Title: A liquid scintillation detector for radioassay of gadolinium-loaded liquid scintillator for the LZ Outer Detector

Abstract

Here, 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 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 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 ultra-low-background PMTs allows the measurement of a variety of radioimpurities. The ¹⁴C/¹²C ratio in the scintillator is measured to be (2.83 ± 0.06(stat.) ± 0.01(sys.)) x 10^–17 . Use of pulse shape discrimination allows the concentration of isotopes throughout the ²³⁸U, ²³⁵U, ,more » and ²³²Th chains to be measured by fitting the collected spectra from α and β events. We reveal that equilibrium is broken in the ²³⁸U and ²³²Th chains and that a crucial portion of the contamination in the GdLS results from decays in the ²²⁷Ac subchain of the ²³⁵U 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 -decays.« less

Authors:

Haselschwardt, S. J. ^[1]; Shaw, S. ^[1]; Nelson, H. N. ^[1]; Witherell, M. S. ^[2]; Yeh, Minfang  ^[3];

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Lesko, K. T. ^[4]; Cole, A. ^[4]; Kyre, S. ^[1]; White, D. T. ^[1]

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+ Show Author Affiliations

1. Univ. of California, Santa Barbara, CA (United States)
2. Univ. of California, Santa Barbara, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
3. Brookhaven National Lab. (BNL), Upton, NY (United States)
4. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

Publication Date:

Wed May 22 00:00:00 EDT 2019

Research Org.:

Brookhaven National Laboratory (BNL), Upton, NY (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); Univ. of California, Santa Barbara, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), High Energy Physics (HEP)

OSTI Identifier:

1550796

Alternate Identifier(s):

OSTI ID: 1526617; OSTI ID: 1600662; OSTI ID: 1777234

Report Number(s):

BNL-211933-2019-JAAM
Journal ID: ISSN 0168-9002

Grant/Contract Number:  

SC0012704; AC02-05CH11231; SC0011702

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment

Additional Journal Information:

Journal Volume: 937; Journal Issue: C; Journal ID: ISSN 0168-9002

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

73 NUCLEAR PHYSICS AND RADIATION PHYSICS; Low-background; Gadolinium; Liquid scintillator