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

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

Signal yields, energy resolution, and recombination fluctuations in liquid xenon

This work presents an analysis of monoenergetic electronic recoil peaks in the dark-matter-search and calibration data from the first underground science run of the Large Underground Xenon (LUX) detector. Liquid xenon charge and light yields for electronic recoil energies between 5.2 and 661.7 keV are measured, as well as the energy resolution for the LUX detector at those same energies. Additionally, there is an interpretation of existing measurements and descriptions of electron-ion recombination fluctuations in liquid xenon as limiting cases of a more general liquid xenon re- combination fluctuation model. Measurements of the standard deviation of these fluctuations at monoenergetic electronic recoil peaks exhibit a linear dependence on the number of ions for energy deposits up to 661.7 keV, consistent with previous LUX measurements between 2-16 keV with $^3$H. We highlight similarities in liquid xenon recombination for electronic and nuclear recoils with a comparison of recombination fluctuations measured with low-energy calibration data.Comment: 11 pages, 12 figures, 3 table

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