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 ≈ 23 kg of LAB-based gadoliniumloaded 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 tag neutron events in the liquid xenon which could mimic a WIMP dark matter signal. To meet the deadtime requirements for the Outer Detector, the radioimpurity levels in the GdLS must be kept below 0.07 mBq/kg. This background level corresponds to a rate of ≈ 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 ultralow-background PMTs allows the measurement of a variety of radioimpurities. The 14 C/ 12 C ratio in the scintillator is measured to be (2.83 ± 0.06(stat.) ± 0.01(sys.)) × 10 −17 . Use of pulse shape discrimination allows the concentration of isotopes throughout the 238 U, 235 U, and 232 Th chains to be measured by fitting the collected spectra from α and β events. We find that equilibrium is broken in the 238 U and 232 Th chains and that a significant portion of the contamination in the GdLS results from decays in the 227 Ac subchain of the 235 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 ± 6.4 Hz, with 65.5 ± 1.9 Hz resulting from α-decays.

Simulations of events for the LUX-ZEPLIN (LZ) dark matter experiment

“…The scintillation light, which ranges in wavelength from 350 nm to 550 nm, can travel through the acrylic and be collected by 120 PMTs situated on a tyvek curtain 115 cm from the outer radius of the tanks. Data from a small prototype detector, the LS Screener [35], which was calibrated with various α-decays, β-decays and γ-rays, was used to finetune a modified version of the Geant4 G4Scintillation code.…”

“…photons/MeV), kB and C are the first and second Birk's law parameters that describe the quenching process for heavily ionizing particles. The parameters used for α-particles, γ-rays and electrons, and the light yield were obtained using the LS Screener calibration data [35]; external 137 Cs and 228 Th γ sources were used to fix Y for all particles and to determine kB for γ/e − , and a 220 Rn source was bubbled through the GdLS to obtain kB and C for α-particles. Parameters for protons were taken from [37], and all are shown in Table I.…”

“…Photons are produced at positions scattered around the energy depositing track, and with a time distribution according to fast and slow time constants that are chosen based on particle type. Once produced, optical photons may be absorbed by the wavelength shifter within the scintillator, after which they have a wavelength-dependent probability to be reemitted at a longer wavelength [35].…”

Measurement of the gamma ray background in the Davis cavern at the Sanford Underground Research Facility

“…A cross-check of these results was performed through simulation of the LS Screener detector, described in Ref. [28], which took data at four positions in z within the water tank. The γ-ray contribution to the rate in the LS Screener from both the cavern and a steel stand originally used to support the LUX detector was simulated within the BACCARAT simulation framework using the event biasing technique.…”

“…Combining this with the prediction for internal rate from LS contaminants in Ref. [28], totalling 6 ± 2 Hz, and less than 12 Hz from simulation of radioactivity from LZ components (see Ref. [5]; an upper limit is given due to the use of upper limits in screening results for materials dominant to this rate), leads to a total rate in the LZ OD of 45 ± 7 Hz, significantly below the 100 Hz needed to maintain less than 5% impact on LZ live time.…”