Engineering topics

Ji, W.

Publications and source records attributed to Ji, W..

At least 37 records · Page 2

DOE OSTI2018

Search for annual and diurnal rate modulations in the LUX experiment

Various dark matter models predict annual and diurnal modulations of dark matter interaction rates in Earth-based experiments as a result of the Earth’s motion in the halo. Observation of such features can provide generic evidence for detection of dark matter interactions. This study reports a search for both annual and diurnal rate modulations in the LUX dark matter experiment using over 20 calendar months of data acquired between 2013 and 2016. This search focuses on electron recoil events at low energies, where leptophilic dark matter interactions are expected to occur and where the DAMA experiment has observed a strong rate modulation for over two decades. By using the innermost volume of the LUX detector and developing robust cuts and corrections, we obtained a stable event rate of 2.3 ± 0.2 cpd / keV ee / tonne , which is among the lowest in all dark matter experiments. No statistically significant annual modulation was observed in energy windows up to 26 keV ee . Between 2 and 6 keV ee , this analysis demonstrates the most sensitive annual modulation search up to date, with 9.2 σ tension with the DAMA/LIBRA result. We also report no observation of diurnal modulations above 0.2 cpd / keV ee / tonne amplitude between 2 and 6 keV ee .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2018

Rapid determination of soil classes in soil profiles using vis–NIR spectroscopy and multiple objectives mixed support vector classification

Summary Visible‐near infrared (vis–NIR) spectroscopy can reveal various soil properties and facilitate soil classification. However, few studies have attempted to classify vertical soil profiles that contain several genetic horizons. Here, we propose the ‘multiple objectives mixed support vector classification’ (MOM–SVC) method to classify soil profiles. A total of 130 soil profiles were collected from genetic horizons in Zhejiang Province, China. After laboratory analysis, soil profiles were classified according to the Chinese Soil Taxonomy system. Vis–NIR spectra were recorded from each genetic horizon of each soil profile and were then pre‐processed. We performed the MOM–SVC method as follows: (i) created a support vector machine (SVM) model (one‐versus‐one approach) using spectral data from all soil horizons in calibration profiles, (ii) applied the SVM model on each horizon of the profile to be predicted, (iii) extracted ‘votes’ from each horizon and mixed (or summarized) them into the votes of each profile to be predicted and (iv) classified each profile by the majority‐voting method. We also investigated whether the additional input of auxiliary soil information (e.g. moist soil colour, soil organic matter and soil texture), which could be measured easily or be well predicted by vis–NIR spectroscopy, could improve the accuracy of soil classification when combined with it. Independent validation results showed that the MOM–SVC method performed better at the soil order level than at the suborder level. Adding auxiliary soil information to the classification model improved the overall accuracy of classification at the soil order level. The proposed MOM–SVC method provides a fast objective diagnostic of soil classes for use in soil surveys and can help to update soil databases when a more objective soil classification system is developed. Highlights The MOM–SVC method can be used to classify soil profiles objectively with a variety of soil horizons. Stratified random sampling was used to quantify prediction uncertainty in classification MOM–SVC can predict soil orders with greater accuracy than suborders. Adding auxiliary soil information into the classification model improved prediction accuracy.

Chen, S.↗

DOE OSTI2018

LUX trigger efficiency

The Large Underground Xenon experiment (LUX) searches for dark matter using a dual-phase xenon detector. LUX uses a custom-developed trigger system for event selection. Here, the trigger efficiency, which is defined as the probability that an event of interest is selected for offline analysis, is studied using raw data obtained from both electron recoil (ER) and nuclear recoil (NR) calibrations. The measured efficiency exceeds 98% at a pulse area of 90 detected photons, which is well below the WIMP analysis threshold on the S2 pulse area. The efficiency also exceeds 98% at recoil energies of 1.3 keV and above, for both ER and NR. In conclusion, the measured trigger efficiency varies between 99% and 100% over the fiducial volume of the detector.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2018

Liquid xenon scintillation measurements and pulse shape discrimination in the LUX dark matter detector

