Gas performance of the ATLAS MDT precision chambers

The ATLAS Muon spectrometer contains more than 350,000, Monitored Drift Tubes (MDTs). The tubes are three cm diameter, one to six meter length, and filled with a three bar mixture of Ar/CO 2 . As the MDT operation is very sensitive to changes in gas composition and environmental conditions, monitoring changes in these parameters is crucial to attain the required MDT spatial resolution of 80 microns. The Michigan-Tel Aviv gas monitoring system is a mini MDT chamber located in the ATLAS gas facility at CERN. The system monitors simultaneously the input feed and exhaust gas to the complete MDT system. By monitoring these two lines it is sensitive to changes of about 1 part per mil to the internal MDT gas composition, and consequently to the drift time to drift radius calibration. As such the system has an important role in the MDT calibration chain. The mini-chamber acquires cosmic ray muon data and monitors continuously the drift times, gas flow rates, temperature and pressure, as well as deriving the radius-time (RT) relations of the drift-tubes. Results of the monitoring are published hourly. This system has been in operation since August 2007. We present a one-year analysis of the results and their correspondence to ambient conditions in the ATLAS cavern as well as interventions in the ATLAS gas facility, demonstrating the sensitivity of the system.