Foil geometry effects on GEM characteristics

We have considered a number of different GEM foil geometries in order to as sess 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 s hapes used in GEM foils, namely, the double-conical, single-conical and cyl indrical. The thick-foil GEM characteristics with cylindrical and double-co nical hole geometries are; studied in detail and their advantages in obtain ing a higher effective gas gain with an improved breakdown endurance are sh own. An effective gas gain of 10(3)-10(4) can be achieved with 200 mum foil s at the voltages of 750-300 V per 50 mum of thickness. No gas gain saturat ion effects have been observed for the foil geometries considered. (C) 2001 Elsevier Science B.V. All rights reserved.