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Tsunefumi Mizuno, Tuneyoshi Kamae, Jonny Ng,
Hiroyasu Tajima (SLAC), John W. Mitchell, Robert
Streitmatter (NASA GSFC), Richard C. Fernholz,
Edward Groth (Princeton University), Yasushi
Fukazawa (Hiroshima Univ), and PoGO collaboration
Comparison between Geant4 and EGS4
(a)
We compared EGS4 and Geant4 to verify them as
toolkits to simulate the polarized Compton
scattering. EGS4 with low energy Compton
scattering extension by Namito, Ban and Hirayama
(Namito, Ban and Hirayama, 1993 NIMA, 332, 277)
and Geant4 ver 5.1 with low energy extension
(Depaola 2003, NIMA 512, 619) were compared. We
found that Geant4 gave less asymmetry than EGS4
for second Compton scattering and fixed the code
to meet the formula in EGS4 documents. After the
modification Geant4 and EGS4 agreed within 10
(Fig. a) and we used the fixed Geant4 to simulate
the beam test. Further study is required to
resolve the remaining difference. These results
were reported to Geant4 team and are to be
verified.
(it was 15 without the modification)
Modulation curves to be observed by PoGO for Crab
spectrum in 25-200 keV. Geometry was simplified
here.
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Experimental Results

• We conducted the beam test at the APS of Argonne
  National Laboratory in last November. We
  irradiated polarized mono-energetic synchrotron
  beam of 60 keV, 73 keV and 83 keV, and the same
  beam was utilized to calibrate the detector
  energy response. Polarization vector is along the
  x-axis and we rotated the detector in 15 degree
  steps (Fig. a) to measure the azimuth angle
  asymmetry. We selected Compton events based on
  the following criteria (for 73.2 keV run)
• Detection threshold of the scintillator was set
  at 2 keV
• Two scintillators detected a hit, of which one
  was the central scintillator
• Deposit energy in the central scintillator was
  below 40 keV and less than half of the total
  deposit energy.
• Total deposit energy was between 45 keV and 100
  keV.
• The selected region of energies is shown as green
  lines in Fig. b and c.

(b)
(c)
total deposit energy of the selected events
73.2 keV run
73.2 keV run
Selected region of energies
Total deposit energy (keV)
Gap between valid compton events and background
events
deposit energy in the central scinti. (keV)
total deposit energy (keV)
During the test, we found that the trigger
efficiency was not 100 and varied among
channels, so we also took the data with
ch4-only-trigger and used the data to evaluate
the relative efficiency of each channel for
coincidence trigger run. The obtained data, after
the trigger efficiency was collected, are shown
by points in Fig. d, e and f. Geant4 predictions
for 100 polarized beam are also shown by dotted
lines. There, the PoGO-fix was applied on Geant4
polarization code (see Comparison between Geant4
and EGS4 above). Small difference between
ch1/ch7, ch2/ch6 and ch3/ch5 could be due to
misalignment. Measured modulation factor is
independent of energy and is 42, whereas that
predicted by Geant4 is 47. We succeeded in
reproducing data with Geant4 simulation within
10. The remaining difference could be explained
by the unpolarized component of the beam and/or
misalignment of scintillators.
(d)
(e)
(f)
ch2
ch1
ch3
ch7
ch5
ch6
Geant4 simulation
Geant4 simulation
Geant4 simulation
Mod. Factor (observed) 43 Mod. Factor (G4
predicted) 47
Mod. Factor (observed) 42 Mod. Factor (G4
predicted) 47
Mod. Factor (observed) 41 Mod. Factor (G4
predicted) 46
Conclusion
To examine the performance of PoGO and verify the
Geant4-based Monte-Carlo simulator, we conducted
the beam test at Argonne National Laboratory. A
prototype consisting of 7 units of plastic
scintillators was irradiated by polarized
synchrotron beam of 60 keV, 73 keV and 83 keV. A
Clear modulation was obtained (modulation factor
was 43) and the data were reproduced by
simulation within 10.