Underground Research Facility Research Articles

The design, implementation, and performance of the LZ calibration systems

LUX-ZEPLIN (LZ) is a tonne-scale experiment searching for direct dark matter interactions and other rare events. It is located at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. The core of the LZ detector is a dual-phase xenon time projection chamber (TPC), designed with the primary goal of detecting Weakly Interacting Massive Particles (WIMPs) via their induced low energy nuclear recoils. Surrounding the TPC, two veto detectors immersed in an ultra-pure water tank enable reducing background events to enhance the discovery potential. Intricate calibration systems are purposely designed to precisely understand the responses of these three detector volumes to various types of particle interactions and to demonstrate LZ's ability to discriminate between signals and backgrounds. In this paper, we present a comprehensive discussion of the key features, requirements, and performance of the LZ calibration systems, which play a crucial role in enabling LZ's WIMP-search and its broad science program. The thorough description of these calibration systems, with an emphasis on their novel aspects, is valuable for future calibration efforts in direct dark matter and other rare-event search experiments.

Methylomonas rivi sp. nov., Methylomonas rosea sp. nov., Methylomonas aurea sp. nov. and Methylomonas subterranea sp. nov., type I methane-oxidizing bacteria isolated from a freshwater creek and the deep terrestrial subsurface.

Four methane-oxidizing bacteria, designated as strains WSC-6T, WSC-7T, SURF-1T, and SURF-2T, were isolated from Saddle Mountain Creek in southwestern Oklahoma, USA, and the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. The strains were Gram-negative, motile, short rods that possessed intracytoplasmic membranes characteristic of type I methanotrophs. All four strains were oxidase-negative and weakly catalase-positive. Colonies ranged from pale pink to orange in colour. Methane and methanol were the only compounds that could serve as carbon and energy sources for growth. Strains WSC-6T and WSC-7T grew optimally at lower temperatures (25 and 20 °C, respectively) compared to strains SURF-1T and SURF-2T (40 °C). Strains WSC-6T and SURF-2T were neutrophilic (optimal pH of 7.5 and 7.3, respectively), while strains WSC-7T and SURF-1T were slightly alkaliphilic, with an optimal pH of 8.8. The strains grew best in media amended with ≤0.5% NaCl. The major cellular fatty acids were C14 : 0, C16 : 1 ω8c, C16 : 1 ω7c, and C16 : 1 ω5c. The DNA G+C content ranged from 51.5 to 56.0 mol%. Phylogenetic analyses indicated that the strains belonged to the genus Methylomonas, with each exhibiting 98.6-99.6% 16S rRNA gene sequence similarity to closely related strains. Genome-wide estimates of relatedness (84.5-88.4% average nucleotide identity, 85.8-92.4% average amino acid identity and 27.4-35.0% digital DNA-DNA hybridization) fell below established thresholds for species delineation. Based on these combined results, we propose to classify these strains as representing novel species of the genus Methylomonas, for which the names Methylomonas rivi (type strain WSC-6T=ATCC TSD-251T=DSM 112293T), Methylomonas rosea (type strain WSC-7T=ATCC TSD-252T=DSM 112281T), Methylomonas aurea (type strain SURF-1T=ATCC TSD-253T=DSM 112282T), and Methylomonas subterranea (type strain SURF-2T=ATCC TSD-254T=DSM 112283T) are proposed.

Monitoring spatiotemporal evolution of fractures during hydraulic stimulations at the first EGS collab testbed using anisotropic elastic-waveform inversion

The EGS Collab project acquired continuous active-source seismic monitoring (CASSM) data before, during, and after hydraulic stimulations at the first testbed at the depth of 4850 ft (1478 m) at the Sanford Underground Research Facility in Lead, South Dakota, for monitoring fracture creation and evolution. CASSM acquisition was conducted using 24 hydrophones, 18 accelerometers, and 17 piezoelectric sources within four fracture-parallel wells and two orthogonal wells. 3D anisotropic traveltime tomography and anisotropic elastic-waveform inversion of the campaign cross-borehole seismic data show that the rock within the stimulation region is a heterogeneous horizontal transverse isotropic medium. We use these inversion results as the initial models and apply 3D anisotropic first-arrival traveltime tomography and 3D anisotropic elastic-waveform inversion to the CASSM data acquired after each stimulation in May, 2018 and December, 2018. We observe the spatiotemporal evolution of seismic velocities and anisotropic parameters caused by hydraulic fracture stimulations, showing the regions of rock alternation caused by hydraulic fracture stimulation.

