Performance of second generation BABAR resistive plate chambers

The BABAR detector has operated nearly 200 Resistive Plate Chambers (RPCs), constructed as part of an upgrade of the forward endcap muon detector, for the past two years. The RPCs experience widely different background and luminosity-driven singles rates (0.01–10 Hz/cm 2 ) depending on position within the endcap. Some regions have integrated over 0.3 C/cm 2 . RPC efficiency measured with cosmic rays is high and stable. The average efficiency measured with beam is also high. However, a few of the highest rate RPCs have suffered efficiency losses of 5–15%. Although constructed with improved techniques and minimal use of linseed oil, many of the RPCs, which are operated in streamer mode, have shown increased dark currents and noise rates that are correlated with the direction of the gas flow and the integrated current. Studies of the above aging effects are presented and correlated with detector operating conditions.