Gas electron multiplier

The Gas Electron Multiplier (GEM) is a proven amplification technique for position detection of ionising radiation such as charged particles, photons, X-rays and neutrons, in gas detectors. The GEM is a detector containing a densely pierced polymer foil coated with electrodes on both sides which is able to achieve high amplification gains and performance at low cost, even under harsh conditions. A GEM consists of a thin, metal-clad polymer foil, chemically pierced by a high density of holes. On application of a difference of potential between the two electrodes, electrons released by radiation in the gas on one side of the structure drift into the holes, multiply and transfer to a collection region. This gas detector is extensively used in High Energy Physics.

This CERN patented technology has more than 50 R&D and commercial licensees around the world. In 2017, CERN experts developed two GEM variants: i) The Optical readout GEM, which is being tailored for online dose imaging in hadron therapy and ii) the GEMpix detector, which has applications in conventional radiotherapy.

Advantages & Applications

Possible applications

  • Medical Imaging.
  • Radiation Therapy Dosimetry.
  • High-Energy Physics.
  • Astronomy/Astrophysics.
  • Material Analysis.
  • Systems for Radiation Detection and Monitoring.


  • Safe non-explosive gas mixtures.
  • Very high radiation rate capability.
  • High sturdiness and reliability.
  • Flexible detector shape and readout patterns.
  • High time and position resolutions.
  • High performance at low cost.
  • High achievable gains in a multiple pre-amplification structure.
  • Can be used as detector on its own.
  • Works in harsh radiation environment.


  • Complexity added by gas chamber compared to solid-state detectors.
  • Low quantum efficiency compared to solid-state detectors.



  • 200 cm x 50 cm currently maximum active area.
  • 1 MHz/mm2 of photon flux rate.
  • 40 micrometers of spatial resolution.
  • 90% achievable gain – 105.
  • 15-20% energy resolution.
  • Holes pitch: 140μm
  • Holes diameter in copper: 70μm +/-5μm
  • Holes diameter in Kapton: 50μm +/-5μm
  • Leakage currents: less than 10 nA at 600V @ 30%HR
  • Breakdown voltage: 650V in air @ 30%HR

Documented material

Technical contacts

  • L. Ropelewski - Technical questions on GEM physics
  • R. De Oliveira - Technical questions on GEM manufacturing and pricing
Patented technology; WO/1999/021211
CERN grants royalty-free licenses for Research and Development use of GEM foils. Any commercial use of GEM foils is subject to acquiring a commercial license from CERN, on conditions to be agreed. In case of direct GEM purchase, a license fee is included