Final results from an extensive ageing test of bakelite Resistive Plate Chambers

We present the final results of an extensive aging test, performed at the CERN Gamma Irradiation Facility on two bakelite Resistive Plate Chambers (RPC) detectors. With a method based on a model describing the behaviour of an RPC under high particle flux conditions, we have periodically measured the electrode resistivity rho of the two RPC prototypes over 3 years. We observed a large spontaneous increase of rho with time, from the initial value of about 10**1**0 Omega cm to more than 200 multiplied by 10**1**0 Omega cm. A corresponding degradation of the RPC rate capabilities, from more than 3 kHz/cm**2 to less than 200 Hz/cm**2, was observed; the reversibility of the process, using a humid gas mixture, has also been studied. We also present a study of the effects of humidity on the bakelite resistivity using two 10 multiplied by 10 cm**2 bakelite samples.