1. PoGO_G4_2005-08-12.ppt1 Study of BGO/Collimator Optimization for PoGO August 8th, 2005 Tsunefumi Mizuno, Hiroshima University/SLAC mizuno@SLAC.Stanford.EDU History of changes; August 12, 2005 updated by T. Mizuno

2. PoGO_G4_2005-08-12.ppt2 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)

3. PoGO_G4_2005-08-12.ppt3 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.

4. PoGO_G4_2005-08-12.ppt4 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 = 2.68cm Gap 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

5. PoGO_G4_2005-08-12.ppt5 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/m 2 ) 100% polarized, 6h exposure Attenuation by air of 4g/cm 2 (atmospheric depth in zenith direction is ~3g/cm 2 and that in line-of-sight direction is 4g/cm 2 ) 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.

6. PoGO_G4_2005-08-12.ppt6 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

7. PoGO_G4_2005-08-12.ppt7 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

8. PoGO_G4_2005-08-12.ppt8 Background gamma-ray spectra primary gamma atmospheric downward gamma (vertical) atmospheric upward gamma (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.

9. PoGO_G4_2005-08-12.ppt9 Side BGO Length 100mCrab (incident) 100mCrab (detected) BG due to atmospheric gamma, Side BGO length=60cm/70cm/80cm atmospheric downward gammaatmospheric 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)

10. PoGO_G4_2005-08-12.ppt10 Side/Bottom BGO Thickness 100mCrab (incident) 100mCrab (detected) BG due to atmospheric gamma, Side/Btm BGO thicknss=3cm/4cm/5cm atmospheric downward gammaatmospheric upward gamma 100mCrab vs. background spectrum Passive collimator: Sn 100um Side BGO length: 60cm No sinificant difference in summed BG below 70 keV

11. PoGO_G4_2005-08-12.ppt11 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

12. PoGO_G4_2005-08-12.ppt12 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.

13. PoGO_G4_2005-08-12.ppt13 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)

14. PoGO_G4_2005-08-12.ppt14 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)

15. PoGO_G4_2005-08-12.ppt15 Expected BG (1) BG due to gamma, Pb 50um/60cm Sn 100um/60cm Pb 50um/60cm + Sn 50um/60cm Pb 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.

16. PoGO_G4_2005-08-12.ppt16 Expected BG (2) BG due to gamma, Total primary gamma atmospheric downward gamma atmospheric upward gamma 100mCrab (incident) 100mCrab (detected ) Shortened Pb collimator with Sn collimator inside Contribution of each component is shown here.

17. PoGO_G4_2005-08-12.ppt17 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)

18. PoGO_G4_2005-08-12.ppt18 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