PolariS (Polarimetry Satellite) is a Japanese small satellite mission dedicated to polarimetry of X-ray and γ-ray sources. The primary aim of the mission is to perform hard X-ray (10-80 keV) polarimetry of sources brighter than 10 mCrab. For this purpose, PolariS employs three hard X-ray telescopes and scattering type imaging polarimeters. PolariS will measure the X-ray polarization for tens of sources including extragalactic ones mostly for the first time. The second purpose of the mission is γ-ray polarimetry of transient sources, such as γ-ray bursts (GRBs). Wide field polarimeters based on similar concept as that used in the IKAROS/GAP but with higher sensitivity will be used, and polarization measurement of 10 GRBs per year is expected.
We present the gain properties of the gas electron multiplier (GEM) foil in pure dimethyl ether (DME) at 190 Torr. The GEM is one of the micro pattern gas detectors and it is adopted as a key part of the X-ray polarimeter for the GEMS mission. The X-ray polarimeter is a time projection chamber operating in pure DME gas at 190 Torr. We describe experimental results of (1) the maximum gain the GEM can achieve without any discharges, (2) the linearity of the energy scale for the GEM operation, and (3) the two-dimensional gain variation of the active area. First, our experiment with 6.4 keV X-ray irradiation of the whole GEM area demonstrates that the maximum effective gain is 2 x 10<sup>4</sup> with the applied voltage of 580 V. Second, the measured energy scale is linear among three energies of 4.5, 6.4, and 8.0 keV. Third, the two-dimensional gain mapping test derives the standard deviation of the gain variability of 7% across the active area.
Polarimetry is a powerful tool for astrophysical observations that has yet to be exploited in the X-ray band. For satellite-borne and sounding rocket experiments, we have developed a photoelectric gas polarimeter to measure X-ray polarization in the 2–10 keV range utilizing a time projection chamber (TPC) and advanced micro-pattern gas electron multiplier (GEM) techniques. We carried out performance verification of a flight equivalent unit (1/4 model) which was planned to be launched on the NASA Gravity and Extreme Magnetism Small Explorer (GEMS) satellite. The test was performed at Brookhaven National Laboratory, National Synchrotron Light Source (NSLS) facility in April 2013. The polarimeter was irradiated with linearly-polarized monochromatic X-rays between 2.3 and 10.0 keV and scanned with a collimated beam at 5 different detector positions. After a systematic investigation of the detector response, a modulation factor ≥35% above 4 keV was obtained with the expected polarization angle. At energies below 4 keV where the photoelectron track becomes short, diffusion in the region between the GEM and readout strips leaves an asymmetric photoelectron image. A correction method retrieves an expected modulation angle, and the expected modulation factor, ~20% at 2.7 keV. Folding the measured values of modulation through an instrument model gives sensitivity, parameterized by minimum detectable polarization (MDP), nearly identical to that assumed at the preliminary design review (PDR).
We report a Monte-Carlo estimation of the in-orbit performance of a cosmic X-ray polarimeter designed to be installed on the focal plane of a small satellite. The simulation uses GEANT for the transport of photons and energetic particles and results from Magboltz for the transport of secondary electrons in the detector gas. We validated the simulation by comparing spectra and modulation curves with actual data taken with radioactive sources and an X-ray generator. We also estimated the in-orbit background induced by cosmic radiation in low Earth orbit.
The Gravity and Extreme Magnetism Small Explorer (GEMS) will realize its scientific objectives through high sensitivity linear X-ray polarization measurements in the 2-10 keV band. The GEMS X-ray polarimeters, based on the photoelectric effect, provide a strong polarization response with high quantum efficiency over a broad band-pass by a novel implementation of the time projection chamber (TPC). This paper will discuss the basic principles of the TPC polarimeter and describe the details of the mechanical and electrical design of the GEMS flight polarimeter. We will present performance measurements from two GEMS engineering test units in response to polarized and unpolarized X-rays and before and after thermal and vibration tests performed to demonstrate that the design is at a technology readiness level 6 (TRL-6).
