Basic properties of gas electron multipliers for cosmic X-Ray polarimeters

A gas electron multiplier (GEM) is a key device for the photoelectric X-ray polarimeters, which will be launched in the near future. Our GEMs have spatial resolution enough for tracking of photoelectrons and low risk of discharge. GEMs need to perform well in all sorts of tolerance tests for space missions before launch. As a result of gain stability test after switching ON/OFF 6500 times repeatedly, our GEM gain variation is within ±2%. This repetition time scale corresponds to 2-year space mission in the Low Earth Orbit. As a result of charged particle (Fe) irradiation to GEM, our GEM survived more than 500 seconds of Fe ion irradiation (flux is 130 cts/cm 2 /s). This corresponds to 40-year irradiation in space. Thus, we conclude that our GEMs are stable enough for space missions and tolerance against discharge which is caused by charged particles.