Foil geometry effects on GEM characteristics

We have considered a number of different GEM foil geometries in order to assess how the hole shape affects the performance of a GEM structure operated with the Ar–70%/CO2–30% gas mixture. The contributions of primary electron collection efficiency, hole multiplication, secondary particle losses and charge distribution in the hole and on the electrodes, to the effective gas gain are calculated and analysed for three classes of amplification hole shapes used in GEM foils, namely, the double-conical, single-conical and cylindrical. The thick-foil GEM characteristics with cylindrical and double-conical hole geometries are studied in detail and their advantages in obtaining a higher effective gas gain with an improved breakdown endurance are shown. An effective gas gain of 103–104 can be achieved with 200 μm foils at the voltages of 250–300 V per 50 μm of thickness. No gas gain saturation effects have been observed for the foil geometries considered.