1. PoGO_G4_2004-11-01.ppt1 Study of optimized fast scintillator length for the astronomical hard X- ray/soft gamma-ray polarimeter PoGO November 1, 2004 Tsunefumi Mizuno mizuno@SLAC.Stanford.EDU

2. PoGO_G4_2004-11-01.ppt2 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) = 5,10,15,20,25 and 30cm Thickness of Sn collimator = 0.01cm Thickness of btm BGO = 2.68cm Length of btm BGO = 3cm (not tapered in simulator for simplicity) Gap between BGOs = 0.5cm (including BaSo4 eflector) Thickness of side Anti BGO = 3cm Length of side Anti BGO = 60cm # of units = 217 (geometrical area of fast scint. not covered by slow scint. = 934.4 cm 2 )

3. PoGO_G4_2004-11-01.ppt3 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 spectra for GLAST BFEM simulation were used as background. Use Geant4 ver5.1. Serious bug in polarized Compton scattering was fixed by user. We also implemented polarized Rayleigh scattering. Fluorescence X-ray are not taken into account in BG simulation.

4. PoGO_G4_2004-11-01.ppt4 Detector Responses 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 BGO and 30 keV for slow scintillator). Energy smearing and poisson fluctuation are not taken into account yet for veto scintillators. Assumed detector responses: 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

5. PoGO_G4_2004-11-01.ppt5 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 Events with measured energy in 20-100keV are selected (see page 6). Study the sensitivity, efficiency and signal/noise ratio as a function of fast scintillator length (see pages***).

6. PoGO_G4_2004-11-01.ppt6 Simulated spectrum/modulation curve for 20cm with atmospheric gamma 100m Crab spectrum (E -2.1 in 20-200keV), 6 hour exposure, 100% polarized Fit the azimuth angle distribution with p0(1+p1*cos(2*phi+pi)) Assumed detector response and event selection criteria are given in pages 4-5 Measured energy is 20-100 keV: In this energy range, signal exceeds the background (see 2 left figures). MF=24.29+-0.673% (36.1sigma) atmospheric downward gamma, Sn collimator of 100um 100mCrab (incident) 100mCrab (detected) gamma energy (keV) Flux(c/s/cm2/keV) 20 100 Background due to atmospheric gamma Eth=10keV, 30keV, 100keV, 300keV and 1MeV atmospheric upward gamma Background due to atmospheric gamma Eth=10keV, 30keV, 100keV, 300keV and 1MeV

7. PoGO_G4_2004-11-01.ppt7 Sensitivity and efficiency (without BG) Although the longer scintillator provides the higher detection efficiency (right figure), the sensitivity will saturate above 20cm (left figure). If we take into account the light attenuation in scintillator, 20cm could be the best.

8. PoGO_G4_2004-11-01.ppt8 Sensitivity and signal/BG ratio Even if we take into account the background, 20cm could be the optimum (assuming that the background flux model is accurate). Signal/BG ratio could be the best at 15cm length.