First constraints on WIMP-nucleon effective field theory couplings in an extended energy region from LUX-ZEPLIN

Following the first science results of the LUX-ZEPLIN (LZ) experiment, a dual-phase xenon time projection chamber operating from the Sanford Underground Research Facility in Lead, South Dakota, USA, we report the initial limits on a model-independent nonrelativistic effective field theory describing the complete set of possible interactions of a weakly interacting massive particle (WIMP) with a nucleon. These results utilize the same 5.5 t fiducial mass and 60 live days of exposure collected for the LZ spin-independent and spin-dependent analyses while extending the upper limit of the energy region of interest by a factor of 7.5 to 270 keV. No significant excess in this high energy region is observed. Using a profile-likelihood ratio analysis, we report 90% confidence level exclusion limits on the coupling of each individual nonrelativistic WIMP-nucleon operator for both elastic and inelastic interactions in the isoscalar and isovector bases. Published by the American Physical Society 2024

Detection of astrophysical neutrinos at prospective locations of dark matter detectors

We study the prospects for detection of solar, atmospheric neutrino, and diffuse supernova neutrino background (DSNB) fluxes at future large-scale dark matter detectors through both electron and nuclear recoils. We specifically examine how the detection prospects change for several prospective detector locations [Sanford Underground Research Facility (SURF), SNOlab, Gran Sasso, China Jinping Underground Laboratory (CJPL), and Kamioka] and improve upon the statistical methodologies used in previous studies. Because of its ability to measure lower neutrino energies than other locations, we find that the best prospects for the atmospheric neutrino flux are at the SURF location, while the prospects are weakest at CJPL because it is restricted to higher neutrino energies. On the contrary, the prospects for the DSNB are best at CJPL, due largely to the reduced atmospheric neutrino background at this location. Including full detector resolution and efficiency models, the CNO component of the solar flux is detectable via the electron recoil channel with exposures of ∼103 ton-yr for all locations. These results highlight the benefits for employing two detector locations, one at high and one at low latitude. Published by the American Physical Society 2024

Detecting fractures and monitoring hydraulic fracturing processes at the first EGS Collab testbed using borehole DAS ambient noise

Enhanced geothermal systems (EGS) require cost-effective monitoring of fracture networks. We validate the capability of using borehole distributed acoustic sensing (DAS) ambient noise for fracture monitoring using core photos and core logs. The EGS Collab project has conducted 10 m scale field experiments of hydraulic fracture stimulation using 50–60 m deep experimental wells at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. The first EGS Collab testbed is located at 1616.67 m (4850 ft) depth at SURF and consists of one injection well, one production well, and six monitoring wells. All wells are drilled subhorizontally from an access tunnel called a drift. The project uses a single continuous fiber-optic cable installed sequentially in the six monitoring wells to record DAS data for monitoring hydraulic fracturing during stimulation. We analyze 60 s time records of the borehole DAS ambient noise data and compute the noise root-mean-square (rms) amplitude on each channel (points along the fiber cable) to obtain DAS ambient noise rms amplitude depth profiles along the monitoring wellbore. Our noise rms amplitude profiles indicate amplitude peaks at distinct depths. We compare the DAS noise rms amplitude profiles with borehole core photos and core logs and find that the DAS noise rms amplitude peaks correspond to the locations of fractures or lithologic changes indicated in the core photos or core logs. We then compute the hourly DAS noise rms amplitude profiles in two monitoring wells during three stimulation cycles in 72 h and find that the DAS noise rms amplitude profiles vary with time, indicating the fracture opening/growth or closing during the hydraulic stimulation. Our results demonstrate that borehole DAS passive ambient noise can be used to detect fractures and monitor fracturing processes in EGS reservoirs.

Cosmogenic background simulations for neutrinoless double beta decay with the DARWIN observatory at various underground sites

Xenon dual-phase time projections chambers (TPCs) have proven to be a successful technology in studying physical phenomena that require low-background conditions. With 40t\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$40\\,\ extrm{t}$$\\end{document} of liquid xenon (LXe) in the TPC baseline design, DARWIN will have a high sensitivity for the detection of particle dark matter, neutrinoless double beta decay (0νββ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0\\upnu \\upbeta \\upbeta $$\\end{document}), and axion-like particles (ALPs). Although cosmic muons are a source of background that cannot be entirely eliminated, they may be greatly diminished by placing the detector deep underground. In this study, we used Monte Carlo simulations to model the cosmogenic background expected for the DARWIN observatory at four underground laboratories: Laboratori Nazionali del Gran Sasso (LNGS), Sanford Underground Research Facility (SURF), Laboratoire Souterrain de Modane (LSM) and SNOLAB. We present here the results of simulations performed to determine the production rate of 137\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}^{137}$$\\end{document}Xe, the most crucial isotope in the search for 0νββ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0\\upnu \\upbeta \\upbeta $$\\end{document} of 136\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}^{136}$$\\end{document}Xe. Additionally, we explore the contribution that other muon-induced spallation products, such as other unstable xenon isotopes and tritium, may have on the cosmogenic background.

