GEM DETECTORS FOR COMPASS

For the small–area tracking of the COMPASS experiment, GEM detectors with an active area of 31 × 31 cm² are employed. These detectors use three cascaded GEM foils with asymmetric voltage sharing and Ar:CO₂ (70/30) as detector gas. The GEMs have a non-uniformity in gain of less than ±15% and achieve an efficiency of 99.0 ± 0.1% and a spatial resolution of 46 ± 3 μm for minimum–ionizing particles at nominal gain of ~ 8000. The narrow charge correlation (σ_rat< 0.1) between the orthogonal coordinates of the 2D projective readout improves the reconstruction capability for multiple hits. High rate tolerance and low discharge probability make the GEM detectors well suited for operation in intense muon and hadron beams.