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 ga s counters'. In beam tests we have verified the excellent efficiency, time resolution and localization accuracy for a GEM with micro-strip r ead-out Efficiency, localization accuracy and operation in strong magn etic fields has been verified; operation at rates above 10(6) Hz/mm(2) and lifetimes corresponding to at least 10 mC/cm of collected charge have been demonstrated. Refinements in the manufacturing technology ha ve permitted the realization of large size detectors (27 by 25 cm(2)), to be used in conjunction with micro-strip gas chambers. With an impr oved design, stable gains above two thousand have been reached (GEM200 0); larger gains can be obtained increasing the thickness of the foils , cascading two GEMs at some distance or in electrical contact. Furthe r developments of the technology and prospective applications are disc ussed.