  We present a conceptual design for an active neutron veto, based on
boron-loaded liquid scintillator, for use in direct-detection dark matter
experiments. The simulated efficiency of a 1 meter thick veto, after including
the effects of neutron captures in the inner detector and inefficiencies due to
feed-throughs into the veto, is greater than 99.5% for background events
produced by radiogenic neutrons, while the background due to externally
produced cosmogenic neutrons is reduced by more than 95%. The ability of the
veto to both significantly suppress, and provide in situ measurements of, these
two dominant sources of background would make the next generation of dark
matter experiments much more robust, and dramatically improve the credibility
of a dark matter detection claim based on the observation of a few recoil
events. The veto would also allow direct extrapolation between the
background-free operation of a small detector and the physics reach of a larger
detector of similar construction.
