1. O 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 Experimental Results: ch1 ch3 ch5 ch2 ch6 ch7 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. Gap between valid compton events and background events Selected region of energies (b) (c) polarization vector 30degree y (vertical axis) x (horizontal axis) Beam Direction 12 345 67 (a) ch1 ch3 ch5 ch2 ch6 ch7 Comparison between Geant4 and EGS4: (d)(e)(f) Geant4 simulation 73.2 keV run deposit energy in the central scinti. (keV) Total deposit energy (keV) total deposit energy (keV) ch1 ch3 ch5 ch2 ch6 ch7 Geant4 simulation Mod. Factor (observed): 43% Mod. Factor (G4 predicted): 47% Mod. Factor (observed): 41% Mod. Factor (G4 predicted): 46% Mod. Factor (observed): 42% Mod. Factor (G4 predicted): 47% 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. (a) total deposit energy of the selected events (it was ~15% without the modification) 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. 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%. Modulation curves to be observed by PoGO for Crab spectrum in 25-200 keV. Geometry was simplified here.