PoGO_G4_2005-08-12.ppt 1

Study of BGO/Collimator Optimization for PoGO

August 8th, 2005Tsunefumi Mizuno, Hiroshima University/SLAC

mizuno@SLAC.Stanford.EDU

History of changes;August 12, 2005 updated by T. Mizuno

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Contents•Objective of this study (p. 3)•Simulation (pp.4-9)

•Geometry (p.4)•Simulation condition (p.5)•Detector response (p.6)•Event selection (p.7)•Gamma-ray background model (p.8)

•BGO/Collimator optimization (pp.9-16)•Side BGO length (p.9)•Side/Bottom BGO thickness (p.10)•Collimator Material (p.11)•Fluorescence X-ray (p.12)•Dual layer collimator (pp.13,14)•Expected BG (pp.15,16)

•Summary (p.17)•Appendix (p.18)

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Objectives•To find an optimum design of BGO and passive collimator regarding to background.•Evaluate the background level with fluorescence X-rays and cosmic X-ray/gamma-ray background (here we call “primary gamma”) taken into account.

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Simulated Geometry•Thickness of fast scint. = 2.63cm

(D = 2.23cm)•W (thickness of slow scint.) = 0.2cm•L1 (slow scint. length) = 60cm•L2 (fast scint. length) = 20cm•Thickness of btm BGO = 2.68cmGap between BGOs = 0.5cm

(including BaSo4 eflector)•# of units = 217 (geometrical area of fast scint. not covered by slow scint. = 934.4 cm2)

•Length of btm BGO = 3/4/5cm(not tapered in simulator for simplicity)•Thickness of side Anti BGO = 3/4/5cm•Length of side Anti BGO = 60/70/80cm•Collimator material = Sn/Pb•single/dual layer collimator

“fixed” parameters

parameters studied here

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Simulation Condition•The same Crab spectrum as that used in Hiro’s EGS4 simulation was simulated here. That is,

•E-2.1 spectrum with 100mCrab intensity, 20-200keV (300.8 c/s/m2)•100% polarized, 6h exposure•Attenuation by air of 4g/cm2 (atmospheric depth in zenith direction is ~3g/cm2 and that in line-of-sight direction is 4g/cm2)

•Atmospheric downward/upward gamma and cosmic X-ray/gamma-ray background gamma (primary gamma) spectra for GLAST BFEM simulation were used as background.•Use Geant4 ver5.1 with PoGO-fix for polarized Compton scattering.

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Detector Resopnses•The same detector responses as those used in Hiro’s EGS4 simulation

•If there is a hit in slow/anti/btm scintillators, event is rejected. (Threshold is 3 keV for anti/btm BGO and 30 keV for slow scintillator. Note that the position dependence has not taken into account yet.). Energy smearing and poisson fluctuation are not taken into account yet for veto scintillators.•Assumed detector resposes:

• 0.5 photo-electron/keV• fluctuated by poisson distribution• smeared by gaussian of sigma=0.5 keV (PMT energy resolution)• minimum hit threshold after three steps above is 3 keV

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Event Analysis•The same as those of Hiro’s EGS4 Simulation

•Use events in which two or three fast scintillators detected a hit.•The largest energy deposit is considered to be photo absorption•The second largest energy deposit is considered to be Compton scattering.•Smallest energy deposit (in case of three scintillators with hit) is ignored.•Smear azimuth angle distribution with Hiro’s resolution function.•No event selection on compton kinematics

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Background gamma-ray spectraprimary gamma atmospheric downward

gamma (vertical)

atmospheric upwardgamma (vertical)•Atmospheric gamma spectral

models are for Palestine, Texas.•We have no data for atmospheric downward gamma below 1MeV, where primary gamma could be dominant.

