First Results of the PixelGEM Central Tracking System for COMPASS

For its physics program with a high-intensity hadron beam of up to 2e7 particles/s, the COMPASS experiment at CERN requires tracking of charged particles scattered by very small angles with respect to the incident beam direction. While good resolution in time and space is mandatory, the challenge is imposed by the high beam intensity, requiring radiation-hard detectors which add very little material to the beam path in order to minimize secondary interactions. To this end, a set of triple-GEM detectors with a hybrid readout structure consisting of pixels in the beam region and 2-D strips in the periphery was designed and built. Successful prototype tests proved the performance of this new detector type, showing both extraordinary high rate capability and detection efficiency. The amplitude information allowed to achieve spatial resolutions about a factor of 10 smaller than the pitch and a time resolution close to the theoretical limit imposed by the layout. The PixelGEM central tracking system consisting of five detectors, slightly improved with respect to the prototype, was completely installed in the COMPASS spectrometer in spring 2008.