Enhancing equity, diversity, and inclusion in physics: perspectives from North American underground laboratories

Equity, Diversity, and Inclusion (EDI) are important to drive innovation in many different fields, including particle physics. Underground labs are working on many different fronts to improve EDI in their host countries and within particle physics collaborations. Laboratories can institute policies to protect their staff and make improvements to their facilities to increase accessibility. Laboratories can encourage the scientific collaborations they host to have policies and plans for increasing EDI. SNOLAB and the Sanford Underground Research Facility (SURF) are each supporting their employees and user-bases in different ways. Some examples are targeted outreach, consultation with experimental collaborations on their own policies, EDI training, and Indigenous cultural recognition. These efforts are intended to enhance the equity and inclusion of their communities.

Kubernetes for the Deep Underground Neutrino Experiment Data Acquisition

The Deep Underground Neutrino Experiment (DUNE) is a next-generation long-baseline neutrino experiment based in the USA which is expected to start taking data in 2029. DUNE aims to precisely measure neutrino oscillation parameters by detecting neutrinos from the LBNF beamline (Fermilab) at the Far Detector, 1,300 kilometres away, in South Dakota at the Sanford Underground Research Facility. The Far Detector will consist of four cryogenic Liquid Argon Time Projection Chamber detectors of 17 kT, each producing more than 1 TB/sec of data. The main requirements for the data acquisition system are the ability to run continuously for extended periods of time, with a 99% up-time requirement, and the functionality to record both beam neutrinos and low energy neutrinos from the explosion of a neighbouring supernova, should one occur during the lifetime of the experiment. The key challenges are the high data rates that the detectors generate and the deep underground environment, which places constraints on power and space. To overcome these challenges, DUNE plans to use a highly optimised C++ software suite and a server farm of about 110 nodes continuously running about two hundred multicore processes located close to the detector, 1.5 kilometres underground. Thirty nodes will be at the surface and will run around two hundred processes simultaneously. DUNE is studying the use of the Kubernetes framework to manage containerised workloads and take advantage of its resource definitions and high up-time services to run the DAQ system. Progress in deploying these systems at the CERN neutrino platform on the prototype DUNE experiments is reported.

Sanford Underground Research Facility’s approach to school education, community activities, and public outreach

The Sanford Underground Research Facility (SURF) is the deepest underground science facility in the United States. SURF hosts world-leading experiments in neutrino, astroparticle and nuclear physics, as well as projects in biology, geology, and engineering, and is home to a major excavation project making space for Fermi National Accelerator Laboratory’s Long-Baseline Neutrino Facility (LBNF), which will power the Deep Underground Neutrino Experiment (DUNE). An emphasis on outreach and education is embedded in SURF’s mission statement: “to advance world-class science and inspire learning across generations.” To achieve this mission, SURF goes beyond established science communication methods, including operating an open-to-the-public visitor center, hosting multiple public outreach events per month, and an annual city-wide science festival. Furthermore, SURF is training K-12 science educators, developing school curriculum units, and providing classroom materials, based on science researched at the laboratory. The strategic approach, specific methods, and successful outcomes of these programs, which are based on SURF’s science, location, and community, may serve as examples for effective science education, public outreach, and community engagement.

Inverse analysis of two in situ heater tests in Boom Clay to identify a range of anisotropic thermal conductivities

Design and safety assessments of geological disposals of heat-emitting radioactive waste require accurate evaluation of the thermal conductivity of the host medium. This paper presents a new approach to evaluate the anisotropic thermal conductivity of Boom Clay, a clayey layer that is studied as a potential host medium in Belgium by ONDRAF/NIRAS. The present approach is based on an inverse analysis of two in situ heater tests (the small-scale ATLAS IV Heater test and the large-scale PRACLAY Heater test) carried out in the HADES underground research facility (URF). The inverse analysis is performed by using the absolute error function and tolerant error-based method based on temperatures measured from 60 sensors across the two tests (i.e., from 29 sensors during the 900-day heating and cooling phase in the ATLAS IV Heater test and from 31 sensors during the nearly 7-year heating phase in the PRACLAY Heater test). In addition, thousands of numerical modeling cases are applied for both tests. The objective is to estimate a reasonable range of thermal conductivity variation that is characteristic of the entire host clay. Finally, a thermal conductivity range (λh, λv) = (1.55–1.95, 1.10–1.25) W/(m⋅K) is identified for the Boom Clay.

