Ion Gating in a GEM

GEM(Gas Electron Multiplier) is superior to other gas detectors because of its high counting rate, excellent spatial resolution, good imaging capability, operability in a magnetic field, and large sensitive area. One of the important features in a GEM is the strong suppression of the feedback of the positive ions, which are generated from the avalanche. GEM can be used in various application areas such as the end cap detector for a TPC(Time Projection Chamber) and the GEM photomultiplier. In these cases, the ion feedback has to be reduced to reduce the field distortion in the TPC and prevent a photocathode degradation in the GEM photomultiplier. The ion feedback in a multi GEM was measured by changing the gas mixture, the drift electric field, the effective gain, the transfer electric field, and the asymmetry of the applied voltage. A simple model is made to explain the ion feedback. One way to reduce the ion feedback is an ion gating. That is, multi-wires are placed in the drift region, and the relative voltage is applied to the wire, which can prevent the ions from arriving at the drift plate. Electric field calculation was done to simulate the ion gating in the GEM. Also, the suppression of the ion feedback was measured with the ion gating method