Aging studies for the muon detector of HERA-B

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 aging studies performed by irradiating aluminum proportional wire chambers filled with Ar/CF4/CH4 (74:20:6), Ar/CF4/CH4 (67:30:3), Ar/CF4/CO2 (65:30:5) Ar/CF4 (70:30), CF4/CH4 (90:10) and CF4/CH4 (80:20) mixtures in three different experimental setups: laboratory conditions (55Fe and 106Ru sources), a 100 MeV α-beam and the high-rate HERA-B environment (secondaries from interactions of 920 GeV protons with target nucleus). Our experience shows that the aging rate depends not only on the total collected charge, but, in addition, on the mode of operation (high voltage and/or gas gain) and area of irradiation. s of CF4 addition on aging performance of gaseous detectors are summarized. Possible application of these results for operation of large scale gaseous detectors in the high rate environment is discussed.