A gas electron multiplier (GEM) is a type of

gaseous ionization detector Gaseous ionization detectors are radiation detection instruments used in particle physics to detect the presence of ionizing particles, and in radiation protection applications to measure ionizing radiation. They use the ionising effect of radia ...

used in nuclear and particle physics and radiation detection. All gaseous ionization detectors are able to collect the

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...

s released by ionizing radiation, guiding them to a region with a large electric field, and thereby initiating an

electron avalanche An electron avalanche is a process in which a number of free electrons in a transmission medium are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium, thereby ionizing them (impact ionizat ...

. 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 collects 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 gaseous detectors Micropatterning is the art of miniaturisation of patterns. Especially used for electronics, it has recently become a standard in biomaterials engineering and for fundamental research on cellular biology by mean of soft lithography. It generally us ...

; this class includes micromegas and other technologies.

History

GEMs were invented in 1997 in the Gas Detector Development GroupThe Gas Detectors Development group. http://gdd.web.cern.ch/GDD/ at CERN by physicist

Fabio Sauli Fabio is a given name descended from Latin '' Fabius'' and very popular in Italy and Latin America (due to Italian migration). Its English equivalent is Fabian. The name is written without an accent in Italian and Spanish, but is usually accented ...

."A GEM of a Detector". CERN Courier, 27 November 1998. http://cerncourier.com/cws/article/cern/27921

Operation

Typical GEMs are constructed of 50–70 micrometre thick

Kapton Structure of poly-oxydiphenylene-pyromellitimide Kapton insulating pads for mounting electronic parts on a heat sink Kapton is a polyimide film used in flexible printed circuits ( flexible electronics) and space blankets, which are used on spa ...

foil clad in copper on both sides. A

photolithography In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer (electroni ...

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. Operation of wire chambers typically involved only one voltage setting: the voltage on the wire provided both the drift field and the amplification field. A GEM-based detector requires several independent voltage settings: a drift voltage to guide electrons from the ionization point to the GEM, an amplification voltage, and an extraction/transfer voltage to guide electrons from the GEM exit to the readout plane. A detector with a large drift region can be operated as a time projection chamber; a detector with a smaller drift region operates as a simple

proportional counter The proportional counter is a type of gaseous ionization detector device used to measure particles of ionizing radiation. The key feature is its ability to measure the energy of incident radiation, by producing a detector output pulse that is ''pro ...

. A GEM chamber can be read-out by simple conductive strips laid across a flat plane; the readout plane, like the GEM itself, can be fabricated with ordinary lithography techniques on ordinary circuit board materials. Since the readout strips are not involved in the amplification process, they can be made in any shape; 2-D strips and grids, hexagonal pads, radial/azimuthal segments, and other readout geometries are possible.

Uses

GEMs have been used in many types of particle physics experiments. One notable early user was the COMPASS experiment at CERN. GEM-based gas detectors have been proposed for components of the

International Linear Collider The International Linear Collider (ILC) is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, with the possibility for a later upgrade to 1000 GeV (1 TeV). Although early proposed ...

, the STAR experiment and PHENIX experiment at the

Relativistic Heavy Ion Collider The Relativistic Heavy Ion Collider (RHIC ) is the first and one of only two operating heavy- ion colliders, and the only spin-polarized proton collider ever built. Located at Brookhaven National Laboratory (BNL) in Upton, New York, and used by ...

, and others. The advantages of GEMs, compared to multiwire proportional chambers, include: ease of manufacturing, since large-area GEMs can in principle be mass-produced, while wire chambers require labor-intensive and error-prone assembly; flexible geometry, both for the GEM and the readout pads; and suppression of positive ions, which was a source of field distortions in time-projection chambers operated at high rates. A number of manufacturing difficulties plagued early GEMs, including non-uniformity and short circuits, but these have to a large extent been resolved.

References

{{authority control Particle detectors Experimental particle physics CERN