The scientific objective of the X-ray Advanced Concepts Testbed (XACT) is to measure the X-ray polarization
properties of the Crab Nebula, the Crab pulsar, and the accreting binary Her X-1. Polarimetry is a powerful tool for
astrophysical investigation that has yet to be exploited in the X-ray band, where it promises unique insights into neutron
stars, black holes, and other extreme-physics environments. With powerful new enabling technologies, XACT will
demonstrate X-ray polarimetry as a practical and flight-ready astronomical technique. Additional technologies that
XACT will bring to flight readiness will also provide new X-ray optics and calibration capabilities for NASA missions
that pursue space-based X-ray spectroscopy, timing, and photometry.
We have developed the gas electron multiplier (GEM) for applying to a cosmic X-ray polarimeter. For a space
use of the GEM, we performed experiments of charged particles irradiation to the GEM as space environmental
tests of cosmic rays. The GEM is irradiated with full-striped iron ions with energy of 500 MeV/n, as a result,
we found that even if a particle deposits an energy of ~ 2 MeV in the detector, it has no direct effect on the
GEM as long as the particle does not hit the GEM directly. In contrast, every time a particle collides with the
GEM, it discharges with a probability of 0.4-40% which depending on the count rate, the applied voltage, and
energy losses, but the mass of particles does not matter. The predicted count rate of discharges in the space
is low enough, so it is negligible compared with a target object. We also found that an irradiation of charged
particles for a certain period causes a destruction of the GEM, but the direct reason remains unclear.
Fine-pitch and thick-foil GEMs have been produced using a laser etching technique for photoelectric X-ray
polarimeters onboard future missions. The finest hole pitch of the thick-foil GEM is 80 μm with a hole diameter
of 40 μm, and a thickness of the insulator is 100 µm. The maximum effective gain in a 70%-30% mixture of argon
and carbon dioxide reaches 3×10<sup>4</sup> at voltage of 750 V between GEM electrodes. No significant gain increase or
decrease was observed during 24 hours test in which applied high voltage was ramped up and down frequently.
The measured gain stability was less than 4%. An accelerated test of the high voltage ramp up and down for
two years LEO operations were carried out. During the 6500 times voltage ramp up and down, the GEM kept
its gain within 4% variation and no unexpected behavior was observed.
We present a performance study of a cosmic X-ray polarimeter which is
based on the photoelectric effect in gas, and sensitive to a few to 30
keV range. In our polarimeter, the key device would be the 50 μm
pitch Gas Electron Multiplier (GEM). We have evaluated the modulation
factor using highly polarized X-ray, provided by a synchrotron
accelerator. In the analysis, we selected events by the eccentricity of the charge cloud of the photoelectron track. As a result, we obtained the relationship between the selection criteria for the eccentricity and the modulation factors; for example, when we selected the events which
have their eccentricity of > 0.95, the polarimeter exhibited with
the modulation factor of 0.32. In addition, we estimated the Minimum
Detectable Polarization degree (MDP) of Crab Nebula with our
polarimeter and found 10 ksec observation is enough to detect the
polarization, if we adopt suitable X-ray mirrors.
We have produced various gas electron multiplier foils (GEMs) by using laser etching technique for cosmic X-ray polarimeters. The finest structure GEM we have fabricated has 30 μm-diameter holes on a 50 μm-pitch. The effective gain of the GEM reaches around 5000 at the voltage of 570 V between electrodes. The gain is slightly higher than that of the CERN standard GEM with 70 μm-diameter holes on a 140 μm-pitch. We have fabricated GEMs with thickness of 100 μm which has two times thicker than the standard GEM. The effective gain of the thick-foil GEM is 104 at the applied voltage of 350 V per 50 μm of thickness. The gain is about two orders higher than that of the standard GEM. The remarkable characteristic of the thick-foil GEM is that the effective gain at the beginning of micro-discharge is quite improved. For fabricating the thick-foil GEMs, we have employed new material, liquid crystal polymer (LCP) which has little moisture absorption rate, as an insulator layer instead of polyimide. One of the thick-foil GEM we have fabricated has 8 μm copper layer in the middle of the 100 μm-thick insulator layer. The metal layer in the middle of the foil works as a field-shaper in the multiplication channels, though it slightly decreases the effective gain.