Readout of GEM detectors using the Medipix2 CMOS pixel chip

We have operated a Medipix2 CMOS readout chip, with amplifying, shaping and charge discriminating front-end electronics integrated on the pixel level, as a highly segmented direct charge collecting anode in a Three-Stage Gas Electron Multiplier (Triple-GEM) to detect the ionization from 55Fe X-rays and electrons from 106Ru. The device allows to perform moderate energy spectroscopy measurements (20% FWHM at 5.9 keV X-rays) using only digital readout and two discriminator thresholds. Being a truly 2D detector, it allows to observe individual clusters of minimum ionizing charged particles in Ar / CO 2 (70:30) and He / CO 2 (70:30) mixtures and to achieve excellent spatial resolution for position reconstruction of primary clusters down to ∼ 50 μ m , based on the binary centroid determination method.