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

Transcription

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 X-ray Polarimeters in Astronomy Bragg Crystal Reflection OSO8,ArielV satellites (1970 s) Compton Scattering Planned for Spectrum X-Gamma X 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) Observations have been almost stagnant since 1970 s for more than 20 years.

3 Rise Time Polarimeter using Gas Proportional Counter Proposed by Riegler et al., Sanford et al., in We re-examined examined the method. (Hayashida et al., 1999, NIMA, 421,p.241) Working principle Anode // ur Anode E Anode ur E ur E X-ray Photon Electron Cloud Rise Time = Short Rise Time = Long

4 (Hayashida et al., 1999, NIMA, 421,p.241) Experiment at Synchrotron Facility ( ) 1993) Ex=10-40keV (P beam ~0.8; measured) Xe proportional counter = Ginga ASM backup Xe:736mmHg + CO 2 :25mmHg Multi cell type; 5cm depth, 5cmx30cm for each cell Anode 50µm m Be; HV=1950V BL14C Vertical Wiggler SOR Vacuum Double Crystal Spectrometer Air 2mmφ Hole Pb Shield Anode Direction Θ Electric Vector // Vertical Direction Xe Gas Proportional Counter Irradiate position =12 mm from anode 2mmφ

5 Rise Time Measurement Sampling the output pulse from CSPA with Digitized Oscilloscope. RT and PH are determined from the waveform data transferred to the computer Shaping Amp Scaler 100 E=40keV Xe Gas Proportional Counter GPIB Personal Computer Digitized Voltage (mv) % of PH 20% of PH PH Pre-Amplifier Digital Storage Oscilloscope RT Time(microsec)

6 Results (Ex=34keV) RT distribution <RT> vs Θ Number of events Number of events 34 kev Θ=0deg keV Q=180deg Number of events Number of events 34keV Q=90deg keV Q=270deg risetime (microsec) kev Θ (deg) (Hayashida et al., 1999, NIMA, 421,p.241)

7 (Hayashida et al., 1999, NIMA, 421,p.241) X-ray Energy 10keV 20keV 30keV dependence Large modulation & longer RT at higher energy Note: Xe-K K edge =34.6keV Events were selected using PH. RT cutoff of 1µs 1 s is employed. Most of the events e with K-fluorescent were likely to be excluded Θ (deg) 34keV Θ (deg) Q (deg) 36keV Q (deg) Q (deg) 40keV Q (deg)

8 Modulation Factor M Figure of merit for polarimter, larger the better. M Modulation Contrast Inciden X-ray Energy (kev) M o o ( Θ= 90 ) RT ( Θ= 0 ) { RT } 2σ RT beam o o ( ) RT ( ) { 2 2 } RT σ = σ Θ= 90 + σ Θ= 0 / 2 σ = RT RT min P : STD of RT distribution cf Minimum Detectable Polarization (MDP) P = M a N event (Hayashida et al., 1999, NIMA, 421,p.241)

9 Extra Experiment for Verification Thin Sn filter was placed at the beam exit, which produced Sn Kα,Kβ fluorescent X-rays X in addition to polarized 36 kev X-rays. 36keV Polarized Sn K a (25.2 kev) Sn K b (28.5 kev) Θ (deg) Q (deg) Q (deg) (Hayashida et al., 1999, NIMA, 421,p.241)

10 Feasibility Study PC polarimeter 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. T=20ks Balloon Observation A pair of 300cm 2 PCs will provide Polarization degrees (if P>5%) for each 5keV band.

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

12 Experiment at Osaka Beam Line Two kinds of Gas PCs Ar gas flow type PC Wire 50µm 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. ( ) 2002) angle 90,270 0, E X-ray X-ray

13 However, Results are Gas Content Xe96.7%+CO 2 3.3% Ar 90% +CH 4 10% Ar99.3%+CH 4 0.7% Incident position 12mm from anode for Xe PC 10mm from anode for Ar PC Contents of Quenching Gas is very sensitive to RT. Ar 99.3% +CH 4 0.7% provided M of Small! More seriously, the same Xe counter used in yielded M of less than Discrepancy!

14 Preliminary Simulation using EGS4 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. Ar-Gas P=1atm Ex=20keV E r Ar-Gas P=1atm Ex=50keV 1mm 10mm

15 *) If multiple scattering does not exist, M would be M expected from EGS4 Simulation Dispersion of the distance to anode (σ ) cm M Size σ (cm) M for each photo-absorption event ( i i) / ( i) ( E ( ) 2 i di d )/( E i) d = E d E σ = Small energy dependence M= for Ar, <0.03 for Xe NOTE: gas processes (diffusion, avalanche etc) is not considered.

16 Capillary Plate Gas Chamber Xrays(90 ) Xrays(0 ) Saito et al, 2001 X Pulse width is measured 0 case PMT Gas Amplification CPGC with Ar gas provided M=21% 20keV in an experiment at Osaka Beam Line Quartz EGS4 simulation predicts M=5% How valid the PMT EGS4 simulation (multiple scattering) is in the low energy end? 90 case PMT

17 What causes the discrepancy? Strong dependence of X-ray X 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. Experiment at Synchrotron Facility is planed in this Nov. risetime (microsec) kev Θ (deg) Anode 12mm

18 Summary Gas proportional counter polarimeter with rise time measurement is introduced. Previous experiment at Synchrotron facility provided M of 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.

19 Xe atm Xe-Gas P=1atm Ex=20keV Xe-Gas P=1atm Ex=50keV 1mm E r 1mm

20 Ne 1 atm Ne-Gas P=1atm Ex=20keV Ne-Gas P=1atm Ex=50keV 10mm E r 10mm

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22 XrayCCD 12 m,6.8 m pixel simulation and Experiment M=(N V -N H )/(N H +N V ) Experimental data Tsunemi et al., 1992; Hayashid et al., 1999; Schmidt et al., 1995

23 Feasibility Study PC polarimter Pmin % (99% confidence) CCD(6micron/50micron/100cm2) CCD(12micron/50micron/100cm2) XePC(4000cm2,20-40keV) ArPC(4000cm2,20-40keV) SXRP(Li) Thomson(Li,628cm2,M=0.5,10-20keV) MPGC+SODART(present,2-10keV) T=100ks flux(crab) Merit of PC polarimter Large effective area is easily realized without mirrors. Suitable for balloon experiment