Atlas Monitored Drift Tube long-term stability and aging study performed for the first time with the gas-recirculation system planned for LHC

The Monitored Drift Tubes (MDTs) of the Atlas muon spectrometer have to sustain count rates up to 1500 Hz/cm and must be able to survive an accumulated charge of up to 0.6 C/cm during 10 years of operation. This paper presents results of aging studies carried out at the CERN Gamma Irradiation Facility GIF for the first time with a final Atlas muon chamber and a prototype of the gas recirculation system planned at LHC. A loss in pulse height was observed for MDTs under gas recirculation after an accumulated charge equivalent to only I year of LHC operation; the effect was traced to a silicone contamination from bad components in the gas recirculation system. Tubes operated with the classical one-pass, flushing mode did not show any evidence of aging, thus (re)validating the materials and components used in the MDT construction. Questions of long-term stability when recycling the major part of the operating gas could also be addressed by the measurements performed; results on this will also be discussed.