Ageing studies for the Atlas muon spectrometer drift tubes

The Atlas muon spectrometer relies on Monitored Drift Tubes (MDTs) for track reconstruction in most regions. Expected count rates due to background photons reach up to 1500 Hz/cm for the innermost MDTs. For a gas gain of 2⋅104 this rate corresponds to an overall accumulated charge of up to 0.6 C/cm in 10 years of LHC operation. The focus of the present ageing studies is on testing complete modules from the ongoing chamber production chain in realistic Atlas conditions. This paper summarizes the results of such a study performed at the CERN Gamma Irradiation Facility GIF. One of the main goals was to evaluate for the first time the principle of gas recirculation with a prototype of the Atlas circulation system.