Humidity dependence of anode corrosion in HERA-B Outer Tracker Chambers operated with Ar/CF4/CO2

The Outer Tracker of the HERA-B experiment consists of drift tubes folded from gold-coated polycarbonate foils (honeycomb) and is operated with the gas mixture Ar/CF4/CO2. The choice of the materials and the drift gas was the result of an extended R&D program launched to avoid both anode aging (gain loss) and cathode aging (Malter effect). However, other aging effects were observed at high irradiation doses: rising conductivity of the wire-supporting strips and wire corrosion. In this paper, we show that these two aging effects could also be prevented by keeping water at a proper concentration (about 300 ppm). In an attempt to interpret the test results, the production and reaction probability of molecular species produced via CF4 dissociation in the avalanche are semi-quantitatively estimated.