Ageing effects observed in the CERN Omega RICH

Abstract We report on the new drift chambers of the Omega RICH used in the hyperon beam experiment WA89 at the CERN-SPS and in the heavy-ion experiment WA94. After four data-taking runs in 1990/91/92 it was found that the anode wires of the chambers were coated with a smooth layer of 1 μm thickness. Pulse heights were found to be reduced considerably with respect to those observed on new wires. Washing with alcohol and water in an ultrasonic bath restored the original pulse heights. In 1993 the RICH is running again in experiment WA89.