A Liquid Scintillation Detector for Radioassay of Gadolinium-Loaded Liquid Scintillator for the LZ Outer Detector

Abstract

We report on the design and performance of the LUX-ZEPLIN (LZ) "Screener", a small liquid scintillator detector consisting of $\approx 23$ kg of LAB-based gadolinium-loaded liquid scintillator (GdLS) to be used in the LZ Outer Detector. The Outer Detector will be filled with 17.3 tonnes of GdLS and will surround the central liquid xenon time projection chamber of LZ. Its primary function will be to veto neutron single-scatter events in the liquid xenon which could mimic a WIMP dark matter signal. To meet the requirements for the Outer Detector, the radioimpurity levels in the GdLS must be kept below $\lesssim 0.07$ mBq/kg. This background level corresponds to a rate of $\approx 50$ Hz above an energy threshold of 100 keV. The Screener was operated in the ultra-low-background environment of the former LUX water shield in the Davis Laboratory at the Sanford Underground Research Facility 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. The ${}^{14}\text{C}$/${}^{12}\text{C}$ ratio in the scintillator is measured to be $(2.83\pm 0.06\text{(stat.)}\pm 0.01\text{(sys.)})\times 10^{-17}$. Use of pulse shape discrimination allows the concentration of isotopes throughout the ${}^{238}\text{U}$, ${}^{235}\text{U}$, and ${}^{232}\text{Th}$ chains to be measured by fitting the collected spectra from $\alpha$ and $\beta$ events. We find that equilibrium is broken in the ${}^{238}\text{U}$ and ${}^{232}\text{Th}$ chains and that a significant portion of the contamination in the GdLS results from decays in the ${}^{227}\text{Ac}$ subchain of the ${}^{235}\text{U}$ series. Predictions for the singles rate in the Outer Detector are presented. The rate from radioimpurities above 100 keV in the GdLS is estimated to be $97.9\pm 6.4$ Hz, with $65.5\pm 1.9$ Hz resulting from $\alpha$-decays.

References in corpus (25)

• Neutrino Physics with JUNO
• Final results from the Palo Verde Neutrino Oscillation Experiment
• Results from the Palo Verde Neutrino Oscillation Experiment
• The Large Underground Xenon (LUX) Experiment
• Projected WIMP sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment
• Observation of Energy and Baseline Dependent Reactor Antineutrino Disappearance in the RENO Experiment
• Measurement of electron antineutrino oscillation based on 1230 days of operation of the Daya Bay experiment
• Improved measurements of the neutrino mixing angle ${\theta }_{13}$ with the Double Chooz detector
• Simultaneous Precision Spectroscopy of $pp$, ${}^7$Be, and $pep$ Solar Neutrinos with Borexino Phase-II
• Production of Gadolinium-loaded Liquid Scintillator for the Daya Bay Reactor Neutrino Experiment
• Lowering the radioactivity of the photomultiplier tubes for the XENON1T dark matter experiment
• 7Be Solar Neutrino Measurement with KamLAND
• ACTIVIA: Calculation of Isotope Production Cross-sections and Yields
• Development of a Gd Loaded Liquid Scintillator for Electron Anti-Neutrino Spectroscopy
• Scintillation decay time and pulse shape discrimination in oxygenated and deoxygenated solutions of linear alkylbenzene for the SNO+ experiment
• Performance of the Hamamatsu R11410 Photomultiplier Tube in cryogenic Xenon Environments
• The veto system of the DarkSide-50 experiment
• An Ultra-Low Background PMT for Liquid Xenon Detectors
• Identification of Radiopure Titanium for the LZ Dark Matter Experiment and Future Rare Event Searches
• Oxygen quenching in LAB based liquid scintillator and nitrogen bubbling model
• The ZEPLIN-III Anti-Coincidence Veto Detector
• Performance of the veto detector incorporated into the ZEPLIN-III experiment
• Measurement of the Gamma Ray Background in the Davis Cavern at the Sanford Underground Research Facility
• Dark Matter Search Backgrounds from Primordial Radionuclide Chain Disequilibrium
• Laboratory Studies on the Removal of Radon-Born Lead from KamLAND's Organic Liquid Scintillator

Cited by in corpus (11)

• First Dark Matter Search Results from the LUX-ZEPLIN (LZ) Experiment
• The LUX-ZEPLIN (LZ) Experiment
• The LUX-ZEPLIN (LZ) radioactivity and cleanliness control programs
• Simulations of Events for the LUX-ZEPLIN (LZ) Dark Matter Experiment
• Background Determination for the LUX-ZEPLIN (LZ) Dark Matter Experiment
• Measurement of the Gamma Ray Background in the Davis Cavern at the Sanford Underground Research Facility
• New constraints on cosmic ray-boosted dark matter from the LUX-ZEPLIN experiment
• The Design, Implementation, and Performance of the LZ Calibration Systems
• Light Yield Quenching and Quenching Remediation in Liquid Scintillator Detectors
• Optical Calibration System for the LUX-ZEPLIN (LZ) Outer Detector
• Remote Reactor Ranging via Antineutrino Oscillations