Long-term irradiation of an ATLAS NSW SM2 Micromegas quadruplet using an AmBe neutron source

The NSW Micromegas chambers in the ATLAS forward muon spectrometer are subject to background rates of 15-20 kHz/cm2 under HL-LHC conditions. The innermost detector area closest to the LHC beam pipe will accumulate a charge of 0.068C cm−2 year−1 under these rates. Due to the late change of the detector gas from non-aging Ar:CO2 93:7 vol% to the more HV stable ternary mixture Ar:CO2:iC4H10 93:5:2 vol% and the known vulnerability of wire chambers to Hydrocarbon-containing gas mixtures a three-year-long aging study has been performed. An SM2 series module of the NSW Micromegas quadruplets was irradiated at LMU in Garching/Munich using a 10 GBq AmBe neutron source emitting 6×105 MeV n/s as well as 3.5×105 4.4 MeV gammas/s and 3.6×109 60 keV gammas/s. The SM2 chamber was irradiated in a region of several 10 cm2 in size with a dose rate well exceeding the HL-LHC equivalent local charge densities for three years. In between the irradiation periods the performance of the SM2 chamber regarding spatial resolution and efficiency on cosmic muon tracking was tested several times. We report on the irradiation and the performance studies of the SM2 Micromegas quadruplet and conclude that no sign of loss in performance has been observed in contradiction to an earlier experience using drift tube wire chambers.