Three-dimensional distributed acoustic sensing at the Sanford Underground Research Facility

Distributed acoustic sensing (DAS) is a valuable tool for monitoring seismic signals as it provides high spatial and temporal resolution strain sensing along the length of a fiber-optic cable. DAS records the seismic wavefields essentially synchronously at each sensing location (i.e., sensing channel with gauge lengths) because the interrogator senses the distributed strain at the speed of light in the fiber. Unlike traditional seismic sensors, DAS has an intrinsic directional sensitivity to the axial strain change along the fiber, leading to difficulties when using standard seismic analysis and interpretations that rely on 3D particle velocity sensing. In addition, cable deployments on the surface can be dominated by high-amplitude wind or urban noise, impeding the detection of low-amplitude distant seismic sources. Here we investigate the capabilities of a unique 3D array with spiral-like portions in the Sanford Underground Research Facility (SURF), the former Homestake Mine, between 1250 m (4100 ft) and 1488 m (4850 ft) in depth for detecting local, regional, and teleseismic sources of ground vibrations. Our pilot array finds that DAS records high frequency (above 5 Hz) vibration sources well, such as mine activities and local and regional blasting events. Furthermore, our deployment method (fiber resting on the surface with rocks placed every meter or so) may contribute to low-frequency noise that contaminates the interpretation of teleseismic waves, particularly lower frequency S-wave arrivals. Nevertheless, this 3D DAS array provides significant data for future analysis as well as the basis for improving and expanding the array in SURF.

Muon energy reconstruction for applications in neutrino astronomy in the DUNE far detector

DUNE (Deep Underground Neutrino Experiment) is a proposed long-baseline neutrino oscillation experiment located in the United States. The main physics objectives of DUNE are to measure neutrino oscillations and interactions, search for nucleon decay, observe supernova neutrino bursts and beyond Standard Model (BSM) effects. The DUNE far detector will be located 4850 feet underground at the Sanford Underground Research Facility in Lead, South Dakota. It will house the world's largest liquid-argon time projection chambers (TPC). The DUNE far detector can detect high-energy leptons that arise from interactions of cosmogenic neutrinos and search for neutrinos originating from the decay of weakly-interactive massive particles (WIMPs) annihilations occurring inside the Sun. Selecting upward-going muons reduces the background from cosmic-ray muons. The muon energy is estimated from the electromagnetic showers accompanying the muon, a technique that allows energy reconstruction up to a few hundred TeV.

Stress field analysis: Construction of site descriptive model of underground research site in KAERI

This study aims to integrate geological investigation data and construct a rock mechanical site descriptive model relative to site selection research for deep geological disposal of spent nuclear fuel. As preliminary research for the model construction process, a sub-divided numerical domain was introduced by adopting existing fracture zone in the site region of an underground research facility in KAERI. The spatial properties of the domain were decided based on the rock classification information of core specimens obtained from several boreholes. Stress field estimation modeling was performed using the constructed domain and the results were compared with the in-situ state of the field test. When the depth-dependent stress ratio was assigned to the model condition, it was confirmed that an appropriate stress state was ensured with the similarity of the in-situ stress along the depths. Additionally, uncertainties of the input properties in the stress estimation model were also assessed. The effect of uncertainties could be considered through a sensitivity analysis of rock mass and fracture zone properties to identify stress dependence on mechanical properties. Significant stress distribution was not indicated with a variation in the rock mass properties. However, weak fracture zone modeling induced a slightly disturbed stress in a shallow depth range. These results indicated that the uncertainty of the rock mechanical site descriptive model could be increased due to the fracture zone properties. This study is a case of prior research in Korea on the site evaluation model that is already applied in leading foreign countries related to site selection research for nuclear fuel disposal. The applicability based on the achievement of this study can be improved through consideration of derived equivalent properties of fractured rock. And more reliable results can be succeeded by assigning properly defined boundary conditions for the simulation of various geological scenarios.

Energy calibration of germanium detectors for the Majorana Demonstrator

The Majorana Demonstrator was a search for neutrinoless double-beta decay (0νββ) in the 76Ge isotope. It was staged at the 4850-foot level of the Sanford Underground Research Facility (SURF) in Lead, SD. The experiment consisted of 58 germanium detectors housed in a low background shield and was calibrated once per week by deploying a 228Th line source for 1 to 2 hours. The energy scale calibration determination for the detector array was automated using custom analysis tools. We describe the offline procedure for calibration of the Demonstrator germanium detectors, including the simultaneous fitting of multiple spectral peaks, estimation of energy scale uncertainties, and the automation of the calibration procedure.

