Ageing effects in drift chambers

We have observed a degradation of efficiencies of large drift chambers. The effect is caused by deposits on the anode wires and, in our case, became significant after approx.= 1017 charges per cm of anode had been collected. The deposits are expected to be some sort of hydrocarbons. However, silicon could be identified as one of the components. These effects, which have also been seen in proportional counters, should be taken into account when designing detectors in which a large number of charges per unit length of the anode wire is expected. (orig.)