RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-7. Astrophysics and Astro-Glaciology

Fabrication of a TPC X-ray Polarimeter and Preliminary Testingwith the Synchrotron Radiation Light Source at Spring-8
A. Hayato,∗1 T. Tamagawa,∗1 T. Kitaguchi,∗1,∗2 E. Teruaki,∗1,∗3 W. B. Iwakiri,∗1Y. Takeuchi,∗1,∗4 M. Kubota,∗1,∗4 and K. Nishida∗1,∗2∗1∗2∗3∗4a)

larized source. The first step of our study is to determine µpol of the RIKEN polarimeter.
We performed two experiments: (a) to measureµ with highly (∼85%) polarized source at BL32B2
SPring-8, and (b) to confirm the flat modulation withunpolarized line emission produced by an X-ray generator. We took measurements at 4.5, 5.5, 6.4, 7.0, and7.5 keV for (a) and 4.5 and 6.4 keV for (b), although
we focus on only the result of the 4.5 keV data here.
Figure 2(a) shows the modulation curve taken withthe well-collimated (200×200 µm2 ) 4.5-keV polarized
X-ray at the middle height of the active volume, and(b) shows that with the 4.5-keV unpolarized X-ray under the same condition. The obtained µpol at 4.5 keV is0.3×0.85/pbeam , where pbeam is the polarization degreeof the X-ray beam, while the observed flat modulation
is 0.00±0.02. Meanwhile, the determination accuracy
for the polarization angle is about 4◦ at a certain drift
height. A detailed comparison with the GEMS polarimeter and other performance studies (e.g., detector
angle dependency for the sensitivity) are in progress.
(a) 4.5 keV(b) 6.4 keV

1 mmCounts

Fig. 1. Charge-weighted photoelectron track image takenby the RIKEN polarimeter with a 6.4-keV X-ray, together with a reconstructed angle with the blue arrow.
800700800(a) Polarized 4.5 keV

600600500500

400400300

300

200200

Nevt = 49980
μ = 26.31 ± 0.62 %Peak = -86.40 ± 0.68 deg
chi2=75.22 (dof=97)

1000(b) Unpolarized 4.5 keV700(X-ray polarimetry is expected to provide unique information about stars, such as strong gravitational
fields around black holes. However, it is far behind intechnology in comparison to radio/optical polarimetry,and the observation of Crab nebula by OSO-8 satellite
40 years ago has been the only detection of X-ray polarization among astronomical objects1) . NASA andRIKEN have been developing the X-ray polarimeterusing the time projection chamber (TPC) techniquewith the sensitivity 10 times greater than the OSO-8
polarimeter, for the Gravity and Extreme Magnetism
Small explorer (GEMS), an X-ray polarization telescope missiona) . Although we have already built theflight model2) , a detailed study is still needed in orderto fully understand the characteristics of the TPC polarimeter. We therefore fabricated an in-house TPCpolarimeter at RIKEN (RIKEN polarimeter) for tests
in various situations. In this report, we present a brief
overview and the basic performance of the RIKEN polarimeter.
The TPC polarimeter is a gas-based detector withsensitivity in the energy range of 2 − 10 keV. The polarimeter can image a photoelectron and reconstruct
its initial direction, where the azimuth angle distribution depends on the linear polarization degree/angle ofthe incoming X-ray3) .
The design of the RIKEN polarimeter is originallybased upon the GEMS polarimeter4) , although handy
materials (e.g., bare aluminum instead of coppertungsten or gold-plated aluminum) are used for easiermachining. Another modification is that we adopted
the Scalable Readout System (SRS), manufactured by
CERN/RD51, instead of a custom-made ASIC as thestrip readout. A SRS consists of APV25(s) with a sampling rate of 40 MHz, ADC card(s), and a Front-EndCard. Figure 1 shows the photoelectron track takenby the RIKEN polarimeter.
What we actually measure with the polarimeter isa modulation curve, an azimuth angle distribution ofthe photoelectron. The modulation factor µ is derived
from (fmax − fmin ) / (fmax + fmin ), where fmax andfmin are the maximum and minimum counts in thecurve. The modulation factor of the polarimeter µpol ,which represents the sensitivity of polarimetry, is defined as µ which should be measured with a 100% po--80-60-40-20Nevt = 50732
μ = 0.00 ± 1.82 %Peak = -45.60 ± 124.63 deg
chi2=129.72 (dof=97)

100020406080Angle (deg)0-80-60-40-20020406080Angle (deg)Fig. 2. Modulation curve for 4.5 keV (a) polarized X-rayand (b) unpolarized X-ray taken with the TPC polarimeter.

References

RIKEN Nishina Center
Department of Physics, Hiroshima University
NASA Goddard Space Flight Center
Department of Physics, Tokyo University of Science
GEMS was canceled in 2012 regarding the cost.

1) M. C. Weisskopf et al.: ApJ, 208, L125 (1976).
2) J. E. Hill et al.: Proc. SPIE, 8433, 84331Q (2012).
3) K. Black et al.: NIMPA, 581, 755 (2007).
4) T. Enoto et al.: Proc. SPIE, 9144, 91444M (2014).

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