Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 48 (2015)

Performance test of TPC Polarimeter for cosmic X-ray sources at BNL NSLS-1
W.B. Iwakiri,＊1 K. Black,＊2 T. Kitaguchi,＊1 J. Hill,＊2 K. Jahoda,＊2 T. Enoto,＊1,＊2 T. Tamagawa,＊1,＊3 Y. Takeuchi,＊1,＊3M. Kubota,＊1,＊3 H. Marlowe,＊4 R.Cole,＊4 K. Nishida,＊1,＊3 and A.Hayato＊1
Cosmic X-ray polarimetry is believed to be a powerfulmeasurement technique for studying the physics in extreme
environments such as strong gravitational fields andmagnetic fields in the universe. However, soft X-ray
polarization has not succeeded so far except for a few
detections in the 1970’s 1). To study such an unexploited
field, NASA and RIKEN have been developing an X-ray
polarimeter that employs the Time Projection Chamber(TPC) technique2). In this progress report, we discuss theperformance test of a TPC polarimeter with a new Read Out
Board (ROB) design3).
When an incident X-ray interacts with a gas atom, aphotoelectron is ejected preferentially in the direction of theelectric field vector of the incident photon according to acosine2 probability distribution. Thus, we can determine thepolarization degree and the phase by obtaining a track
image and initial direction of the photoelectron. A
schematic view of TPC polarimeter is shown in Fig 1. To
ensure high efficiency for the polarization signal, the charge
detection plane of the TPC polarimeter consists of the Gas
Electron Multiplier (GEM) designed by RIKEN4) mounted
over strip anodes parallel to the incident X-rays.
Two-dimensional images of photoelectron are created using
a one-dimensional strip readout and by timing the arrival ofcharge2). For a small satellite mission of cosmic X-ray
polarimetry, we have proposed two different ROB designs.
One design uses two gold-plated titanium frames with“tongue in groove” to secure the strips under tension3). The
performance of the polarimeter using this type of ROB has
been verified and meets all mission requirements5). To
reduce cost and risk by further simplifying the assembly
more, the other design one has used epoxy adhesives tohold the strips under tension to the lower frame.
We carried out the performance test for the polarimeter at
the Brookhaven National Laboratory (BNL) SynchrotronLight Source-1 facility in September 2014. The polarimeter
was filled with 190 torr dimethyl ether (DME). To make thepixels of the track image square-shaped, the electrical drift
region was selected as 196 V/cm, which is the drift velocity
equal to the 121 μm divided by the 50 ns sampling time.
The transfer field between the GEM and the ROB wasselected as 660 V/cm considering the collection efficiencyof the charge. The detector was irradiated withliner-polarized monochromatic X-rays at eight energy bands.
The total number of the events is 5 × 104 for individual
measurement. Further, to calibrate the polarimeter, we alsoacquired 2.7 keV unpolarized events.
Fig.2 shows the derived modulation curves for threeenergy bands and the 2.7 keV unpolarized data at 8 mmfrom GEM, which corresponds to the detector optical axis.
The results have subtracted the pedestal and common modenoise and deconvolved the electronics response. The S/Nachieved 5 σ levels. To correct the asymmetry caused by thedifference of the drift velocity between the drift and thetransfer region, the results are corrected using Gaussianconvolution in the time axis. The Gaussian σ value is 46 μm
which is optimized using the 2.7 keV unpolaraized data5).
As an indicator of the polarization sensitivity, we introduce
a modulation factor μ , which is defined as

μ = (fmax – fmin) / (fmax + fmin),where fmax and fmin are maximum and minimum values ofthe modulation curve, respectively. The derived modulation
factors of 2.7 keV, 4.5 keV, and 6.4 keV are 21.3±0.6%,
37.9±0.6%, and 46.6±0.6%, respectively. The results areconsistent with previous ROB design5) and meet themeasurement requirement. The estimation of the systematicerror for the detector position is still under discussion.

Fig. 2. Modulation at three energy bands measured at BNL
using TPC polarimeter.
Fig. 1. Schematic view of the TPC polarimeter2,3)
RIKEN Nishina Center＊2
NASA Goddard Space Flight Center＊3
Department of Physics, Tokyo University of Science＊4
Department of Physics and Astronomy, University of Iowa
＊1

References

1) M.C.Weisskopf et al: ApJL, 220, L117 (1978)
2) K. Black et al: NIMPA, 581, 755 (2007)
3) J. Hill et al: Proc. of the SPIE, 9144 , 91441N (2014)4) T. Tamagawa et al: NIMPA, 608, 390 (2009)5) T. Enoto et al: Proc. of the SPIE, 9144, 91444M (2014)- 175 -

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