MICROMEGAS

Micromegas, short for micro mesh gaseous structure, is a type of micropattern gaseous detector used to detect charged particles with high spatial resolution and rate capability. It was developed in 1996 by Giomataris and colleagues as a compact, robust alternative to traditional gas detectors.

The detector consists of a drift region, typically a few millimeters to several centimeters thick, above a thin amplification gap of about 50 to 100 micrometers. A micromesh electrode separates the two regions and is held at a fixed distance from a readout plane by a regular array of microscopic pillars. A uniform electric field in the drift region guides primary electrons toward the mesh, while a strong field in the amplification gap produces gas amplification as electrons pass through the mesh holes and avalanche near the readout plane. The resulting charge is collected on a segmented readout (strips, pads, or pixels), enabling precise two-dimensional reconstruction.

Two broad fabrication approaches exist: bulk Micromegas, where the mesh is bonded to the readout board with lithographically defined pillars; and microbulk Micromegas, in which the mesh and spacer are produced from copper-clad polyimide foils. Microbulk devices tend to offer very uniform gain, low radioactivity, and excellent energy resolution, though they require more specialized manufacturing.

Performance characteristics include high granularity, good position resolution (tens of micrometers to a few hundred micrometers), fast timing, and high rate capability. Gains of 10^4–10^5 are common in standard gas mixtures such as Ar/CO2 or Ar/isobutane. Spark protection can be enhanced with resistive layers or segmentation.

Applications are concentrated in high-energy physics experiments for tracking and triggering. Notable deployments include the ATLAS experiment’s New Small Wheel upgrade, where Micromegas modules provide precision muon tracking, along with use in other experiments such as COMPASS and various large-area tracking projects.