1. ArgonneResult_2004-06-10.ppt1 Comparison of data and simulation of Argonne Beam Test July 10, 2004 Tsunefumi Mizuno mizuno@SLAC.Stanford.EDU

2. ArgonneResult_2004-06-10.ppt2 Experimental Setup beam direction x y Objective: to examine the performance of PoGO and verify the Geant4-based Monte-Carlo Simulator with a PoGO prototype PoGO: 200-400 units of fast/slow plastic scintillators and BGOs Prototype: 7 units of fast scintillators (detection part) Polarization vector was along the horizontal axis (x-axis) Irradiated polarized synchrotron beam of 60 keV, 73 keV and 83 keV at the center of the central scintillator. The same beam was used to calibrate the energy response. Rotated the detector in x-y plane in 15 degree steps.

3. ArgonneResult_2004-06-10.ppt3 Definitions polarization vector 30degree y x Beam Direction 12 345 67 We defined that xy plane is normal to the scintillator principle axis. Scintillators are numbered from 1 to 7. Central scintillator is number 4. Beam goes from +z to -z. Polarization vector is along x-axis. We rotated the detector. When we rotated it by 30 degree, scintillator number 2 was along the y axis.

4. ArgonneResult_2004-06-10.ppt4 Run Summary Coincidence Trigger (in 15 degree steps): run048-073: 83.5 keV run run106-149: 60.2 keV run run164-190: 73.2 keV run Ch4 Trigger (in 30 degree steps. They don’t cover the whole azimuth angle): run191-197: 73.2 keV run run210-212: 83.5 keV run Calibration Run: run088,090-096: 83.5 keV run run090-105,150-156: 60.2 keV run run157-163: 73.2 keV run In this report, we use only ch4 trigger run data. (Coincidence run data gave similar modulation factor.)

5. ArgonneResult_2004-06-10.ppt5 Calibration Constants We had been using calibration constants (channel/energy conversion factors) derived from the calibration run, but they gave ~5% lower energy deposition. For reference, please look at http://www.slac.stanford.edu/~mizuno/PoGO/Argonne/report/Argonne Result_2004-02-07.ppt. I suppose that shaper output was slightly affected by the trigger condition (due to the change of input impedance?), and determined the conversion factors so that total energy deposition is equal to the beam energy. http://www.slac.stanford.edu/~mizuno/PoGO/Argonne/report/Argonne Result_2004-02-07.ppt Below we list channel/energy conversion factors of 73.2 keV beam. ch1: f=4.20 (it was 4.46 when we used the calibration run data) ch2: f=4.32 (it was 4.53) ch3: f=4.35 (it was 4.58) ch4: f=4.39 (it was 4.34) ch5: f=4.39 (it was 4.79) ch6: f=4.17 (it was 4.45) ch7: f=4.85 (it was 5.15)

6. ArgonneResult_2004-06-10.ppt6 Geant4 vs. EGS4 We have compared the scattering process of polarized photons between Geant4 and EGS4, and found that G4 gave less asymmetry. There are two reasons for this: 1) In Geant4, polarization vector after the Compton scattering is excessively randomized. 2) Rayleigh scattering in Geant4 does not take into account the polarization. We have fixed them and obtained a good agreement (within a few %) between two simulation toolkits. Simulated modulation curves observed by PoGO (geometry is simplified) for Crab spectrum in 25-200 keV. 0.2184+-0.0022(EGS4) 0.2223+-0.0023(Geant4)

7. ArgonneResult_2004-06-10.ppt7 73.2 keV Ch4 Trigger Run (1) Event selection criteria: Detection threshold was 2 keV 2 scintillators detected a hit (one was the central scintillator) Deposit energy of the central scintillator was below 40 keV and less than half of the total deposit energy. total deposit energy = 73.2+-25keV deposit energy in the central scinti. (keV) Total deposit energy (keV)

8. ArgonneResult_2004-06-10.ppt8 73.2 keV Ch4 Trigger Run (2) ch1 ch7 ch3 ch5 ch2 ch6 We fitted data with a sinusoidal curve. Modulation Factor (an average of 6 channels) is 0.430+-0.006. On the other hand, MF predicted by G4 is 0.500+-0.005. Difference between data and simulation is ~15%.

9. ArgonneResult_2004-06-10.ppt9 Effects that affect the MF Passive materials If we put sensor mount and table (see page 2), MF predicted by G4 decreases from 0.5 to 0.48 (i.e., MF decrease by ~4%). This is mostly due to the splash component from the table. Accuracy of the simulation If we regard EGS4 simulation to be correct, G4 overpredicts the MF by ~2% (see page 6). Polarization degree of the beam According to Doug Robinson, the calculated source polarization was 98-99%. This can reduce the MF by 1-2%. Alignment of sensors If we shorten the distance between sensors by 2mm (from 2.22cm to 2.02cm), predicted MF decreases by ~1%. If we use an extended beam (7mm diameter) instead of a pencil beam, MF predicted decreases by ~1%. Considering them, we expect that misalignment of sensors could explain 2-3% difference of MF. There still be ~5% difference. What else do we have?

10. ArgonneResult_2004-06-10.ppt10 Comparison between data and simulation Below we compare between data and simulation and can see the differences. One difference which could affects the MF is that, data seems to be contaminated by background. To examine how this influences the MF, we change the selection criteria and revaluate the MF (next page). deposit energy in the central scinti. (keV) Total deposit energy (keV) deposit energy in the central scinti. (keV) 73.2keV run, 0degree, data 73.2keV run, 0degree, simulation

11. ArgonneResult_2004-06-10.ppt11 Ratio of MF factor We selected the event based on criteria given in page 7, but changed a range of total deposit energy. Narrower range (high S/N ratio) gives closer values of Modulation Factor between data and simulation. Background could explain the difference of 3-5%. totE=73+-25keV, +-20keV, +-15keV, +-10keV, +-5keV Ratio of the MF (data to simulation), as a function of the range of total deposit energy. Passive materials (sensor mount and table) are already taken into account in simulation.

12. ArgonneResult_2004-06-10.ppt12 Summary We have fixed the Geant4 physics processes (polarized Compton and Rayleigh scattering), and obtained a good agreement with EGS4 prediction (page6). Modulation Factor observed in Argonne beam test data was ~0.43, whereas that predicted by Geant4 was ~0.50. There are ~15% difference (page 8). Passive materials, accuracy of the simulation toolkit, depolarization of the beam and instrumental effect (misalignment of sensors) could explain ~10% difference (page9). There seems to be background component of the beam (continuum under the 73.2keV line?). If this explains the 5% difference, we obtain a good agreement with data (pages 10 and 11).