A triple GEM detector with two dimensional readout

2000-56 The triple GEM detector is a micropattern gas detector which consists of a primary ionisation gap and three consecutive gas electron multiplier (GEM) foils. A printed circuit board with readout strips detects the current induced by the drifting electron cloud originating from the last GEM stage. Thus the gas amplification and the signal readout are completely separated. Triple GEM detectors are being developed as a possible technology for the inner tracking in the LHCb experiment. In an earlier note (LHCb TRAC-99/024) we have reported first experience with such a detector in a test beam at PSI. Here we describe the construction of an improved version (thinner transfer gaps, segmented GEM foils, two dimensional readout). Results from performance measurements are presented using intense hadronic beams as well as cosmic ray data.