A large ultra-clean gas system with closed loop for the high-rate Outer Tracker at HERA-B

The gas system for the Outer Tracker of the HERA-B experiment at DESY produces the desired counting gas mixture Ar/CF4/CO2 65:30:5 and circulates it through the detector at a flow rate of 20 m3/h, i.e. ∼1 vol/h. It controls flows and regulates pressures in all 26 OTR half-superlayers, purifies the gas upon return from the detector, and automatically performs a quantitative analysis of main and trace (O2, N2, H2O) gas components for the common input and the outputs of all half-superlayers. The first running experience and the strategies employed during system construction to avoid any detector aging possibly induced by the gas system are discussed. The large system with major gas purification stations was constructed using only non-outgassing, “clean” materials and devices, such as stainless steel, PEEK, baked Viton, and metal bellows pumps. An epoxy glue was used extensively as a non-outgassing sealing material in applications with up to 100 bar pressure.