Laboratory aging studies for the HERA-B muon chambers

The severe radiation environment of the HERA-B experiment leads to a maximum accumulated charge on a wire, within the muon detector, of 200 mC/cm wire. For operation in this high-intensity environment, the main criteria for the gas choice turned out to be stability against aging. We report recent results of laboratory aging studies performed by irradiating proportional wire chambers filled with Ar/CF4/CH4 (74:20:6), Ar/CF4/CH4 (67:30:3), and Ar/CF4/CO2 (65:30:5) mixtures. The penetration of water and oxygen through the walls of plastic tubes has also been investigated. Water can be introduced indirect to the gas mixture by using polyamide (nylon) pipes for gas supply lines.