Experience with the BaBar Resistive Plate Chambers

The BaBar detector has operated over 2000 m2 of Resistive Plates Chambers (RPCs) as muon and neutral hadron detectors since 1999. Most of the original RPC production have lost significant efficiency and many are now completely inefficient. Both the linseed oil used to coat the inner surfaces and the graphite coating on the outer surfaces are implicated as contributors to the efficiency loss which was accelerated by the operation of the RPCs at 29 to 34 C during the first summer. RPCs from 2 more recent production runs have been installed and tested. The most recent RPCs have exhibited stable efficiencies and high voltage plateaus during the first 8 months of service. Some have shown increased dark currents and noise rates