Study of electrostatic charge-up phenomena in Gas Electron Multipliers

Gas Electron Multipliers (GEM) are widely used as amplification stage in gaseous detectors exposed to high rates, e.g. in the Time Projection Chamber of the ALICE (A Large Ion Collider Experiment) experiment after its upgrade. The GEM consists of a polyimide foil which is coated by two thin copper layers. GEM have a high density of holes, where charges are multiplied if suitable voltages are applied. One critical property is the electrostatic charge-up of GEM. It occurs when the trajectories of drifting ions or electrons end on the polyimide surface. Since polyimide is a very good insulator, the charge remains there. The charge-up influences key properties of the GEM, e.g. the gain. The characteristics of the charge-up process are studied in simulations. The electrostatic environment is calculated with a finite-element method. Afterwards, the movement of single particles in the electrostatic environment is simulated. The possibility to study the behaviour of single charges gives insight on the charge-up process. Furthermore, the simulated properties are cross-checked with measurements. Results of the measurements and simulations of the charge-up effect are presented. Additionally, the influence of different parameters, e.g. the rate of incoming radiation, on the time constant of the charge-up process is discussed.