Measurements of the Charging-Up Effect in Gas Electron Multipliers

Abstract

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 foils are known to collect charges on the polyimide part in its holes during the amplification process. This is accompanied by a change of the effective gain. The effect is commonly known as the "charge-up effect". This work presents two different methods to investigate the charge-up effect. Both methods were conducted with a single GEM as amplification stage. The first one is based on a current measurement while the second one relies on the analysis of 55 Fe spectra over time. Both methods combined give a quantitative result of the time constant on which the charge-up effect occurs. It can be shown that the characteristic time constant is of the order of 5 × 10 5 electrons/hole if the GEM voltage is set to 400 V. With the first method, two measurements at a GEM voltage of 350 V were conducted as well. Here, the time constant is approximately a factor ten higher.

Study of charging up effect in a triple GEM detector

“…7 (b) because the HV is different. The residual voltage dependence on the charging-up effect is also seen in Ref [21].…”