Aging and performance of BaBar RPCs

For the past five years the BaBar detector has operated over 200 resistive plate chambers(RPCs) in the forward endcap. Instalfed as part of an 2002 upgrade of the forward muon detection system these 2nd generation RPCs share many construction details with the LHC RPCs. The streamer rates produced by backgrounds and signals from normal BaBar running vary considerably (0.1->20 Hz/cm²) depending on the layer and position of the chambers within the endcap, providing a broad spectrum test of RPC performance and aging. The lowest rate chambers have performed well with efficiencies averaging 95%, but show significant increases in noise rate and HV current. Rate-dependant inefficiencies due to Bakelite drying were reversed by the introduction of humidified gases for the last two years of operation. RPC inefficiencies in the highest rate regions did not improve with humidified gas. 24 of these RPCs were converted to avalanche mode operation and display significantly improved efficiencies.