A gas electron multiplier (GEM) is a type of gaseous ionization detector used in nuclear and particle physics and radiation detection.
GEMs were invented in 1997 in the Gas Detector Development Group[1] at CERN by physicist Fabio Sauli.
[2] Typical GEMs are constructed of 50–70 micrometre thick Kapton foil clad in copper on both sides.
For operation, a voltage of 150–400 V is placed across the two copper layers, making large electric fields in the holes.
A number of manufacturing difficulties plagued early GEMs, including non-uniformity and short circuits, but these have to a large extent been resolved.