Weakly Interacting Massive Particles (WIMPs) are a leading candidate for dark matter and are expected to produce nuclear recoil (NR) events within liquid xenon time-projection chambers. Here, we present a measurement of liquid xenon scintillation characteristics in the LUX dark matter detector and develop a pulse shaped based discrimination parameter to be used for particle identification. To accurately measure the scintillation characteristics, we develop a template-fitting method to reconstruct the detection time of photons. Analyzing calibration data collected during the 2013-16 LUX WIMP search, we measure a singlet-to-triplet scintillation ratio for electron recoils (ER) that is consistent with existing literature, and we make a first-ever measurement of the NR singlet-to-triplet ratio at recoil energies below 74 keV. A prompt fraction discrimination parameter exploits the difference of the photon time spectra for NR and ER events and is optimized to have the least number of ER events that occur in the 50% NR acceptance region. When this discriminator is used in conjunction with charge-to-light discrimination on the calibration data, the signal-to-noise ratio in the NR dark matter acceptance region increases by up to a factor of two.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2018

Calibration, event reconstruction, data analysis, and limit calculation for the LUX dark matter experiment

Here, the LUX experiment has performed searches for dark matter particles scattering elastically on xenon nuclei, leading to stringent upper limits on the nuclear scattering cross sections for dark matter. Here, for results derived from ${1.4}\times 10^{4}\;\mathrm{kg\,days}$ of target exposure in 2013, details of the calibration, event-reconstruction, modeling, and statistical tests that underlie the results are presented. Detector performance is characterized, including measured efficiencies, stability of response, position resolution, and discrimination between electron- and nuclear-recoil populations. Models are developed for the drift field, optical properties, background populations, the electron- and nuclear-recoil responses, and the absolute rate of low-energy background events. Innovations in the analysis include in situ measurement of the photomultipliers' response to xenon scintillation photons, verification of fiducial mass with a low-energy internal calibration source, and new empirical models for low-energy signal yield based on large-sample, in situ calibrations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2018

Metallic wire grid behavior and testing in a low pressure gaseous noble elements detector

High voltage performance has been a challenge for noble element detectors. One piece of this challenge is the emission of electrons from metal electrodes when applying high voltage. This has become a major concern for low-background detectors such as LUX-ZEPLIN (LZ). LZ is a liquid xenon Time Projection Chamber (TPC) searching for Weakly Interactive Massive Particles (WIMPs). In this work, we demonstrate a method to measure electron emission from metallic electrode grids via detection of proportional scintillation light. As a result, we find consistency with Fowler-Nordheim emission with a surface parameter β = 1988 after electro-polishing treatment of a stainless steel grid.

47 OTHER INSTRUMENTATION↗

DOE OSTI2018

Position reconstruction in LUX

The (x, y) position reconstruction method used in the analysis of the complete exposure of the Large Underground Xenon (LUX) experiment is presented. The algorithm is based on a statistical test that makes use of an iterative method to recover the photomultiplier tube (PMT) light response directly from the calibration data. The light response functions make use of a two dimensional functional form to account for the photons reflected on the inner walls of the detector. To increase the resolution for small pulses, a photon counting technique was employed to describe the response of the PMTs. The reconstruction was assessed with calibration data including 83m Kr (releasing a total energy of 41.5 keV) and 3 H (β - with Q = 18.6 keV) decays, and a deuterium-deuterium (D-D) neutron beam (2.45 MeV). Within the detector's fiducial volume, the reconstruction has achieved an (x, y) position uncertainty of σ = 0.82 cm and σ = 0.17 cm for events of only 200 and 4,000 detected electroluminescence photons respectively. Such signals are associated with electron recoils of energies ~0.25 keV and ~10 keV, respectively. Lastly, the reconstructed position of the smallest events with a single electron emitted from the liquid surface (22 detected photons) has a horizontal (x, y) uncertainty of 2.13 cm.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2017

