  The ZEPLIN-III experiment is operating in its second phase at the Boulby
Underground Laboratory in search of dark matter WIMPs. The major upgrades to
the instrument over its first science run include lower background
photomultiplier tubes and installation of a plastic scintillator veto system.
Performance results from the veto detector using calibration and science data
in its first six months of operation in coincidence with ZEPLIN-III are
presented. With fully automated operation and calibration, the veto system has
maintained high stability and achieves near unity live time relative to
ZEPLIN-III. Calibrations with a neutron source demonstrate a rejection of 60%
of neutron-induced nuclear recoils in ZEPLIN-III that might otherwise be
misidentified as WIMPs. This tagging efficiency reduces the expected untagged
nuclear recoil background from neutrons during science data taking to a very
low rate of ~0.2 events per year in the WIMP acceptance region. Additionally,
the veto detector provides rejection of 28% of gamma-ray induced background
events, allowing the sampling of the dominant source of background in
ZEPLIN-III - multiple scatter gamma-rays with rare topologies. Since WIMPs will
not be tagged by the veto detector, and tags due to gamma-rays and neutrons are
separable, this population of multiple scatter events may be characterised
without biasing the analysis of candidate WIMP signals in the data.
