PoGO_G4_2004-02-03.ppt 1

Study on Key Properties of PoGO by Geant4 Simulator

January 28, 2004Tsunefumi 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

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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 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 cm2) or 217 (934.4 cm2)

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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/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 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.).

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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). 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 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

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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

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Incident energy distribution of atmospheric gammas

atmospheric downward gamma atmospheric 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)

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Collimator thickness dependence of the background(1)

atmospheric downward gamma

397 units

100mCrab (incident)100mCrab (detected)

Background due to atmospheric gamma10um thickness25um thickness50um 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

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Collimator thickness dependence of the background(2)

217 unitsatmospheric upward gamma

100mCrab (incident)100mCrab (detected)

Background due to atmospheric gamma10um thickness25um thickness50um 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

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Slow scint. energy threshold dependence of the background

atmospheric downward gamma atmospheric upward gamma

100mCrab (incident)100mCrab (detected)

Background due to atmospheric gammaEth=3keV, 5keV, 10keV and 1MeV

Veto threshold of slow scintillator does not affect the background so much.

Reduced Design (217 units)

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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

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Predicted Effective Area

397 units

•Apply the same event selection as that for polarization measurement (p11).

217 units

Maximum at 40-50 keV, 230cm2 (217 units)/460cm2 (397 units)