Measurements of the charge-up effect in Gas Electron Multipliers

Gas Electron Multipliers (GEM) are widely used as an 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 coated by two thin copper layers. A GEM foil has 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 drifting ions or electrons end up on the polyimide surface. Since polyimide is a very good electrical insulator, the charge remains there and changes the configuration of the electrostatic field which in turn influences key properties of a GEM such as the effective gain. This effect was investigated through simulations and measurements. In this talk, the results of dedicated measurements with a test-detector are presented and compared to the outcome of the simulations. The influence of the rate of incoming radiation on the time constant of the charge-up process is discussed. A special focus lies on the investigation of the charge-up effect in GEM foils with different hole shapes (e.g. single-conical GEM).