A gaseous muon detector at the HERA-B experiment

The muon detector of the HERA-B experiment at DESY (Hamburg, Germany) is a gaseous detector that provides moon identification in a high-rate environment. Pairs of moons with an invariant mass in the region of the J/psi meson ensure the first-level trigger for the experiment. Three different types of gas proportional chambers are employed: tube, pad, and pixel. The muon detector is fully installed and has been continuously running since November 1999. A clear signal for J/ psi --> mu mu is reconstructed from data taken during the commissioning run. Design of the moon system as well as experience gained with detector commissioning are presented in this paper. The severe radiation environment of the HERA-B experiment leads to a maximum charge deposit on a wire, within the muon detector, of 200 mC/cm per year. We report recent results of aging studies performed by irradiating proportional wire chambers filled with Ar/CF4/CH4 and Ar/CF4/CO2 mixtures under various conditions. These results show that aging rates obtained by irradiating a small region of the wire with a radioactive source cannot be extrapolated to the irradiation of large areas in an environment of hadronic interactions. Our experience shows that the aging rate depends not only on the total collected charge but also on the mode of operation and area of irradiation.