A Study of Gain Stability and Charging Effects in GEM Foils

A study has been carried out to investigate the gain stability and charging effects in Gas Electron Multipliers (GEMs). It was found that the gain of a GEM typically rises after the initial application of high voltage across the foil, but the degree and rate of rise can depend on a variety of different factors. These factors include the geometry of the foil (inner and outer hole diameter), the rate of ionizing radiation applied to the foil, the amount of water content of the operating gas, and possibly various other factors related to the manufacturing process. We have studied GEMs produced by several different manufacturers and compared their gain behavior as a function of time, after applying and removing the high voltage, gain as a function of rate, and the degree of initial charge up as a function of water content in the gas. While GEMs from all manufacturers exhibited an initial charge up to some degree, there was a large variation in this effect between different foils. The ratio of inner to outer hole diameter and the amount of water content in the gas seem to be two principle factors affecting the degree and rate of the initial charge up effect observed in certain foils.