GEM detectors for COMPASS

For the small-area tracking of particles within COMPASS (common muon and proton apparatus for structure and spectroscopy), a new fixed target experiment at CERN/SPS, several large-size(31×31 cm² ) detectors based on the gas electron multiplier (GEM), have been built. These new devices, consisting of several GEM amplification stages with a two-coordinate readout, combine good spatial resolution with high rate capability, which is required by the large particle flux near the beam. At the same time, the material exposed to the beam is minimized in order not to spoil the mass resolution of the spectrometer. The first detectors out of a total of 20 were subject to extensive tests in the beam and in the laboratory, showing that full (i.e., >98%) detection efficiency for minimum ionizing particles can be achieved at a total effective gain of 6000. Ongoing research work focuses on discharges triggered by heavily ionizing particles entering the detectors. Systematic studies of the energy released in discharges and their probability of occuring at all as a function of a variety of parameters suggest several means to minimize their impact on detector performance. First results of the operational characteristics of these detectors in the real COMPASS beam are presented