Triple GEM tracking detectors for COMPASS

The small area tracker of COMPASS, a high-luminosity fixed target experiment at CERN's SPS, includes a set of 20 large-size ($31\times 31\,\cm^2$) Gas Electron Multiplier (GEM) detectors. Based on gas amplification in three cascaded GEM foils, these devices permit to obtain high gain and good spatial resolution even at very high particle fluxes. A two-coordinate projective readout yields, for each track, highly correlated signal amplitudes on both projections, allowing to resolve multiple hits in high occupancy regions close to the central deactivated area of $5\,\cm$ diameter. At the same time the material exposed to the beam is minimized. Splitting the amplification in three cascaded stages permits to achieve a gain of $\sim 8000$, necessary for efficient ($>98\%$) detection of minimum ionizing particles on both coordinates, already at relatively moderate voltages across individual GEM foils. As a consequence, the probability of a gas discharge to occur when a heavily ionizing particle enters the detector volume, is reduced by more than an order of magnitude at a given gain compared to the initially foreseen double GEM structure. In conjunction with other strategies resulting from extensive R\&D on discharge phenomena, we were able to further reduce both the triggered by heavily ionizing particles entering the detector volume, this helped to drastically reduce both the energy and the probability of such breakdowns. In order to completely exclude permanent damage to the front-end chip by the rare event of a discharge fully propagating to the readout strips, an external electronic protection circuit is used. The operational characteristics of these detectors were examined both in the laboratory and in the beam, where a spatial resolution for minimum ionizing particles of $(46\pm 3)\,\mum$ and a time resolution of $\sim 15\,\ns$ were achieved. For the 2001 run of COMPASS, a total of 14 triple GEM detectors have been installed. First results from the commissioning phase in the high-intensity $\mu$ beam are presented.