First results of the PixelGEM central tracking system of COMPASS

For its physics program with a high-intensity hadron beam of up to 2 • 10 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 triple-GEM detectors with a hybrid readout structure consisting of 1 x 1 mm2 pixels in the beam region and 2-D strips with a pitch of 400 //m in the periphery was designed and built. The 2048 channels of the detector are read out by 16 APV25-Sl analog sampling ASICs. The amplitude information thus obtained is used to cluster neighboring hits and allows us to achieve spatial resolutions which are about a factor of 10 smaller than the strip or pixel pitches. In addition, a pulse-shape analysis of the sampled signal yields a good time resolution of about 7.5 ns. After successful prototype tests, which proved the performance of this new detector type, the PixelGEM central tracking system consisting of five detectors was completely installed in the COMPASS spectrometer in spring 2008.