1. 1 Report on analysis of PoGO Beam Test at Spring-8 Tsunefumi Mizuno mizuno@SLAC.Stanford.EDU July 15, 2003 July 21, 2003 revised August 1, 2003 updated

2. 2 Detector/Beam configuration (1) Incident beam goes from +y to –y Polarization vector of incident beam: along x-axis Beam size (for detector): a circle of 2.68cm diameter Expected beam energy (for detector):83.6 keV (incident beam of 100keV is scattered at 90 degree; this direction picks up the minor component of incident beam whose polarization vector is along z-axis) Expected polarization vector (for detector): along z-axis We used an Al block instead of a foil to get an appropriate S/N ratio, and decreased the beam rate by a factor of ~100 (from ~10^13Hz down to ~10^11Hz) to prevent pile-up (<=5kHz at the center scintillator). Trigger rate was 150-320 Hz and the background rate (measured with Pb block placed in front of the collimator) was 70-200 Hz. (Rate had changed as incident beam rate varied, I think.) Beam(100keV) polarization vector Collimator (Pb with smaller hole) Collimator (Pb with bigger hole) x y Scatterer (Al block) (Pb sheet)

3. 3 Detector/Beam configuration (2) y z Id:1 2 34 5 67 Expected polarization vector 26.8mm 49mm hexagonal scintillators 20cm long Beam direction Scattered beam goes from +x to –x Beam size: a circle of 2.68cm diameter Expected polarization vector: along z-axis Expected beam energy:83.6 keV (incident beam of 100keV is scattered at 90 degree) Right figure corresponds to the rotation angle of 0 degree. We rotated the detector. rotation angle

4. 4 Detector/Beam configuration (3) left) detectors mounted and collimators. This photo was taken outside the beam area. bottom) Al block scatterer. Photos are collected in http://www.slac.stanford.edu/~mizuno/Photos/Spring8/thumbnail.html

5. 5 DAQ block diagram Trigger condition: any one of surrounding scintillators are with hit. Trigger thresholds were set at ~20 keV. Following 7 were counted with visual scaler. Live time is calculated as (7)*(2)/(1) 1)OR output 2)Gata Generator output 3)PMT2 discriminator output 4)PMT3 discriminator output 5)PMT5 discriminator output 6)PMT6 discriminator output 7)CLK(100kHz)

6. 6 Run summary Data are stored in http://www.slac.stanford.edu/~mizuno/PoGO/Spring8http://www.slac.stanford.edu/~mizuno/PoGO/Spring8 calibration with 241Am for PMT1 (before beam test): run16_p.txt. Shaper gains (for all PMTs) were adjusted to give ~3V for 60keV. 0 degree run/bg: run17_p.txt and run18_p.txt 30 degree run/bg: run19_p.txt and run20_p.txt 15 degree run/bg: run21_p.txt and run22_p.txt 180 degree run/bg: run23_p.txt and run24_p.txt bg without beam: run25_p.txt calibration with 241Am (after beam test): run26_p.txt scaler data: Scaler.dat Data files with _p are peak data and without are waveform data (binary). Integration time was ~1200s for all runs, ~600s for bg. runs (Pb block was placed in front of the collimator) and ~300s for bg. run without beam.

7. 7 Calibration run (1) PMT/scint 1 PMT/scint 2 PMT/scint 3 PMT/scint 4 A gain of PMT1 might be shifted during the test. (from ~3V to ~2.7V)

8. 8 Calibration run (2) PMT/scint 5 PMT/scint 6 PMT/scint 7 PMT/scint 4 PMT/scintillator of #4 is taken by ch4(upper card) and ch8(lower card) and gives consistent spectra -> DAQ itself works well.

9. 9 Energy spectrum of each scint. (1) sim. of 0 degree Event selection: Central scintillator and one of surrounding scintillators are with hit (detection threshold is set at 3keV). Total deposit energy is above 45 keV. Geant4 simulation results of 1M run with 100% polarized 83keV beam are presented. Here, energy resolution (FWHT) is assumed to be 35% at 60keV, typical values seen in the calibration run (run26). As far as I know, Geant4 (of version 4) has minor bug in polarization process and scattered X-ray behaves as if it were not polarized. Since this bug hampers the modulation, we had expected that we could observe the difference in spectra of at least by a factor of 2 in the beam test. ch1 ch2 ch3 ch5 ch6 ch7

10. 10 Energy spectrum of each scint. (2) 0 degree 30 degree Energy scale of ch1 might have been shifted during the test and not appropriate for 0 degree run (see page 5). For other PMT/scintillators, no significant difference (modulation) is observed. ch1 ch2 ch3 ch1 ch2 ch3 ch5 ch6 ch7 ch5 ch6 ch7 Event selection: Central scintillator and one of surrounding scintillators are with hit (detection threshold is set at 3keV). Total deposit energy is above 45 keV.

11. 11 Energy spectrum of each scint. (3) 15 degree 180 degree No significant difference (modulation) is observed. ch1 ch2 ch3 ch1 ch2 ch3 ch5 ch6 ch7 ch5 ch6 ch7 Event selection: Central scintillator and one of surrounding scintillators are with hit (detection threshold is set at 3keV). Total deposit energy is above 45 keV.

12. 12 Total energy deposition Beam energy was expected to be 83.6 keV (100keV is scattered at 90 degree), but was ~70 keV. -> main component might be double scattered photons or something unexpected and be unpolarized. G4 simulationrun23: 180 degree Event selection: Central scintillator and one of surrounding scintillators are with hit (detection threshold is set at 3keV)

13. 13 Summary Gain of PMT/scint. #1 seemed to have changed during the test; We have adjusted shaper gain so that 60keV peak comes to 3V, but that of PMT#1 was 2.7V in the calibration run (run26). For other PMT/scintillators, no significant difference (modulation) is observed in spectra. This might be due to that the beam (after 90degree scattering) was not what we expected. Mean energy(70keV) is lower than calculation (100keV->83.6keV).