DOI: 10.4018/978-1-4666-6014-4.ch004
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This chapter describes another type of micropattern detector invented in 1995 by G. Charpak and collaborators, a micromesh gaseous detector. This detector is in fact a parallel-plate avalanche counter with a very small gap (50-100 µm) between a special cathode mesh and the anode plate. This feature offers excellent position resolution, down to 30 µm in conventional gas mixtures and close to 14 µm in some special gas mixture in which diffusion of electrons is very low. Initially, small prototypes of MICROMEGAS were made by hand. Later, microelectronic technology was used in their manufacturing, allowing the building of individual modules with active areas up to 40x40 cm2. The results from detailed studies of maximum achievable gain, rate characteristics, time-, position-, and energy resolutions of this detector are presented in this chapter, as well as a comparison with classical parallel-plate avalanche counters. Nowadays, this detector conquers more and more applications in high-energy physics and other applications.
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2. Detector Design And Main Characteristics

The MICROMEGAS is actually a PPAC with a very small gap between the cathode mesh and the anode plate. Normally the distance is only 50-100 μm. Recall that in usual PPAC the gas gap is around 3-5 mm. Its design is shown schematically in Figure 1.

Figure 1.

Artistic view of a MICROMEGAS. Primary electrons created in the drift region (the region between the drift electrode and the mesh) drift towards the cathode mesh. They are transmitted through the mesh into the gap between the cathode mesh and the anode plane where they produce electron avalanches (courtesy of Giles Barouch, 2014).

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