Scott Haselschwardt (UCSB) " Expected Performance of the LZ Outer Detector and a Radioassay of its Gadolinium-Loaded Liquid Scintillator"

ABSTRACT:
The LZ (LUX-ZEPLIN) experiment is a second generation direct dark matter detector under construction at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. LZ will use a 7 tonne central liquid xenon target, arranged in a dual-phase time projection chamber (TPC), to seek evidence for nuclear recoils from a hypothesized galactic flux of weakly interacting massive particles (WIMPs). Surrounding the LZ TPC will be an outer detector (OD) consisting of 17.3 tonnes of LAB-based gadolinium-loaded liquid scintillator (GdLS). The OD’s primary functions will be to tag neutron single-scatter events in the liquid xenon which could mimic a WIMP dark matter signal and to characterize the radiation environment of LZ. In this talk, I summarize the expected performance of the OD and report on the design and results of the “Screener”, a small liquid scintillator detector consisting of ≈ 23 kg of the GdLS to be used in the OD. The Screener was operated in the ultra-low-background environment of the former LUX water shield in the Davis Laboratory at SURF for radioassay of the GdLS. Careful selection of detector materials and use of ultra-low-background PMTs allows the measurement of a variety of radioimpurities. In particular, the 14C/12C ratio in the scintillator is measured, while the use of pulse shape discrimination allows the concentration of isotopes throughout the 238U, 235U, and 232Th chains to be measured by fitting the collected spectra from α and β events. The GdLS is found to meet the requirements for LZ, however, more aggressive purification is being implemented for the final GdLS product that ensures the OD will successfully carry out its role in the hunt for WIMP dark matter.