What is the real gas gain of a standard GEM?

We have observed very high gains (up to 7000) from GEMs with 'standard' par ameters (kapton thickness 50 mu m, pitch 120 mu m, copper hole diameter 65 mu m, kapton hole diameter 30 mu m) This was achieved using GEMs coupled to a simple array of copper read-out strips. From the measurements of the cur rent on all the electrodes, we conclude that the high observed gains are fu lly attributable to electron multiplication in the holes of the mesh, and n ot to electronics related effects as had been previously suggested. Further more, we report that this large gain may only be fully exploited when the f ield in the second GEM gap is high. The effect on the gain of coupling a GE M to another charge amplifying device was investigated using a GEM-PMGC com bination. (C) 1998 Elsevier Science B.V. All rights reserved.