GEM-MIGAS: Charge gain characteristics for the induction gap in the 50–300 μm range

A conventional gas electron multiplier with its induction gap set around 50 μm results in a two stage electron amplification device called the gas electron multiplier with micro-induction gap amplifying structure (GEM-MIGAS). We present results on the charge gain characteristics of GEM-MIGAS induction gap as a function of induction gap thickness, using the single electron response method. In the present study, the induction gap was varied between 50 and 300 μm and the GEM-MIGAS was operated in gas flow mode using He/iso-C4H10 (85/15%). The charge gain response agreed well with the classical parallel plate gain formula. From the experimental data, it was possible to derive the approximate values of the ionisation potential threshold and the ionisation mean free path, which exhibits an approximately linear dependence on the induction gap thickness. The optimum gap thickness region, where the charge gain was at a maximum for a given induction voltage and where it was least sensitive to variations in the gap thickness, was found to be in the 50–70 μm range. Nevertheless, the highest charge gains, achieved before the onset of detector instabilities, were obtained with the gap thickness set between 150 and 300 μm with values reaching the 106 region, owing to the higher voltages that can be applied to the device.