Study of the gain uniformity and high-voltage tolerance of the GEM for a space application

Toshio Nakano

Polarimetry is a powerful tool for astronomy in addition to imaging, timing and spectroscopy. However, it is still unachieved in the soft X-ray band except for a few detections, because of difficulty in detector technology. Recent developed X-ray polarimeters for spacecrafts employ Gas Electron Multipliers (GEMs), and images trajectories of the photoelectrons that reflects polarization of the incident X-ray photons. For example, IXPE (Imaging X-Ray Polarimetry Explorer), the next SMEX mission to be launched in 2020, carries Gas Pixel Detectors employing a GME. PRAXyS (The Polarimetry for Relativistic Astrophysical X-ray Sources) was another candidate for the SMEX, and designed to achieve high sensitivity polarimetry with Time Projection Chamber polarimeters using GEMs.

For the PRAXyS mission, we developed a GEM with a hole pitch of  140 um, a hole diameter of 70 um and insulator thickness of 100 um. Using collimated  8 keV X-ray, we scanned the GEM with 2mm intervals and measured its gain uniformity. In order to space-application,  we tested its tolerance for the on/off cycles of the applied high-voltage. We report the results of these tests.