The BaBar instrumented flux return performance: lessons learned.

The BaBar Collaboration has operated an instrumented flux return (IFR) system covering over 2000 m2 with resistive plate chambers (RPCs) for nearly 3 years. The chambers are constructed of bakelite sheets separated by 2 mm . The inner surfaces are coated with linseed oil. This system provides muon and neutral hadron detection for BaBar. Installation and commissioning were completed in 1998, and operation began mid-year 1999. While initial performance of the system reached design, over time, a significant fraction of the RPCs demonstrated significant degradation, marked by increased currents and reduced efficiency. A coordinated effort of investigations have identified many of the elements responsible for the degradation. This article presents our current understanding of the aging process of the BaBar RPCs along with the action plan to combat performance degradation of the IFR system.