Ageing test of ATLAS RPCs at CERN's gamma irradiation facility

An ageing test of three ATLAS production RPC stations is in progress at X5 GIF, the CERN irradiation facility. The chamber efficiency as a function of the operating voltage is measured at different source intensities, up to a maximum counting rate of about 700 Hz/cm/sup 2/. All along the test, plate resistivity has been monitored using two different methods: by measuring the I-V characteristics in pure Ar, and by comparing the efficiency vs high voltage curves at different source intensities. The increase in plate resistivity, which is one of the dominant ageing effects for RPCs with phenolic-melaminic electrodes, has been shown to be strongly dependent on environmental relative humidity. Indeed, after setting to values between 40% and 50% the relative humidity of both the operating gas and the air surrounding the chambers, a significant decrease of the plate resistivity has been observed.