The Gas Electron Multiplier (GEM) is a gaseous-ionisation detector – a type of detector used in nuclear physics, particle physics and radiation detection.

It works by collecting the free electrons released when ionising radiation passes through a medium (usually a gas). An electric field is applied to the medium to accelerate free electrons. They collide with atoms in the medium, ionising them. The ionisation releases more electrons that accelerate and collide with further atoms, releasing more electrons in a chain reaction known as an electron avalanche. The avalanche produces enough electrons to create a detectable current, and the affected region of gas is turned into electrically conductive plasma.