Development of the gas electron multiplier (GEM)

We describe recent developments of the Gas Electron Multiplier (GEM), a thin composite mesh acting as proportional avalanche amplifier in gas counters. In beam tests we have verified the excellent efficiency, time resolution and localization accuracy for a GEM with micro-strip read-out. Efficiency, localization accuracy and operation in strong magnetic fields has been verified; operation at rates above 106 Hz/mm2 and lifetimes corresponding to at least 10 mC/cm of collected charge have been demonstrated. Refinements in the manufacturing technology have permitted the realization of large size detectors (27 by 25 cm2), to be used in conjunction with micro-strip gas chambers. With an improved design, stable gains above two thousand have been reached (GEM2000); larger gains can be obtained increasing the thickness of the foils, cascading two GEMs at some distance or in electrical contact. Further developments of the technology and prospective applications are discussed.