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 collect 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 gas detectors; this class includes micromegas and other technologies.

GEMs were invented in 1997 in the Gas Detector Development Group at CERN by physicist Fabio Sauli.

Typical GEMs are constructed of 50–70 micrometre thick Kapton foil clad in copper on both sides. A photolithography and acid etching process makes 30–50 micrometer diameter holes through both copper layers; a second etching process extends these holes all the way through the kapton. The small holes can be made very regular and dimensionally stable. For operation, a voltage of 150–400 V is placed across the two copper layers, making large electric fields in the holes. Under these conditions, in the presence of appropriate gases, a single electron entering any hole will create an avalanche containing 100–1000 electrons; this is the "gain" of the GEM. Since the electrons exit the back of the GEM, a second GEM placed after the first one will provide an additional stage of amplification. Many experiments use double- or triple-GEM stacks to achieve gains of one million or more.

...
Wikipedia