Effect of mesh geometry on resistive Micromegas for the ATLAS experiment

The ATLAS Experiment will use resistive Micromegas detectors for the upgrade of the forward Muon System (NSW, for New Small Wheel). With the test of the first production modules, instabilities in the detector operation have been observed, leading to a systematic revision of the selected construction components and working parameters. In particular, the effect of the mesh geometry with respect to the discharge behaviour was studied using a special Micromegas detector designed and built by the ATLAS CERN group in 2014. The detector has a Micromegas structure similar to the one later on adopted by the NSW project; moreover it provides the possibility to easily replace the mesh. The test procedure consisted in measuring current, gain and counting rate from an 55Fe X-ray source, as a function of the amplification voltage up to the discharge limit. The systematic analysis of the data allowed to conclude on the stability interval of six different types of mesh, including the one used for the NSW, consistent with the expectations. Results suggested that the mesh selected for the NSW (among the few available in the requested big size), was not optimal for operation with a safe plateau at high gain before discharges. After the present study, cost constraint prevented the replacement of the one already chosen and available.