F. Sauli

CERN

Summary: The Gas Electron Multiplier (GEM) is a new concept in proportional gas counters. The basic element is a thin insulating foil, metal-coated on both sides, with a high density of microscopic holes, typically 50 to 100 per mm². On application of a suitable voltage difference between the two sides, electrons released on one side are drifted into the holes, multiplied and released on the other side for further handling; proportional gains of several thousand can be obtained in a large range of operating gases and conditions. Originally made by chemical etching of copper-clad kapton, GEM electrodes have been manufactured by different technologies on a variety of supports including glass and ceramics, more suited for certain applications. Several GEM foils can be cascaded, a unique feature of the technology, permitting to achieve very large gains even in harsh radiation environments; this has motivated the development and construction of multi-GEM devices for high rate particle physics experiments, such as the inner tracker for HREA-B and COMPASS at CERN. Thanks to the localization of the avalanches and the suppression of photon and ion feedback in these structures, high gains can also be attained in pure noble gases and their mixtures, making it possible to couple the multiplier to a photocathode, a very promising direction of development towards large area position-sensitive photomultipliers. In particle physics, developments are on the way for high multiplicity read-out of Time Projection Chambers and photon detection for Cherenkov Ring Imaging. Various applications of the technology are being developed in other fields and will be described: very high rate plasma diagnostics, x-ray polarimetry in astrophysics, neutron detection and localization, high rate medical diagnostics and portal imaging.