High Rate and Ageing Studies for the Drift Tubes of the ATLAS Muon Spectrometer

The muon spectrometer of Atlas, one of the 4 experiments currently under construction at the Large Hadron Collider LHC, relies on Monitored Drift Tubes (MDTs) for track reconstruction in most of its regions. The MDTs will 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 thesis presents results of high rate and ageing studies conducted at the CERN Gamma Irradiation Facility GIF; a series production Atlas muon chamber and a prototype of the gas recirculation system planned at LHC were used for the first time. Test beam muons were utilized to evaluate the MDT high rate behaviour; questions of resolution, efficiency and changes in the drift properties of the operating gas were addressed. The measurements were complemented by simulations with the program GARFIELD. For the ageing study the MDTs were irradiated over a period of several months, in which their performance was monitored weekly with cosmic muons. A loss in pulse height was observed for MDTs under gas recirculation after an accumulated charge equivalent to only 1 year of LHC operation, while tubes operated without gas recirculation did not show any evidence of ageing. Extensive material analyses were performed in order to determine the origin of the performance degradation.