Ageing studies on 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 photons reach up to 1500 Hz/cm for the innermost MDTs. For a gas gain of 2 · 10⁴ this corresponds to an overall accumulated charge of up to 0.6 C/cm which the MDTs must be able to withstand in 10 years of LHC operation without a degradation in their performance. With the main MDT parameters and the design frozen and chamber production well under way the focus of present ageing studies is on testing final Atlas chambers instead of using small, dedicated setups under very well controlled conditions as in the past. This paper summarizes the results of such a study performed at the CERN Gamma Irradiation Facility, GIF, that aims to emulate LHC background conditions. One of the main goals was to evaluate the principle of gas recirculation and a prototype of the Atlas circulation system.