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 seperations (0--600~$\mu$m) 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 maintaine d using an acceptable drift voltage (around 4000~V). Very high combined gains (up to 3~$\times$10$^5$) 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 10,000.