A gas monitoring chamber for ATLAS MDTs

As one of the four experiments at the Large Hadron Collider (LHC), the ATLAS experiment features the largest muon tracker in terms of volume. The main detecting elements of the muon tracker are Monitored Drift Tubes (MDTs) which measure the drift times of the electrons created by a traversing muon ionizing the operating gas. The trajectory of the muon is reconstructed from the drift times according to the so-called space-to-drift time relation (rt-relation). The design goal of the performance of the muon tracker is a stand-alone transverse momentum resolution of approximately 10 % for tracks of 1 TeV particles. This requires the MDT detector to have a spacial resolution better than 50 micrometer within a volume of a length of 40 m and a diameter of 25 m. The ATLAS MDTs use Ar:CO2 93:7 plus a few hundred ppm water vapour as the operating gas. The types of the gas components and their ratios should be kept very steady in order to have a stable rt-relation. In this thesis, the influences of the fluctuations of the mixture ratio on the electron velocity are discussed. Gas Monitoring Chambers (GMCs) have been designed and implemented to monitor the gas of the ATLAS MDTs by measuring the electron drift velocity in the operating gas as a function of the reduced electric field (v-r.e.f relation). The performance of the GMC has been tested. It has a better resolution than that of the commercial gas mixture we can get. With the GMC even a small change of the mixture ratio can be detected in a short time. Additionally, an empirical method based on artificial neural network with multilayer perceptron has been developed to analyse the mixture ratio from a measured v-r.e.f relation. With this method a fall in water vapour content in the ATLAS MDT gas in the end of 2010 can be seen.