The ATLAS experiment at CERN undergoes a series of upgrades, in line with the upgrades (Phase-I and Phase-II) of the Large Hadron Collider (LHC), to cope with the steadily increasing instantaneous luminosity that ultimately is expected to reach 7.5×1034cm−2 s−1 at the High Luminosity LHC (HL-LHC) era. The most challenging upgrade project of the ATLAS experiment concerns its Muon Spectrometer’s inner wheel-shaped detection stations (Small Wheels) located close to and on either side of the pp beams’ interaction point. Another type of detection system (New Small Wheels) replaced the two Small Wheels so that the ATLAS maintains its performance while preserving the acceptance of critical physics signatures under the harsh conditions of higher data and radiation rates (up to 25kHz/cm2 at the HL-LHC accelerator). The NSWs are a complex detection system (each wheel comprises 2.4x106 readout channels)that employ two novel micro-pattern gaseous detector technologies, the Micromesh Gaseous Structure (Micromegas, MM) and the small strip Thin Gap Chambers (sTGCs), mainly to improve the trigger performance and to provide precise spatial measurements. New electronic boards residing on the detectors were designed and manufactured, hosting high radiation and magnetic field tolerant custom-made ASICs.This paper presents extensive tests carried out at the new CERN Gamma Irradiation Facility (GIF++) by combining a muon beam (of 12kHz) with a 137Cesium radiation source of adjustable rate (up to ten times the expected rate at the HL-LHC conditions), to assess the performance of the Micromegas chambers and their readout electronics. Data from NSW Micromegas modules of different detection areas, having different positions on the wheels, operating with different high voltage and filling gas have been read out and analyzed by placing the electronics on PCBs located at various distances from the pp beam pipe. The spatial resolution of the precision coordinate is found to be well below the required 100μm, and the excellent alignment of the precision layers has been verified. Results confirm that all types of Micromegas modules tested have a linear response up to a rate around four times the expected at HL-LHC.

No abstract