A comprehensive study of the discharge formation mechanisms in Micro-Pattern Gaseous Detectors

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During the first funding period, we investigated the intrinsic stability limits of GEM, Thick GEM, and Micromegas detectors upon irradiation with alpha particles. The measurements were performed in Ar- and Ne- based mixtures with different CO2 content to study the influence of the gas on discharge probability and critical charge limits. The latter were evaluated by comparing the experimental data to results obtained within a Geant4 simulation framework. The measurements provided a direct comparison between GEMs and THGEMs and allowed us to evaluate the influence of geometrical parameters, such as hole size, pitch, and (TH)GEM thickness, on the stability of a structure and the resulting critical charge value, estimated to be within a range of (3-7)106 electrons. Surprisingly, the results for both amplification structures nicely agree with each other, in spite of the clear geometrical differences and different electric field configurations inside GEM and THGEM holes. We observe that the breakdown limit is strongly dependent on the gas, and that a higher amount of quencher in the mixture does not necessarily correlate with higher stability. The results obtained with Micromegas detectors confirmed the observed gas dependency on the discharge stability. In addition, we observed discharge probability scaling with the wire pitch which suggests that a Micromegas mesh cell can be treated as an independent amplification unit, similar to a hole in a GEM foil. The outcome of these studies provides valuable input for further optimization of MPGD detectors, multi-layer stacks in particular. In addition, we have investigated the stability of (TH)GEMs coated with different materials using discharge light spectroscopy methods. We have identified Molybdenum electrodes particularly resistant to so-called secondary discharges. The results did not so far provide a complete understanding of all discharge processes in MPGDs but they opened new perspectives for the development of GEM-based gaseous detectors capable of sustaining extreme high voltage settings without damage. In this sense, a continuation of the so far successful program is proposed hereafter. We plan to study structures with new electrodes and MPGD-hybrid stacks to be used in future TPCs, trackers, or photodetectors which require operation in high-rate radiation environment under extreme HV settings: allowing for high-gain and/or low ion backflow performance.