A two-stage, high gain micro-strip detector

A two stage position-sensitive gas proportional counter has been constructed by tightly coupling a Gas Electron Multiplier (GEM) with a Micro-Groove Detector (MGD). The GEM was used as the first amplifying stage and was optimised to transmit close to 100% of the primary charge even at very high drift fields (10 kV/cm). Very narrow GEM-MGD separations (0–600 μm) were used so that the active volume of the detector is still very thin (3–3.6 mm) and the required drift field could be maintained using an acceptable drift voltage (around 4000 V). Very high combined gains (up to 3×105) were obtained with this system. The detector was found to be spark-free in the presence of HIPs (alpha particles) up to gains in excess of 20 000.