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

Why would you put a flashlight in a dark matter detector?

Silicon photomultipliers (SiPMs) are solid-state, single-photon sensitive, pixelated sensors whose usage for scintillation detection has rapidly increased over the past decade. It is known that the avalanche process within the device, which renders a single photon detectable, can also generate secondary photons which may be detected by a separate device. This effect, known as external crosstalk, could potentially degrade the science goals of future xenon dark matter experiments. In this article, we measure the effect of external crosstalk in a dual-phase, liquid xenon time projection chamber fully instrumented with SiPMs. We then consider the implications for a future xenon dark matter experiment utilizing SiPMs and discuss possible solutions.Comment: 12 pages, 6 figure

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

On planetary surfaces, pressurized human habitable volumes will require a means to carry equipment around 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. The overhead system consists of two concentric circle tracks that have a movable beam between them. The beam has a hoist carriage that can move back and forth on the beam. Therefore, the entire system acts like a bridge crane curved around to meet itself in a circle. The novelty of the system is in its configuration, and how it interfaces with the volume of the HDU habitat. Similar to how a bridge crane allows coverage for an entire rectangular volume, the RIMS system covers a circular volume. The RIMS system is the first generation of what may be applied to future planetary surface vertical cylinder habitats on the Moon or on Mars

First measurement of discrimination between helium and electron recoils in liquid xenon for low-mass dark matter searches

We report the first measurement of discrimination between low-energy helium recoils and electron recoils in liquid xenon. This result is relevant to proposed low-mass dark matter searches which seek to dissolve light target nuclei in the active volume of liquid-xenon time projection chambers. Low-energy helium recoils were produced by degrading $\alpha$ particles from $^{210}$Po with a gold foil situated on the cathode of a liquid xenon time-projection chamber. The resulting population of helium recoil events is well separated from electron recoils and is also offset from the expected position of xenon nuclear recoil events.Comment: 4 pages, 3 figure

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

Projected sensitivity of the LUX-ZEPLIN experiment to the 0νββ decay of Xe 136

The LUX-ZEPLIN (LZ) experiment will enable a neutrinoless double β decay search in parallel to the main science goal of discovering dark matter particle interactions. We report the expected LZ sensitivity to Xe136 neutrinoless double β decay, taking advantage of the significant (>600 kg) Xe136 mass contained within the active volume of LZ without isotopic enrichment. After 1000 live-days, the median exclusion sensitivity to the half-life of Xe136 is projected to be 1.06×1026 years (90% confidence level), similar to existing constraints. We also report the expected sensitivity of a possible subsequent dedicated exposure using 90% enrichment with Xe136 at 1.06×1027 years

Projected WIMP sensitivity of the LUX-ZEPLIN dark matter experiment

LUX-ZEPLIN (LZ) is a next-generation dark matter direct detection experiment that will operate 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. Using a two-phase xenon detector with an active mass of 7 tonnes, LZ will search primarily for low-energy interactions with weakly interacting massive particles (WIMPs), which are hypothesized to make up the dark matter in our galactic halo. In this paper, the projected WIMP sensitivity of LZ is presented based on the latest background estimates and simulations of the detector. For a 1000 live day run using a 5.6-tonne fiducial mass, LZ is projected to exclude at 90% confidence level spin-independent WIMP-nucleon cross sections above 1.4×10-48 cm2 for a 40 GeV/c2 mass WIMP. Additionally, a 5σ discovery potential is projected, reaching cross sections below the exclusion limits of recent experiments. For spin-dependent WIMP-neutron(-proton) scattering, a sensitivity of 2.3×10-43 cm2 (7.1×10-42 cm2) for a 40 GeV/c2 mass WIMP is expected. With underground installation well underway, LZ is on track for commissioning at SURF in 2020

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

The LUX-ZEPLIN dark matter search aims to achieve a sensitivity to the WIMP-nucleon spin-independent cross-section down to (1–2)×10−12 pb at a WIMP mass of 40 GeV/c2. This paper describes the simulations framework that, along with radioactivity measurements, was used to support this projection, and also to provide mock data for validating reconstruction and analysis software. Of particular note are the event generators, which allow us to model the background radiation, and the detector response physics used in the production of raw signals, which can be converted into digitized waveforms similar to data from the operational detector. Inclusion of the detector response allows us to process simulated data using the same analysis routines as developed to process the experimental data