  The LUX-ZEPLIN experiment will search for dark matter particle interactions
with a detector containing a total of 10 tonnes of liquid xenon. Surrounding
the liquid xenon cryostat is an outer detector veto system with the primary aim
of vetoing neutron single-scatter events in the liquid xenon that could mimic a
weakly interacting massive particle (WIMP) dark matter signal. The outer
detector consists of approximately 17 tonnes of gadolinium-loaded liquid
scintillator confined to 10 acrylic tanks surrounding the cryostat and 228,000
litres of water as the outermost layer. It will be monitored by 120
inward-facing 8-inch photomultiplier tubes. An optical calibration system has
been designed and built to calibrate and monitor these photomultiplier tubes
allowing the veto system to reach its required efficiency and thus ensuring
that LUX-ZEPLIN meets its target sensitivity.
