A triple-GEM detector with pixel readout for high-rate beam tracking in COMPASS

For its physics program with a high-intensity hadron beam of 2 · 10⁷ 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 minimise secondary interactions. To this end, a set of triple-GEM detectors with pixel readout in the beam region and 2-D strip readout in the periphery is currently being built. The readout structure has been realised on a polyimide printed circuit foil of only 100 μm total thickness with three conductive layers. The pixel size has been chosen to be 1×1 mm², which constitutes a compromise between the spatial resolution achievable and the number of readout channels. A 2-D strip readout structure with a pitch of 400 μm surrounds the pixel area. In total an active area of 10 × 10 cm² is covered using 2048 readout channels. Analogue readout by the APV25 ASIC has been chosen in order to profit from amplitude measurements which help to improve the spatial resolution by clustering neighbouring hit strips or pixels. A detector prototype has been tested successfully in the 5 · 10⁷ particles/s COMPASS muon beam, as well as in a focused hadron beam. The design of the detector and first results concerning its performance as a beam tracker will be presented.