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)
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.

2. 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.

3. 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 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.

4. 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)

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

6. 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)

7. 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)

8. 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)

9. 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)

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 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.

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
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.

12. 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 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.

13. 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 Apparently discrepancy.
Contents of Quenching Gas is very sensitive to RT.
Ar 99.3% +CH4 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
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.

15. 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 for Ar. For Xe, it is less than There is little energy dependence. Simulation looks roughly consistent with the recent experiment.
Small energy dependence
M= for Ar, <0.03 for Xe
NOTE: gas processes (diffusion, avalanche etc) is not considered.

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
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

17. 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

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

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

22. 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

23. 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