Drift time spectrum and gas monitoring in the ATLAS Muon Spectrometer precision chambers

The ATLAS Muon Spectrometer incorporates 354 000 drift tubes assembled into 1200 Monitored Drift Tube (MDT) precision chambers, with a total gas volume of 723m3. This MDT gas, Ar 93% and CO2CO2 7% at 3 bar, is cycled through the spectrometer at a rate of one total detector volume per day. Achieving the 80μm drift tube design resolution requires stringent gas quality control as a fundamental component of the MDT calibration program. We report on the design, deployment and performance of a dedicated MDT mini-chamber conceived for continuous monitoring and drift time calibration of the ATLAS MDT operating gas. This chamber enables measurement of the drift spectra from which gas properties relevant to MDT calibrations and stable operating conditions are determined. Located in the ATLAS gas facility at CERN, the mini-chamber produces hourly drift spectra which are automatically analyzed. Results are published online and disseminated to the ATLAS muon system conditions and calibration databases in real time.