Gas Electron Multiplier
Encyclopedia
The Gas Electron Multiplier (GEM) is a type of gaseous ionization detector used in nuclear and particle physics and radiation detection.
All gaseous ionization detectors are able to collect the electrons released by ionizing radiation
, guiding them to a region with a large electric field
, and thereby initiating an electron avalanche
. 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 collect the electrons from the avalanche and guides them towards the readout electronics. GEMs create the large electric field in a 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 gas detectors; this class includes Micromegas, microstrip detector
s, and other technologies.
by physicist Fabio Sauli.
foil clad in copper on both sides. A photolithography and acid etching process makes 30-50 micrometre 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
.
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.
. GEM-based gas detectors have been proposed for components of the International Linear Collider
, the STAR experiment and PHENIX experiment at the Relativistic Heavy Ion Collider
, 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.
All gaseous ionization detectors are able to collect the electrons released by ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
, guiding them to a region with a large electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
, and thereby initiating an electron avalanche
Electron avalanche
An electron avalanche is a process in which a number of free electrons in a medium are subjected to strong acceleration by an electric field, ionizing the medium's atoms by collision , thereby forming "new" electrons to undergo the same process in successive cycles...
. The avalanche is able to produce enough electrons to create a current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
or charge
Charge (physics)
In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
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 collect the electrons from the avalanche and guides them towards the readout electronics. GEMs create the large electric field in a 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 gas detectors; this class includes Micromegas, microstrip detector
Microstrip detector
A microstrip detector is a particle detector designed to consist of a large number of identical components laid out along one axis of a two-dimensional structure, generally by lithography...
s, and other technologies.
History
GEMs were invented in 1997 in the Gas Detector Development Group at CERNCERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
by physicist Fabio Sauli.
Operation
Typical GEMs are constructed of 50-70 micrometre thick KaptonKapton
Kapton is a polyimide film developed by DuPont which can remain stable in a wide range of temperatures, from -273 to +400 °C...
foil clad in copper on both sides. A photolithography and acid etching process makes 30-50 micrometre 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
Time projection chamber
In physics, a time projection chamber is a particle detector invented by David R. Nygren, an American physicist, at Lawrence Berkeley Laboratory in the late 1970s...
; a detector with a smaller drift region operates as a simple proportional counter
Proportional counter
A proportional counter is a measurement device to count particles of ionizing radiation and measure their energy.A proportional counter is a type of gaseous ionization detector. Its operation is similar to that of a Geiger-Müller counter, but uses a lower operating voltage. An inert gas is used to...
.
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 experimentCOMPASS experiment
The NA58 experiment, or COMPASS is a fixed-target particle physics experiment at the Super Proton Synchrotron, a particle accelerator at the European Organization for Nuclear Research...
. GEM-based gas detectors have been proposed for components of the International Linear Collider
International Linear Collider
The International Linear Collider is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, and, if approved after the project has published its Technical Design Report, planned for 2012, could be completed in the late 2010s. A later upgrade to 1000...
, the STAR experiment and PHENIX experiment at the Relativistic Heavy Ion Collider
Relativistic Heavy Ion Collider
The Relativistic Heavy Ion Collider is one of two existing heavy-ion colliders, and the only spin-polarized proton collider in the world. It is located at Brookhaven National Laboratory in Upton, New York and operated by an international team of researchers...
, 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.