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33. Detectors at accelerators
Figure 33.9: Normalized gas gain as a function of particle rate for MWPC [72]
and GEM [86].
last GEM.
The micro-mesh gaseous structure (Micromegas) is a thin parallel-plate avalanche
counter, as shown in Fig. 33.11 [90]. It consists of a drift region and a narrow
multiplication gap (25–150 µm) between a thin metal grid (micromesh) and the readout
electrode (strips or pads of conductor printed on an insulator board). Electrons from the
primary ionization drift through the holes of the mesh into the narrow multiplication gap,
where they are amplified. The electric field is homogeneous both in the drift (electric
field ∼ 1 kV/cm) and amplification (50–70 kV/cm) gaps. In the narrow multiplication
region, gain variations due to small variations of the amplification gap are approximately
compensated by an inverse variation of the amplification coefficient, resulting in a more
uniform gain. The small amplification gap produces a narrow avalanche, giving rise to
excellent spatial resolution: 12 µm accuracy, limited by the micro-mesh pitch, has been
achieved for MIPs, as well as very good time resolution and energy resolution (∼ 12%
FWHM with 6 keV x rays) [91].
The performance and robustness of GEM and Micromegas have encouraged their use
in high-energy and nuclear physics, UV and visible photon detection, astroparticle and
neutrino physics, neutron detection and medical physics. Most structures were originally
optimized for high-rate particle tracking in nuclear and high-energy physics experiments.
COMPASS, a high-luminosity experiment at CERN, pioneered the use of large-area
(∼ 40 × 40 cm2 ) GEM and Micromegas detectors close to the beam line with particle
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