Simulation study on impact of GEM geometry for gas gain uniformity

Gas Electron Multiplier (GEM) is one of the devices for gas amplification and available as an amplification part of detectors for many experiments. A GEM with a 50 $\mu m$ thick insulator is popular and widely used in many experimental fields. However it is necessary to use several layers of them to get sufficient gas gain because gas gain which is provided with only one 50 $\mu m$ thick GEM is only several times ten. On the other hand, a thick GEM whose amplification area is expanded from 50 to 100 $\mu m$ to get higher gas gain which can be sufficient even with a double stack configuration. But the measurement of gas gain uniformity using a large area 100 $\mu m$ thick GEM reported here observed sizable non-uniformity which reached more than 50\%. We investigated gas gain uniformity which is derived from geometries of the GEM using $\rm{garfield^{++}}$ and considered an optimal geometry for the 100 $\mu m$ thick GEM.