Measurements of the charging-up effect in Gas Electron Multipliers

Gas Electron Multipliers (GEM) are widely used as amplification stages in gaseous detectors exposed to high rates. The GEM consists of a polyimide foil which is coated by two thin copper layers. Charges collected into the holes or created during the amplification process may adhere to the polyimide part inside the holes. This is often accompanied by a change of the effective gain. The effect is commonly known as the "charging-up effect''. This work presents a systematic investigation of the effect under well-controlled and monitored conditions for a single standard GEM foil in Ar/CO2 (90/10) gas for varying rates of X-ray interactions in the detector and for different gains of the foil. In order to cover a wide range of different rates, we apply two different methods. The first one is based on a current measurement while the second one relies on the analysis of 55Fe spectra over time. Both methods give consistent results, showing an initial increase of the effective gain with time and an asymptotic saturation, which can be well described by a single-exponential function. We find that the characteristic time constants extracted from our measurements scale inversely proportional to the rate of incoming electrons for a given GEM voltage. Introducing characteristic quantities, which describe either the number of incoming electrons per hole or the total number of electrons in a hole per characteristic time, we find consistent numbers for measurements taken at the same GEM voltages. For measurements taken at different GEM voltages, however, also the characteristic total number of electrons inside the hole, which is supposed to take into account the different effective gains, is found to be higher by a factor of about 3.5 for U-GEM = 350V (n(tot) = 8.8 x 10(8)) compared to 400V (n(tot) = 2.4 x 10(8)). This hints at a residual dependence of the charging-up characteristics on the GEM voltage.