1. PoGO_G4_2004-02-03.ppt1 Study on Key Properties of PoGO by Geant4 Simulator January 28, 2004 Tsunefumi Mizuno mizuno@SLAC.Stanford.EDU History of changes: February 3, 2004: line-of-sight atmospheric depth for Crab observation was changed from 3g/cm2 to 4g/cm2

2. PoGO_G4_2004-02-03.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) = 20cm Thickness of W collimator = 0.0025cm 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 = 397 (geometrical area of fast scint. not covered by slow scint. = 1709 cm 2 ) or 217 (934.4 cm2)

3. PoGO_G4_2004-02-03.ppt3 Simulation Condition The same Crab spectrum as that used in Hiro’s EGS4 simulation is 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 are used as background. Use Geant4 ver5.1. Possible minor bug of polarization vector after scattering was fixed by user (found by Y. Fukazawa @ Hiroshima Univ.).

4. PoGO_G4_2004-02-03.ppt4 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). 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

5. PoGO_G4_2004-02-03.ppt5 Event Analysis The same as those of Hiro’s EGS4 Simulation Use events in which two or three fast scintillators are with hit. The largest energy deposit is considered to be photo absorption 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

6. PoGO_G4_2004-02-03.ppt6 Incident/measured gamma-ray energy distributions Incident gamma energy, 2 or 3 hits in fast scint. no hits in veto scinti. (Eth=3keV) after attenuated by air of 3g/cm2 measured gamma energy, detector responses are convoluted 100mCrab (20-200keV), 6 hour exposure, 217 units

7. PoGO_G4_2004-02-03.ppt7 Incident energy distribution of atmospheric gammas atmospheric downward gammaatmospheric upward gamma gammas that hit 2 or 3 fast scintillators gammas that pass the event selection (background) Downward atmospheric gammas below a few MeV and upward ones between a few hundred keV to a few MeV contribute to background. Reduced Design (217 units)

8. PoGO_G4_2004-02-03.ppt8 Collimator thickness dependence of the background(1) atmospheric downward gamma 397 units 100mCrab (incident) 100mCrab (detected) Background due to atmospheric gamma 10um thickness 25um thickness 50um thickness # of units does not affect S/N ratio very much Collimator (W) of 25um could be optimum (see the next slide). Signal>background below 100 MeV. 217 units

9. PoGO_G4_2004-02-03.ppt9 Collimator thickness dependence of the background(2) 217 units atmospheric upward gamma 100mCrab (incident) 100mCrab (detected) Background due to atmospheric gamma 10um thickness 25um thickness 50um thickness # of units does not affect the S/N ratio very much Collimator (W) of 25um could be optimum (see the previous slide). Signal>background below 100 MeV. 397 units

10. PoGO_G4_2004-02-03.ppt10 Slow scint. energy threshold dependence of the background atmospheric downward gammaatmospheric upward gamma 100mCrab (incident) 100mCrab (detected) Background due to atmospheric gamma Eth=3keV, 5keV, 10keV and 1MeV Veto threshold of slow scintillator does not affect the background so much. Reduced Design (217 units)

11. PoGO_G4_2004-02-03.ppt11 Predicted Azimuth Angle Distribution 397 units 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 MF=24.4% Sensitivity; 24.4/0.83=29sigma 217 units MF=25.2% Sensitivity; 25.2/0.58=43sigma

12. PoGO_G4_2004-02-03.ppt12 Predicted Effective Area 397 units Apply the same event selection as that for polarization measurement (p11). 217 units Maximum at 40-50 keV, 230cm 2 (217 units)/460cm 2 (397 units)