GAS ELECTRON MULTIPLIER (GEM)

The Gas Electron Multiplier (GEM) consists of a thin, metal-clad polymer foil, chemically pierced by a high density of narrow holes (typically 50 to 100 per mm²). On application of appropriate differences of potential between the two metal electrodes, electrons released in the gas by radiation on one side of the structure drift into the holes, multiply and transfer to a collection region; each hole acts as an individual proportional amplifier. The multiplier can be used as detector on its own, or as a preamplifier in multiple cascaded structures; in this case, it permits to reach large overall gains in harsh radiation environments. The separation between amplification stages and read-out contribute to the safety of operation and permit freedom of design of the charge collecting electrode. Detailed studies of gain and discharge voltage at high rate and in presence of heavily ionizing tracks have demonstrated the superiority of multiple GEM structures to other micro-pattern devices. A set of twenty large size (30x30 cm²) Triple-GEM detectors is used for the detection and localization of charged particle trackers for COMPASS, a high rate experiment at CERN LHC; a similar setup with half-moon shaped detectors is used for for TOTEM (forward trackers in CMS). A technologyhas been developed to fabricate very large size GEM foils (a square meter or more); a set of large, Triple GEM detectors is in construction for the upgrade of the CMS forward muon tracker. Other developments in particle physics include an improved readout of Time Projection Chambers for the Linear Collider project at DESY and for the ALICE TPC upgrade at CERN's LHC, a fast muon trigger system for LHCb, neutron detection and other experiments. Numerous applications in medicine, astrophysics, dosimetry are under development.

The GEM technology and applications are protected by US and WCT patents. While the use of the technology is encouraged and free in the research and development domains, commercial exploitation has to be licensed by CERN.
For more information, contact CERN's Knowledge Transfer Department.

The main characteristics and performances of GEM detectors are:
- Operation in most gas fillings, including pure noble gases
- Proportional gains above 10⁵
- Energy resolution 18% FWHM at 5.9 keV
- Space localization accuracy 60µm rms or better
- Rate capability above 10⁶ counts/mm²sec
- Active areas up to 1000 cm²
- Flexible detector shape and readout patterns
- Robust, Low cost

For a description of performances,detectors manufacturing, GEM framing and assembly see https://gdd.web.cern.ch/gem

See also PUBLICATIONS; a recent review by the author is available.

A list of major GEM-based experimental setups is given HERE.

Field lines and equipotentials near the GEM holes on application of a voltage difference between the two metal sides.	Electron microscope image of a cut GEM electrode, etched on a metal-clad, 50 µm-thick polymer foil. Hole's diameter and pitch are 70 and 140 µm.
Schematics of a single-GEM detector. Electrons relased in the upper gas volume drift to and multiply in the holes. The amplified electron charge is collected on the anode, with 1- or 2-dimensional projective strips readout.	A Triple-GEM detector: ionization electrons drift to the first GEM foil, multiply and transfer to the cascaded multiplying electrodes.
Charge amplification factor for Single, Double and Triple-GEM devices.	Energy resolution of a GEM detector for 5.9 keV X-rays.
Localization accuracy in one direction for fast charged particles, with rms of ~70 µm.	Correlation between the detected charge on X and Y projections for fast tracks.
X-ray absorption radiography of a small mammal recorded with a two-dimensional GEM detector
The TOTEM Semi-Circular Triple-GEM module	The TOTEM T2 Tracker.

Fabio Sauli with a 30x30 cm GEM electrode	The COMPASS Triple-GEM Spectrometer

KLOE-2 Cylindrical GEM Inner Tracker

Production of the large GEM foils for the CMS Forward Muon Triple'GEM upgrade

ALICE GEM-TPC Module	COMPASS RICH-2 GEM'MICROMEGAS

GEM X-Ray Polarimeter	5.9 KeV photoelectron track

Optical GEM Schematics	Hadronic shower recorded with the Optical GEM

CYGNO Optical GEM for directional Dark Matter Search	Image of an Am-Be Source