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Quality Control of ATLAS Muon Chambers

ATLAS is a general-purpose experiment for the future Large Hadron Collider (LHC) at CERN. Its Muon Spectrometer will require ∼ 5500m of precision tracking chambers to measure the muon tracks along a spectrometer arm of 5m to 15m length, embedded in a magnetic field of ∼ 0.5T. The precision tracking devices in the Muon System will be high pressure drift tubes (MDTs). Approximately 370,000 MDTs will be assembled into ∼ 1200 drift chambers. The performance of the MDT chambers is very much dependent on the mechanical quality of the chambers. The uniformity and stability of the performance can only be assured providing very high quality control during production. Gas tightness, high-voltage behaviour and dark currents are global parameters which are common to gas detectors. For all chambers, they will be tested immediately after the chamber assembly at every production site. Functional tests, for example radioactive source scans and cosmic-ray runs, will be performed in order to establish detailed performance maps. One major issue of the quality control, the gas tightness, is the topic of this thesis. The gas pressure inside the tubes will be 3 bar, the drift gas will be Ar/CO2 93/7. The high spatial resolution made it necessary to define the maximum allowed leak rate for MDT chambers to 2 ∗ 10−8 bar l s . This is equivalent to a pressure drop of about 1 mbar for MDT chambers within one day at the given working point. Several methods had been considered in the past. This thesis deals with an innovative method. The gas gain and hence the signals are a strong function of the gas density. The test on the gas tightness using the tubes itself as sensitive devices makes it possible to determine the leak rate with a sensitivity better than 10−4 within one day. The principle of this method will be discussed and demonstrated. Also the quality control of single tubes can be realized with it. The method allows easy-to-use series controls with integrated test of other chamber properties.