The Mu2e experiment is designed to search for the charged-leptonflavor-
violating process, μ− to a e−, with unprecedented sensitivity. The
single 105-MeV electron that results from this process can be mimicked
by electrons produced by cosmic-ray muons traversing the detector. An
active veto detector surrounding the apparatus is used to detect incoming
cosmic-ray muons. To reduce the backgrounds to the required level
it must have an efficiency of about 99.99% as well as excellent hermeticity.
The detector consists of four layers of scintillator counters, each
with two embedded wavelength-shifting fibers, whose light is detected
by silicon photomultipliers. The design and expected performance of
the cosmic ray veto detector will be described.