The LUX-ZEPLIN Skin Detector

The LUX-ZEPLIN (LZ) experiment is a dark matter detector centered on a dual-phase xenon time projection chamber operating at the Sanford Underground Research Facility in Lead, South Dakota, USA. Immediately surrounding LZ's 7-ton TPC is a 2-ton liquid xenon 'skin'. This acts as an HV standoff between the TPC and cryostat, and is instrumented with 131 PMTs to act as an anticoincidence detector for additional background rejection. The skin is a powerful tool to understand and characterize LZ's backgrounds. In particular, the atomic de-excitations following Xe127 electron capture (EC) decays in the TPC can be tagged when the associated gamma(s) deposit energy in the skin. This allows us to confirm LZ's sensitivity down to 1keV, and inform our modeling of Xe124 ECEC decays, another background of xenon detectors such as LZ and one of the rarest decays ever measured.