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) My name is Kiyohsi Hayashida, from Osaka Unviersity. I am going to present about an approach of X-ray polarimerty with conventional gas proportional counters.

X-ray Polarimeters in Astronomy 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) We can divide the X-ray polarimeters for astronomy which were used or have been developed into three categories. Among these, polarimeters using the anisotropy in the photo-elecetron emission is studied extensively these years. The rise time polarimeter using gas proportional counter, I am talking about also belongs to the third category. Observations have been almost stagnant since 1970’s for more than 20 years.

Rise Time Polarimeter using 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) Anode Electron Cloud Rise Time = Short Rise Time = Long X-ray Photon Working principle This figure illustrate the working principle of the rise time polarimter using a gas proportional counter. Considering an X-ray with the polarization vector of this direction, the direction parallel to the anode direction, comes to the screen and is absorbed in the counter. The electron cloud produced should elongate like this. This will make a short rise time for the output signal, since drifting time of the electrons must align. On the contarary, if the polarization vector is perpendicular to the anode, the rise time should be longer. As fas as I know, the idea was first proposed in 1970. We recently examined this method and got some results as appeared in this paper. After the publication, I heard from several persons that they have tested the method before us but could no get positive results.

Experiment at Synchrotron Facility (1992-1993) 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 Irradiate position =12 mm from anode 2mmf We did the experiment in a Synchrotron facility in KEK Japan, neasly 10 years ago. We employed a Xe gas proportional counter. That is a backup model of the same counter for the All sky monitors onboard the Ginga satellite. The detector contains the multi-cell, but we used one of them. X-rays from Synchrotron beam comes from this direction. We measureed its polarization degree to be about 80% with a scattering method. We set the detector on a rotational stage. Note that we irradiated the X-ray beam at a small portion, about 2mm diameter, 12mm apart from the anode. (Hayashida et al., 1999, NIMA, 421,p.241)

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 We digitally sampled the output pulses from the counter. Rise time and pulse height were determined from the wavefomr data.

Results (Ex=34keV) <RT> vs Q RT distribution Here, I show the result for 34keV incidence. For this energy, we took the data . You can see the distribution is sharper for 0 deg, 180 deg incidence. The right panel shows the average rise time as a function of eenrgy. Modulation is visible as expected. (Hayashida et al., 1999, NIMA, 421,p.241)

X-ray Energy dependence 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. We repeated similar experiment for various X-ray energies, though only from 0deg to 90deg. You can see modulations particularly in higher energy incidence. The rise time itself is longer for higher energy incidence. Note that we selected the events. As noted in our paper, large modulation above the K-edge energy was against our expectation. However, It might be due to most of the events with K-fluorescent are excluded. We could apparently observe two-step type pulse profile. (Hayashida et al., 1999, NIMA, 421,p.241)

Modulation Factor M Figure of merit for polarimter, larger the better. We summarized the results in terms of the modulation factor defined with this formulae. It ranges from about 0.1 to 0.3. (Hayashida et al., 1999, NIMA, 421,p.241)

Extra Experiment for Verification 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. We did extra experiment to verify the modulation is due to polarization. We places a thin Sn filter at the beam exit. It produce Sn Ka, Kb fluoresenct X-rays in addition to polarized 36 keV X-rays. We separate those events by their PH. Only polarized X-ray showed the modulation. We convinced that we observed the polarization. (Hayashida et al., 1999, NIMA, 421,p.241)

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 300cm2 PCs will provide Polarization degrees (if P>5%) for each 5keV band. One of the merit of the PC rise time polarimter is its simplicity, if it works. We only need to add some signal processing unit to conventional gas proportional counters. Large effective area is easily realized without mirros. Pluse the effective energy range of above 20 keV suggests us to apply in a balloon experiment without mirrors. In fact, a pair of small proportional counter of 300 cm2 and 20 ks exposure will allow us to measure the X-ray polarization of Crab with enough sensitivity.

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 We recently started the experiment for the rise time polarimeter, agin. But in this case, we used the X-ray beam line we constructed in Osaka. This use a conventional X-ray generator and a monochrometer to get monochromatic X-ray beam of a polarization degree of 40-50%. 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.

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 0.270 0.265 0.260 0.250 0.255 0.245 We employed two kinds of proportional counters in the experiment. One is the same Xe counter used in the experiments in 1992-1993. We are happy that the counter works as well as 10 years ago. We obtained the rise time modulation like this. The rise time of the Ar counter is almost the half of the Xe counter.

However, Results are … Gas Content Xe96.7%+CO23.3% Ar 90% +CH410% Incident position 12mm from anode for Xe PC 10mm from anode for Ar PC 　Here, I summarized the results in terms of M. We considered the polarization degree of the beam. As you see, The M is at most 0.1 for this energy range. More seriously, the same Xe counter yielded M of less than 0.05. Apparently discrepancy. 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!

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 Ar-Gas P=1atm Ex=50keV 10mm 1mm We thus started a simulation using EGS4 code. That is a code for electron and photon intercactions in matter, developed and mainly used in high energy physics or gamma-ray community.

M expected from EGS4 Simulation *) 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 The right panel shows the M caliculated. Red, Green, Purple correspond to Ne, Ar , Xe gas, respectively. M ranges between 0.03-0.06 for Ar. For Xe, it is less than 0.03. There is little energy dependence. Simulation looks roughly consistent with the recent experiment. Small energy dependence M=0.03-0.06 for Ar, <0.03 for Xe NOTE: gas processes (diffusion, avalanche etc) is not considered.

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 Gas Amplification time 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 ? However, we have to note about a recent experiment of a gas chamber CPGS at Osaka beam line. The CPGC is a chmaber developed by Yamagata grooup using a capillary plate. Quartz PMT Out PMT time

What causes the discrepancy ? 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. Anode Experiment at Synchrotron Facility is planed in this Nov. 12mm

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.

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

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

XrayCCD（12μm,6.8μm pixel）simulation and Experiment M=(NV-NH)/(NH+NV) Experimental data Tsunemi et al., 1992; Hayashid et al., 1999; Schmidt et al., 1995

Feasibility Study PC polarimter Pmin % (99% confidence) T=100ks Merit of PC polarimter Large effective area is easily realized without mirrors. Suitable for balloon experiment