The gas system of the drift tube detector of the neutrino experiment OPERA

The gas system of the drift tube detector of the neutrino experiment OPERA is presented. The drift tube detector, called precision tracker (PT), consists of 10 000 thinwalled aluminum tubes of 38 mm diameter and 8 m length filled with an 80% Ar 20% CO 2 gas mixture. The total gas volume amounts to 80 m 3 . During stable operation the gas is exchanged once per 80 h and not recycled. Thus about 1 m3 per hour is consumed. Argon and CO 2 are stored and supplied in liquid form. Subsequently they are mixed and the composition ratio is regulated with a precision better than 1% per component. Inside the drift tubes the gas is kept at a constant absolute pressure within ± 0.5 % . Since the experiment is operated in the low activity environment of the Gran Sasso underground laboratory, monitoring of the gas quality with a radioactive source is prohibited. Instead the oxygen content of the used gas is measured. We report on the first experience with the performance of the gas system.