Drift properties of Monitored Drift Tubes chambers of the ATLAS Muon Spectrometer

The ATLAS Muon Spectrometer relies on Monitored Drift Tube (MDT) chambers for precision tracking. In standard conditions, MDTs are operated with an Ar 93% - CO₂ 7% mixture at 3 bar absolute pressure. Any variation of the gas mixture has an impact on the detector performance. During a beam test at CERN in summer 2002, the drift properties of several production MDT chambers were studied as a function of temperature, composition of the gas mixture, water vapour content, applied voltage and discriminator threshold. A second beam test still at CERN in July 2002 was devoted to studying the performance of MDT chambers in the presence of a high photon background.