Further advances in aging studies for RPCs

Aging phenomena in RPCs have been studied since 1996 in the framework of the ATLAS experiment with small-size detectors, irradiated with a **6**0Co source. The results evidenced a decrease of the detector rate capability at fixed electric field, due to an increase of the total resistance of electrodes. This was confirmed by an extensive aging test performed on the ATLAS RPC "module-0" at GIF-X5, the CERN irradiation facility. A primary cause for this effect was previously shown to be the degradation of the anodic graphite coating, which distributes the electric field on the bakelite electrode. We present here a systematic study of the graphite aging which fully confirms this interpretation. Moreover, we show that detectors with improved graphite coating allow to gain a factor of at least two in lifetime. In the framework of these tests, we also show that the aging behavior of a detector working at high current induced by heavy irradiation can be reproduced by operating the detector, filled with pure Argon, in "self-sustaining" streamer mode.