Ultralow energy calibration of LUX detector using Xe 127 electron capture

We report an absolute calibration of the ionization yields (Q y ) and fluctuations for electronic recoil events in liquid xenon at discrete energies between 186 eV and 33.2 keV. The average electric field applied across the liquid xenon target is 180 V/cm. The data are obtained using low energy 127 Xe electron capture decay events from the 95.0-day first run from LUX (WS2013) in search of weakly interacting massive particles. The sequence of gamma-ray and x-ray cascades associated with 127 I deexcitations produces clearly identified two-vertex events in the LUX detector. We observe the K-(binding energy, 33.2 keV), L-(5.2 keV), M-(1.1 keV), and N-(186 eV) shell cascade events and verify that the relative ratio of observed events for each shell agrees with calculations. In conclusion, the N-shell cascade analysis includes single extracted electron (SE) events and represents the lowest-energy electronic recoil in situ measurements that have been explored in liquid xenon.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2017

83⁢m Kr calibration of the 2013 LUX dark matter search

LUX was the first dark matter experiment to use a 83⁢m Kr calibration source. In this study, we describe the source preparation and injection. We also present several 83⁢m Kr calibration applications in the context of the 2013 LUX exposure, including the measurement of temporal and spatial variation in scintillation and charge signal amplitudes, and several methods to understand the electric field within the time projection chamber.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2017

3D modeling of electric fields in the LUX detector

This work details the development of a three-dimensional (3D) electric field model for the LUX detector. The detector took data to search for weakly interacting massive particles (WIMPs) during two periods. After the first period completed, a time-varying non-uniform negative charge developed in the polytetrafluoroethylene (PTFE) panels that define the radial boundary of the detector's active volume. This caused electric field variations in the detector in time, depth and azimuth, generating an electrostatic radially-inward force on electrons on their way upward to the liquid surface. To map this behavior, 3D electric field maps of the detector's active volume were generated on a monthly basis. This was done by fitting a model built in COMSOL Multiphysics to the uniformly distributed calibration data that were collected on a regular basis. The modeled average PTFE charge density increased over the course of the exposure from -3.6 to -5.5 μC/m 2 . Here, from our studies, we deduce that the electric field magnitude varied locally while the mean value of the field of ~200 V/cm remained constant throughout the exposure. As a result of this work the varying electric fields and their impact on event reconstruction and discrimination were successfully modeled.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2017

Chromatographic separation of radioactive noble gases from xenon

The Large Underground Xenon (LUX) experiment operates at the Sanford Underground Research Facility to detect nuclear recoils from the hypothetical Weakly Interacting Massive Particles (WIMPs) on a liquid xenon target. Liquid xenon typically contains trace amounts of the noble radioactive isotopes 85 Kr and 39 Ar that are not removed by the in situ gas purification system. The decays of these isotopes at concentrations typical of research-grade xenon would be a dominant background for a WIMP search exmperiment. To remove these impurities from the liquid xenon, a chromatographic separation system based on adsorption on activated charcoal was built. 400 kg of xenon was processed, reducing the average concentration of krypton from 130 ppb to 3.5 ppt as measured by a cold-trap assisted mass spectroscopy system. Finally, a 50 kg batch spiked to 0.001 g/g of krypton was processed twice and reduced to an upper limit of 0.2 ppt.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗

DOE OSTI2017

Identification of Radiopure Titanium for the LZ Dark Matter Experiment and Future Rare Event Searches

The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a detector containing a total of 10 tonnes of liquid xenon within a double-vessel cryostat. The large mass and proximity of the cryostat to the active detector volume demand the use of material with extremely low intrinsic radioactivity. We report on the radioassay campaign conducted to identify suitable metals, the determination of factors limiting radiopure production, and the selection of titanium for construction of the LZ cryostat and other detector components. This titanium has been measured with activities of 238 U e < 1.6 mBq/kg, 238 U l < 0.09 mBq/kg, 232 Th e = 0.28 ± 0.03 mBq/kg, 232 Th l = 0.25 ± 0.02 mBq/kg, 40 K < 0.54 mBq/kg, and 60 Co < 0.02 mBq/kg (68% CL). Such low intrinsic activities, which are some of the lowest ever reported for titanium, enable its use for future dark matter and other rare event searches. Monte Carlo simulations have been performed to assess the expected background contribution from the LZ cryostat with this radioactivity. As a result, in 1,000 days of WIMP search exposure of a 5.6-tonne fiducial mass, the cryostat will contribute only a mean background of 0.160 ± 0.001(stat) ± 0.030(sys) counts.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2017

