A study of ageing in timing RPCs

The present study concerns the search for ageing effects in 0.3 mm gap timing RPCs. Counters were made with glass and aluminium electrodes and kept under a continuous gas flow of tetrafluorethane with 10% sulphur hexafluoride, 5% isobutane and water at 10% relative humidity. After 500 days of operation at the saturation current an accumulated charge of 800 mC was collected, equivalent to over 8 years of continuous operation at 300 Hz/cm2. No evidence was found of any long-term increase of dark current, suggesting that the ageing effect in timing RPCs is less severe than in streamer-mode glass RPCs. However, a visual inspection of the electrodes confirmed the existence of a deposit over the glass electrodes that was analysed by Electron Probe Microanalysis.