Mapping study of gain and hole diameter of Japanese GEMs

The Gas Electron Multiplier (GEM) is a new device that amplifies an input electron cloud, while keeping its shape unchanged [1]. We have developed various type GEMs using a laser etching technique. Those GEMs had different geometry (pitch, hole diameter and thickness) and materials (polyimide; PI or liquid crystal polymer; LCP). We have applied the GEMs to a photoelectric polarimeter that measures polarization of X-rays emitted from stellar objects. To obtain the polarization accurately, it is necessary for GEMs to have good uniformity in two-dimension. As a result of gain mapping, the standard deviation of gain variation of our standard GEM was ~ 5%. Our GEMs showed enough uniformity to detect X-ray polarization. Comparing hole diameter and gain maps of the same region, there was no correlation between these two maps. Comparing maps of different type GEMs, there was no difference among them and PI GEMs were more uniform than LCP ones.