Study of ATLAS monitored drift tube flushing

The ATLAS monitored drift tube (MDT) muon tracking detector system consists of about 1,200 large-scale MDT chambers built from some 400,000 individual aluminum tubes of 3 cm diameter, and lengths ranging from approximately 1 to 6 meters. This MDT system has a total gas volume of 800 m³. Both during the many years of chamber preparation, testing, commissioning and after their later installation in ATLAS, the chamber gas has to be refreshed or replaced periodically. In cases where the chamber gas shall be replaced, it would be of great advantage (cost, time) to know how to remove the old gas in the most efficient manner. Frequently chambers are pressurized and then depressurized three or more times. We have examined an alternate method of just flushing at atmospheric pressure and determined how many ´volume exchanges´ of gas are needed until the ´new gas´ will have replaced the ´old gas´ during the flushing process to a very high degree of purity. It is shown that after 1.8 volume exchanges (perhaps even less) at atmospheric pressure the gas is replaced at the 99.9%-level. Results of these studies and a method of visualization of the flushing process, shown in real-time, how the old gas is replaced by the new gas in an MDT-like setup, are presented.