Systematic investigation of critical charge limits in Thick GEMs

Impact of the gas choice and the geometry on the breakdown limits in Micromegas detectors

“…Hence, for electron transparencies of <100% the actual (absolute) gain value is larger than the effective one. A clear gas dependency can be observed in the order of discharge curves, pointing to lighter gases as more stable ones, as observed in the studies with GEMs [4] and THGEMs [5]. For both, Neand Ar-based mixtures an anti-correlation can be observed between the stability of the Micromegas and the quencher content of the gas.…”

“…Thus, all alpha tracks are confined between the source and the mesh and even the highest primary charge densities obtained around the Bragg peak will be reduced during the electron drift toward the amplification structure. As discussed in [4,5], the discharge probability for these distances drops by several orders of magnitude, close to the background level. The results with the Micromegas detectors are presented in figure 3.…”

“…Therefore, one can ask whether mesh cells can be considered independent amplification units, similar to GEM holes. Following the GEM and THGEM comparison [5], the discharge probability shall scale with the Micromegas cell size, i.e. higher discharge rate is expected for large-𝑎 wire meshes (small LPI value) which presumably collect more primary charges in a single cell than denser meshes, where the primary charge cloud is shared among many cells.…”

“…The minimum avalanche size necessary to trigger a spark varies between 10 6 𝑒, and 10 7 𝑒 and concerns all MPGD-type detectors (see e.g. [3][4][5][6]).…”

“…A proper understanding of the fundamental limits of mesh structures may provide important input for designing Micromegas-based detectors. For completeness of our discharge studies, conducted initially with (TH)GEMs and summarised in [4,5], we have launched a dedicated discharge investigation campaign with several types of Micromegas detectors. The main motivation of the studies is to measure stability dependency on the mesh geometry, varying the diameter of its wires (𝑑 wire ) and the distance between them (𝑎 wire ), thus varying its optical transparency.…”