Performance Studies of the Mu2e Cosmic Ray Veto Detector

The cosmic ray veto (CRV) detector of the Mu2e experiment consists of four layers of plastic scintillation counters that surround the detector solenoid. These counters are embedded with wavelength-shifting fibers and are read out by silicon photomultipliers (SiPMs). The performance of a subset of the CRV counters was studied in a cosmic-ray test stand. Using data taken over a two-year period, we report the single-layer muon detection efficiency and the rate at which the light yield degrades due to the aging of the plastic scintillation counters.