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Side BGO Length

100mCrab(incident)

100mCrab(detected)

BG due to atmospheric gamma,Side BGO length=60cm/70cm/80cm

atmospheric downward gamma atmospheric upward gamma

•100mCrab vs. background spectrum•Passive collimator: Sn 100um•Side/Bottom BGO thickness: 3cm

•No sinificant difference in summed BG below 40keV and above 100keV•Longer BGO reduces the background in 50-100 keV. (Pb collimator can also do. See p. 11)

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Side/Bottom BGO Thickness

100mCrab(incident)

100mCrab(detected)

BG due to atmospheric gamma,Side/Btm BGO thicknss=3cm/4cm/5cm

atmospheric downward gamma atmospheric upward gamma

•100mCrab vs. background spectrum•Passive collimator: Sn 100um•Side BGO length: 60cm

•No sinificant difference in summed BG below 70 keV

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Collimator Material

primary gamma

atmospheric downward gamma

atmospheric upward gamma

BG due to gamma,Collimator = Pb 50um/Sn100 um

100mCrab(incident)

100mCrab(detected)

•100mCrab vs. background spectrum•Side BGO length: 60cm•Side/Btm BGO thickness: 3cm•Standard process (no fluorescence X-ray)

•Pb collimator reduces summed BG above 50 keV

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Effect of Fluorescence X-ray

primary gamma

atmospheric downward gamma

atmospheric upward gamma

BG due to gamma,Collimator = Pb 50um/Sn100 um

100mCrab(incident)

100mCrab(detected)

•100mCrab vs. background spectrum•Side BGO length: 60cm•Side/Btm BGO thickness: 3cm•Low energy process (fluorescence X-ray)

•BG below 30 keV for Pb collimator is worse than that for Sn collimator, due to fluorescence X-rays from Pb.

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Dual Layer Collimator (1)•Due to fluorescence X-rays, BG level for Pb collimator becomes higher than that for Sn collimator below 30keV.•Dual layer collimator could reduce the BG; outer collimator (Pb) eliminates contamination from primary gammas and downward atmospheric gammas, and inner collimator (Sn) eliminates fluorescence X-rays from Pb collimator.•We tested two configurations. The idea of shortened Pb collimator is, to make the pass length in Sn collimator long enough to absorb fluorescent X-rays from Pb.

Fast/slow scintillator

Pb collimator(50um, 60cm)Sn collimator(50um, 60cm)

a: long pass length

b: short pass length

normal configuration shortened Pb collimator

Pb collimator(50um, 50cm)Sn collimator(50um, 60cm)

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Dual Layer Collimator (2)

primary gamma

atmospheric downward gamma

atmospheric upward gamma

BG due to gamma,Pb collimator, standard process(solid line) lowE process(dotted line)Dual layer collimator, normal configuration shortened Pb collimator

100mCrab(incident)

100mCrab(detected)

•BGO configuration is the same as p.12

•Dual collimator reduces BG below 30 keV. No significant difference in summed BG between normal configuration and shortened Pb collimator below 60 keV. (see next)

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Expected BG (1)

BG due to gamma,Pb 50um/60cmSn 100um/60cmPb 50um/60cm + Sn 50um/60cmPb 50um/50cm + Sn 50um/60cm

100mCrab(incident)

100mCrab(detected)

primary gamma + downward/upward atmospheric gamma

•No significant difference among Sn and dual layer collimators below 60 keV.•Dual collimator with shortened Pb gives the lowest BG in high energy.

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Expected BG (2)

BG due to gamma, Totalprimary gammaatmospheric downward gammaatmospheric upward gamma

100mCrab(incident)

100mCrab(detected)

Shortened Pb collimator with Sn collimator inside

Contribution of each component is shown here.

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Summary•BG dependence on BGO length/thickness and collimator configuration are studied.•3 components of gamma-ray background (primary, atmospheric downward/upward) and fluorescence X-rays are taken into account.•Longer side BGO reduces BG above 50 keV (p.9). Pb collimator instead of Sn can also do this. (p.11)•Thicker side/bottom BGO reduces BG above 80 keV. (p.10)•Dual layer collimator with shortened Pb gives the lowest BG. Below 60keV, there is no significant difference among Sn collimator and dual collimators (normal configuration and shortened Pb). (pp.11-15)

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Appendix: Energy of incident gamma which contribute to BG

primary gamma

atmospheric downward gamma

atmospheric upward gamma

2 or 3 fast scintillators have a hit

Events that contribute BG

•Contamination in FOV.•Penetrate BGO without interaction, hit fast scintillators and absorbed by collimator.

•Energy distribution of incident gamma clearly shows the process how they contribute to BG.•Pb collimator of 50um is assumed here