Gas electron multiplier

A gas electron multiplier (GEM) is a type of gaseous ionization detector used in nuclear and particle physics and radiation detection.

All gaseous ionization detectors are able to collect the electrons released by ionizing radiation, guiding them to a region with a large electric field, and thereby initiating an electron avalanche. The avalanche is able to produce enough electrons to create a current or charge large enough to be detected by electronics. In most ionization detectors, the large field comes from a thin wire with a positive high-voltage potential; this same thin wire collects the electrons from the avalanche and guides them towards the readout electronics. GEMs create the large electric field in small holes in a thin polymer sheet; the avalanche occurs inside of these holes. The resulting electrons are ejected from the sheet, and a separate system must be used to collect the electrons and guide them towards the readout.

GEMs are one of the class of micropattern gaseous detectors; this class includes micromegas and other technologies.

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