First Searches for Axions and Axionlike Particles with the LUX Experiment

The first searches for axions and axionlike particles with the Large Underground Xenon experiment are presented. Under the assumption of an axioelectric interaction in xenon, the coupling constant between axions and electrons g Ae is tested using data collected in 2013 with an exposure totaling 95 live days ×118 kg. A double-sided, profile likelihood ratio statistic test excludes g Ae larger than 3.5 × 10 –12 (90% C.L.) for solar axions. Assuming the Dine-Fischler-Srednicki-Zhitnitsky theoretical description, the upper limit in coupling corresponds to an upper limit on axion mass of 0.12 eV/c 2 , while for the Kim-Shifman-Vainshtein-Zhakharov description masses above 36.6 eV/c 2 are excluded. For galactic axionlike particles, values of g Ae larger than 4.2 × 10 –13 are excluded for particle masses in the range 1–16 keV/c 2 . As a result, these are the most stringent constraints to date for these interactions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2017

Limits on spin-dependent WIMP-nucleon cross section obtained from the complete LUX exposure

We present experimental constraints on the spin-dependent WIMP-nucleon elastic cross sections from the total 129.5 kg yr exposure acquired by the Large Underground Xenon experiment (LUX), operating at the Sanford Underground Research Facility in Lead, South Dakota (USA). A profile likelihood ratio analysis allows 90% C.L. upper limits to be set on the WIMP-neutron (WIMP-proton) cross section of σ n = 1.6 × 10 –41 cm 2 (σ p = 5 × 10 –40 cm 2 ) at 35 GeV c –2 , almost a sixfold improvement over the previous LUX spin-dependent results. Finally, the spin-dependent WIMP-neutron limit is the most sensitive constraint to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2017

Searching for new spin-dependent interactions with SmCo 5 spin sources and a spin-exchange-relaxation-free comagnetometer

We propose a novel method to search for possible new macroscale spin- and/or velocity-dependent forces (SVDFs) based on specially designed SmCo 5 spin sources and a spin exchange relaxation-free (SERF) comagnetometer. A simulation shows that, by covering a SmCo 5 permanent magnet with a layer of pure iron, a high net spin density source of about $1×10^{22}/cm^3$ could be obtained. Taking advantages of the high spin density of this iron-shielded SmCo 5 and the high sensitivity of the SERF, the proposed method could set up new limits of greater than 10 orders of magnitude more sensitive than those from previous experiments or proposals in exploring SVDFs in force ranges larger than 1 cm.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2017

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

This study 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 recombination 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 and 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.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2017

Results from a search for dark matter in the complete LUX exposure

Here, we report constraints on spin-independent weakly interacting massive particle (WIMP)-nucleon scattering using a 3.35 ×10 4 kg day exposure of the Large Underground Xenon (LUX) experiment. A dual-phase xenon time projection chamber with 250 kg of active mass is operated at the Sanford Underground Research Facility under Lead, South Dakota (USA). With roughly fourfold improvement in sensitivity for high WIMP masses relative to our previous results, this search yields no evidence of WIMP nuclear recoils. At a WIMP mass of 50 GeV c –2 , WIMP-nucleon spin-independent cross sections above 2.2 ×10 –46 cm 2 are excluded at the 90% confidence level. When combined with the previously reported LUX exposure, this exclusion strengthens to 1.1 × 10 –46 cm 2 at 50 GeV c –2 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2016

FPGA-based trigger system for the LUX dark matter experiment

LUX is a two-phase (liquid/gas) xenon time projection chamber designed to detect nuclear recoils resulting from interactions with dark matter particles. Signals from the detector are processed with an FPGA-based digital trigger system that analyzes the incoming data in real-time, with just a few microsecond latency. The system enables first pass selection of events of interest based on their pulse shape characteristics and 3D localization of the interactions. It has been shown to be >99% efficient in triggering on S2 signals induced by only few extracted liquid electrons. It is continuously and reliably operating since its full underground deployment in early 2013. This document is an overview of the systems capabilities, its inner workings, and its performance.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