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 environme nt. 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 det ector is fully installed and has been continuously running since November 1 999. A clear signal for J/ psi --> mu mu is reconstructed from data taken d uring the commissioning run. Design of the moon system as well as experienc e gained with detector commissioning are presented in this paper. The sever e 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 repo rt recent results of aging studies performed by irradiating proportional wi re chambers filled with Ar/CF4/CH4 and Ar/CF4/CO2 mixtures under various co nditions. These results show that aging rates obtained by irradiating a sma ll region of the wire with a radioactive source cannot be extrapolated to t he irradiation of large areas in an environment of hadronic interactions. O ur experience shows that the aging rate depends not only on the total colle cted charge but also on the mode of operation and area of irradiation.