design Considerations Light Yield and Photoelectrons Detector Geometry, EGS Simulations, Linearity Decay time Crystal Properties

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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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Probability of getting n pe photoelectrons from Compton Photons of energy E i photons giving n pe photoelectrons Convolution of two gaussians gives variance for n pe,i : If energy independent, error on summed energy is: Finite photoelectron term small if E max large

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Measured Energy Deposited (MeV) 20 MeV 5 MeV 1MeV Measured energy deposited for 1 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 only photoelectron statistics and PMT gain variance

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Detector Geometry, EGS Simulations, Linearity EGS simulation by Brian Quinn MeV photons ISaint-Gobain BrilLanCe 380 LaBr 3 (Cd) Density: 5.29 g/cm 3 1 inch diam. 4 inch thick (~ 5.3 rad lengths) Energy Deposited 511 keV escape 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 in analyzing power 1 MeV Analyzing Power of summed Deposited Energy as function of Deposited Energy Threshold % change in Analyzing Power 1.5% 3.0% E Dep 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 BGOGSOCeF 3 BriLanCe 380 PreLude 420 Density (6/cm 3 ) Rad Length (cm) ~ Moliere Radius (cm) ?? Decay time (ns) : Light output (% NaI) 0.4%9%45%6.6%165%84% photoelectrons (# / MeV) $$$ 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

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