The LUX-ZEPLIN Outer Detector PMT System and its performance during Science Run I

The LUX-ZEPLIN (LZ) experiment is centered on a liquid xenon time projection chamber (LXe-TPC) searching for nuclear recoils induced by interactions with Weakly Interacting Massive Particles (WIMPs). One of the most important backgrounds is neutrons, as they also result in nuclear recoils in the TPC. 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. The Outer Detector consists of approximately 17t of gadolinium-loaded liquid scintillator confined to acrylic tanks surrounding the cryostat and 238t of high purity water as the outermost layer. The volume is monitored by 120 Hamamatsu R5912 photomultiplier tubes (PMTs). The PMTs are calibrated and monitored using an Optical Calibration System (OCS) which is situated within the array. I will present an overview of the two systems and a review of their performance during LZ's first science run. I will also describe the operating parameters chosen during commissioning and the impact of environmental interference on the PMT system.