Aging Studies of 2nd Generation BaBar RPCs

The BaBar detector, operating at the PEPII B factory of the Stanford Linear Accelerator Center (SLAQ, installed over 200 2nd generation Resistive Plate Chambers (RPCs) in 2002. The streamer rates produced by backgrounds and signals from normal BaBar running vary considerably (0.1 rarr20 Hz/cm²) depending on the layer and position of the chambers, thus providing a broad spectrum test of RPC performance and aging. The lowest rate chambers have performed very well with stable efficiencies averaging 95%. Other chambers had rate-dependant inefficiencies due to Bakelite drying which were reversed by the introduction of humidified gases. RPC inefficiencies in the highest rate regions of the higher rate chambers have been observed and also found to be rate dependant. The inefficient regions grow with time and have not yet been reduced by operation with humidified input gas. Three of these chambers were converted to avalanche mode operation and display significantly improved efficiencies. The rate of production of HF in the RPC exhaust gases was measured in avalanche and streamer mode RPCs and found to be comparable despite the lower current of the avalanche mode RPCs.