Radioassay of Gadolinium-Loaded Liquid Scintillator and Other Studies for the LZ Outer Detector

It is now well established that over 80\% of the matter in our Universe is comprised of a non-luminous substance known as dark matter. By far the most popular dark matter candidate is the weakly interacting massive particle (WIMP). Attempting to discover the nature of WIMP dark matter through direct detection has been a central activity of experimental physics for at least the last 20 years. To date, no conclusive signal consistent with WIMP interactions has been observed.The LZ (LUX-ZEPLIN) experiment is a second generation direct dark matter detector under construction one mile underground in the Davis Laboratory of 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 WIMPs. Two active detector elements will surround the TPC: a layer of liquid xenon, the xenon skin, optimized to detect $\gamma$'s, and the outer detector (OD), optimized to detect neutrons. Together, these detectors will tag backgrounds to the sought-after WIMP signal and characterize the background environment around LZ.The OD is comprised of acrylic tanks filled with 17.3 tonnes of LAB-based gadolinium-loaded liquid scintillator (GdLS) that will surround the central cryostat of LZ in a near-hermetic fashion. Its primary function will be to tag neutron single-scatter events in the liquid xenon which could mimic a WIMP dark matter signal. I summarize simulation studies of the OD expected performance as a neutron veto and expected light collection.The rate of single background pulses in the OD is also discussed. The three primary sources of rate in the OD are identified as: LZ detector components, $\gamma$-rays from the Davis Laboratory walls, and the radioimpurities in the GdLS. The radioimpurities in the GdLS are particularly troublesome because the OD is sensitive to both the $\alpha$ and $\beta$/$\gamma$ decays of these isotopes. To meet the requirements for the OD, the radioimpurity levels in the GdLS must be kept below $\lesssim0.07$ mBq/kg. This background level corresponds to a rate of $\approx50$ Hz above an energy threshold of 100 keV.I report on the design and performance of the ``Screener", a small liquid scintillator detector consisting of $\approx23$ 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. The $^{14}\textrm{C}$/$^{12}\textrm{C}$ ratio in the scintillator is measured to be $(2.83\pm0.06\textrm{(stat.)}\pm0.01\textrm{(sys.)}) \times 10^{-17}$. Use of pulse shape discrimination allows the concentration of isotopes throughout the $^{238}\textrm{U}$, $^{235}\textrm{U}$, and $^{232}\textrm{Th}$ chains to be measured by fitting the collected spectra from $\alpha$ and $\beta$ events. It is found that equilibrium is broken in the $^{238}\textrm{U}$ and $^{232}\textrm{Th}$ chains and that a significant portion of the contamination in the GdLS results from decays in the $^{227}\textrm{Ac}$ subchain of the $^{235}\textrm{U}$ series.Predictions for the singles rate in the OD are presented. The rate from radioimpurities above 100 keV in the GdLS is estimated to be $97.5\pm6.3$ Hz, with $65.0\pm1.4$ Hz resulting from $\alpha$-decays

Toward a neutrino-limited dark matter search with crystalline xenon

Experiments searching for weakly interacting massive particle dark matter are now detecting background events from solar neutrino-electron scattering. However, the dominant radioactive background in state-of-the-art experiments such as LZ and XENONnT is beta decays from radon contamination. In spite of careful detector material screening, radon progenitor atoms are ubiquitous and long-lived, and radon is extremely soluble in liquid xenon. We propose a change of phase and demonstrate that crystalline xenon offers more than a factor ×500 exclusion against radon ingress, compared with the liquid state. This level of radon exclusion would allow crystallized versions of existing experiments to probe spin-independent cross sections near 10-47 cm2 in roughly 11 years, as opposed to the 35 years required otherwise

Observation of low-lying isomeric states in 136^{136}Cs: a new avenue for dark matter and solar neutrino detection in xenon detectors

We report on new measurements establishing the existence of low-lying isomeric states in $^{136}$Cs using $\gamma$ rays produced in $^{136}$Xe(p,n)$^{136}$Cs reactions. Two states with $\mathcal{O}(100)$~ns lifetimes are placed in the decay sequence of the $^{136}$Cs levels that are populated in charged-current interactions of solar neutrinos and fermionic dark matter with $^{136}$Xe. Xenon-based experiments can therefore exploit a delayed-coincidence tag of these interactions, greatly suppressing backgrounds to enable spectroscopic studies of solar neutrinos and dark matter.Comment: Supplemental material available upon request. Version accepted by Phys.Rev.Let

Radial Internal Material Handling System (RIMS) for Circular Habitat Volumes

A Radial Internal Material Handling System (RIMS) has been developed to service a circular floor area in variable gravity. On planetary surfaces, pressurized human habitable volumes will require a means to carry heavy equipment between various locations within the volume of the habitat, regardless of the partial gravity (Earth, moon, Mars, etc). On the NASA Habitat Demonstration Unit (HDU), a vertical cylindrical volume, it was determined that a variety of heavy items would need to be carried back and forth from deployed locations to the General Maintenance Work Station (GMWS) when in need of repair, and other equipment may need to be carried inside for repairs, such as rover parts and other external equipment. The vertical cylindrical volume of the HDU lent itself to a circular overhead track and hoist system that allows lifting of heavy objects from anywhere in the habitat to any other point in the habitat interior. In addition, the system is able to hand off lifted items to other material handling systems through the side hatches, such as through an airlock. This paper describes the RIMS system which is scalable for application in a variety of circular habitat volumes

Solar neutrino detection in liquid xenon detectors via charged-current scattering to excited states

We investigate the prospects for real-time detection of solar neutrinos via the charged-current neutrino-nucleus scattering process in liquid xenon time projection chambers. We use a nuclear shell model, benchmarked with experimental data, to calculate the cross sections for populating specific excited states of the cesium nuclei produced by neutrino capture on 131Xe and 136Xe. The shell model is further used to compute the decay schemes of the low-lying 1+ excited states of 136Cs, for which there is sparse experimental data. We explore the possibility of tagging the characteristic deexcitation γ rays/conversion electrons using two techniques: spatial separation of their energy deposits using event topology and their time separation using delayed coincidence. The efficiencies in each case are evaluated within a range of realistic detector parameters. We find that the topological signatures are likely to be dominated by radon backgrounds, but that a delayed-coincidence signature from long-lived states predicted in 136Cs may enable background-free detection of CNO neutrino interactions in next-generation experiments with smaller uncertainty than current measurements. We also estimate the sensitivity as a function of exposure for detecting the solar-temperature-induced line shift in 7Be neutrino emission, which may provide a new test of solar models.peerReviewe