X-ray Polarimetry with gas proportional counters through rise-time

1. X-ray Polarimetry with gas proportional counters through rise-time K. Hayashida, T. Horikawa, Y. Nakashima, H. Tsunemi (Osaka University, Japan) Acknowledge to Y. Namito (KEK, Japan) N.Tokanai (Yamagata Univ, Japan) B.Paul (TIFR, India)

2. Bragg Crystal Reflection OSO8,ArielV satellites (1970’s) Compton Scattering Planned for Spectrum X-Gamma mission (1990’s) Tracking Photo-electron Emission Direction X-ray CCD (Tsunemi et al., 1992) Micro electrodes Gas Chambers (Costa et al., 2001; Tanimori et al., 1999; Sakurai et al., 1996) X-ray Polarimeters in Astronomy Observations have been almost stagnant since 1970’s for more than 20 years.

3. Anode Anode X-rayPhoton Anode Electron Cloud Rise Time = Long Rise Time = Short Rise Time Polarimeterusing Gas Proportional Counter • Proposed by Riegler et al., Sanford et al., in 1970. • We re-examined the method. (Hayashida et al., 1999, NIMA, 421,p.241) Working principle

4. Ex=10-40keV (Pbeam~0.8; measured) Xe proportional counter = Ginga ASM backup Xe:736mmHg + CO2:25mmHg Multi cell type; 5cm depth, 5cmx30cm for each cell Anode 50mm Be; HV=1950V Experiment at Synchrotron Facility (1992-1993) Irradiate position =12 mm from anode 2mmf (Hayashida et al., 1999, NIMA, 421,p.241)

5. Sampling the output pulse from CSPA with Digitized Oscilloscope. RT and PH are determined from the waveform data transferred to the computer Rise Time Measurement

6. Results (Ex=34keV) <RT> vs Q RT distribution (Hayashida et al., 1999, NIMA, 421,p.241)

7. Large modulation & longer RT at higher energy Note: Xe-K edge =34.6keV Events were selected using PH. RT cutoff of 1ms is employed. Most of the events with K-fluorescent were likely to be excluded. X-ray Energy dependence (Hayashida et al., 1999, NIMA, 421,p.241)

8. Modulation Factor M Figure of merit for polarimter, larger the better. M (Hayashida et al., 1999, NIMA, 421,p.241)

9. Thin Sn filter was placed at the beam exit, which produced Sn Ka,Kb fluorescent X-rays in addition to polarized 36 keV X-rays. Extra Experiment for Verification (Hayashida et al., 1999, NIMA, 421,p.241)

10. Merit of the PC rise time polarimeter Only need to add RT processing unit to conventional PCc Large effective area is easily realized without mirrors. Feasibility Study PC polarimeter T=20ks Balloon Observation A pair of 300cm2 PCs will provide Polarization degrees (if P>5%) for each 5keV band.

11. Polarized X-ray Beam Line at Osaka Monochromatic beam with linear polarization degree of 40-50% is obtained. (Koike et al., 2000, SPIE4012, p.414) Electron Impact type X-ray Generator Measurement Chamber Double Crystal Monochrometer Pickup X-rays whose energy (Ex keV) is close to applied High Voltage (HV kV) Bremstrahlung X-rays are partially polarized.

12. 0.270 0.265 0.260 0.255 0.250 0.245 Experiment at Osaka Beam Line (2001-2002) angle E X-ray • Two kinds of Gas PCs • Ar gas flow type PC • Wire 50mm • Quenching gas (CH4) of various contents were examined. • Xe gas shield type PC (the same one used in Synchrotron Facility) • Data taking system was upgraded. 90,270 0,180 X-ray

13. Gas Content Xe96.7%+CO23.3% Ar 90% +CH410% Ar99.3%+CH40.7% However, Results are … • Contents of Quenching Gas is very sensitive to RT. • Ar 99.3% +CH4 0.7% provided M of 0.05-0.1. Small! • More seriously, the same Xe counter used in 1992-1993 yielded M of less than 0.05. Discrepancy! Incident position 12mm from anode for Xe PC 10mm from anode for Ar PC

14. EGS4 (Electron Gamma Shower Simulator ver4) + KEK low energy extensions Energy deposit and path of photoelectrons are simulated. Large effect of multiple scattering is observed. 1mm 10mm Preliminary Simulation using EGS4 Ar-Gas P=1atm Ex=20keV Ar-Gas P=1atm Ex=50keV

15. *) If multiple scattering does not exist, M would be 0.78. M expected from EGS4 Simulation Dispersion of the distance to anode (s ) cm M Size s (cm) M • Small energy dependence • M=0.03-0.06 for Ar, <0.03 for Xe • NOTE: gas processes (diffusion, avalanche etc) is notconsidered.

16. Capillary Plate Gas Chamber Pulse width is measured Saito et al, 2001 0˚ case Xrays(90˚) Electron Cloud Electric Vector Xray Detection Layer Xrays(0˚) PMT Out time Gas Amplification 90 ˚ case Capillary Plate CPGC with Ar gas provided M=21%＠20keV in an experiment at Osaka Beam Line EGS4 simulation predicts M=5% How valid the EGS4 simulation (multiple scattering) is in the low energy end ? Quartz PMT Out PMT time

17. Strong dependence of X-ray incident position on RT was observed. Misalignment of rotation axis could have made an artifact modulation. But, the modulation should have 1 peak / 360˚, not like observed 2 peaks / 360˚. Sn-filter result was not explained by the misalignment, either. What causes the discrepancy ? Anode Experiment at Synchrotron Facility is planed in this Nov. 12mm

18. Summary • Gas proportional counter polarimeter with rise time measurement is introduced. • Previous experiment at Synchrotron facility provided M of 0.1-0.3 with Xe PC, while recent experiment (Xe PC, Ar PC) provides much smaller M. • Preliminary simulation also suggests M <0.1 It also contradicts the CPGC result of M=0.2. • No firm solution has not yet been obtained for the discrepancy. Sorry for inconclusive, confusing results.