Speaker
Description
The NSW Micromegas chambers in the ATLAS forward muon
spectrometer are subject to background rates of 15-20 kHz/cm$^2$
under HL-LHC conditions.
Typical anode currents will be around 6 $\mu$A on an area of 1440 cm$^2$
in the innermost part closest to the beamline.
Due to the late change of the detectorgas from non-ageing Ar:CO2 93:7
to the more HV stable ternary mixture Ar:CO2:isobutane 93:5:2
and the known vulnerability of wire chambers to
hydrocarbon containing gas mixtures
a three year long ageing study has been performed.
At CERN an SM1 and LM2 chamber underwent
intense gamma irradiation of 662 keV gammas in GIF++
and an SM2 series module of the NSW Micromegas quadrupets
was irradiated at LMU in Munich using a 10 GBq strong AmBe neutron source
emitting 6*10e5 MeV n/s as well as 3.5E6 4.4 MeV gammas/s
and 3.6E9 60keV gammas/s. Using this cocktail the
SM2 chamber was irradiated on a several cm$^2$ large region 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 come to the conclusion that no sign of loss in performance
has been observed in contradiction to an earlier experience using
drift tube wire chambers.