Excavation Damage Zone fracture modelling for seismic tomography: A comparison of explicit fractures and effective medium approaches

We model the full wavefield produced by a seismic velocity survey and optimise the representation of the fracture zone to best match field waveforms. The velocity survey was part of a mapping study on fractures in the Excavation Damage Zone (EDZ) of ONKALO underground research facility at Olkiluoto. The EDZ results from excavation of the rock mass, which modifies stress conditions changing the nature and behaviour of pre-existing fractures and generating new fracturing. These fractures act as the main transport pathways for contaminants both in and out of a geological disposal facility (GDF). Our goal is to test different representations of the fracture zone and to determine which models most successfully improve the interpretation of the fracture zone, producing estimates of a key unknown parameter, fracture stiffness, in addition to fracture sizes, fracture geometry, fracture density and crack density. We use modelling techniques previously tested in theoretical and laboratory studies and assess their performance on a real engineering problem. The paper introduces the field experiment and relevant information from the GDF in Finland. It describes the methodologies used for representing the fracture networks in the models — Explicit Fracture models with two approximations called Pixelised Fracture Model (PFM) and Equivalent Discrete Fracture Medium (EDFM), the Effective Medium (EM) model, and two versions of the Localised Effective Medium (LEM) model (LEM fine, LEM thick). These alternative representations were used within models of the field experiment and the calculated waveforms were used in an iterative inversion for fracture stiffness. Results show that the EM model and the EDFM model were unsuccessful in matching recorded waveforms. The fine LEM model and the explicit PFM model produced the best results especially after iterative optimisation of the fracture stiffness, giving confidence that further optimisation will lead to improved characterisation of the fracturing from the full waveform data.

First Dark Matter Search Results from the LUX-ZEPLIN (LZ) Experiment.

The LUX-ZEPLIN experiment is a dark matter detector centered on a dual-phase xenon time projection chamber operating at the Sanford Underground Research Facility in Lead, South Dakota, USA. This Letter reports results from LUX-ZEPLIN's first search for weakly interacting massive particles (WIMPs) with an exposure of 60 live days using a fiducial mass of 5.5t. A profile-likelihood ratio analysis shows the data to be consistent with a background-only hypothesis, setting new limits on spin-independent WIMP-nucleon, spin-dependent WIMP-neutron, and spin-dependent WIMP-proton cross sections for WIMP masses above 9 GeV/c^{2}. The most stringent limit is set for spin-independent scattering at 36 GeV/c^{2}, rejecting cross sections above 9.2×10^{-48} cm at the 90% confidence level.

Using in-situ strain measurements to evaluate the accuracy of stress estimation procedures from fracture injection/shut-in tests

Fracture injection/shut-in tests are commonly used to measure the state of stress in the subsurface. Injection creates a hydraulic fracture (or in some cases, opens a preexisting fracture), and then the pressure after shut-in is monitored to identify fracture closure. Different interpretation procedures have been proposed for estimating closure, and the procedures sometimes yield significantly different results. In this study, direct, in-situ strain measurements are used to observe fracture reopening and closure. The tests were performed as part of the EGS Collab project, a mesoscale project performed at 1.25 and 1.5 km depth at the Sanford Underground Research Facility. The tests were instrumented with the SIMFIP tool, a double-packer probe with a high-resolution three-dimensional borehole displacement sensor. The measurements provide a direct observation of the fracture closure signature, enabling a high-fidelity estimate of the fracture closure stress (ie, the normal stress on the fracture). In two of the four tests, injection created an opening mode fracture, and so the closure stress can be interpreted as the minimum principal stress. In the other two tests, injection probably opened preexisting natural fractures, and so the closure stress can be interpreted as the normal stress on the fractures. The strain measurements are compared against different proposed methods for estimating closure stress from pressure transients. The shut-in transients are analyzed with two techniques that are widely used in the field of petroleum engineering – the ‘tangent’ method and the ‘compliance’ method. In three of the four tests, the tangent method significantly underestimates the closure stress. The compliance method is reasonably accurate in all four tests. Closure stress is also interpreted using two other commonly-used methods – ‘first deviation from linearity’ and the method of (Hayashi and Haimson, 1991). In comparison with the SIMFIP data, these methods tend to overestimate the closure stress, evidently because they identify closure from early-time transient effects, such as near-wellbore tortuosity. In two of the tests, microseismic imaging provides an independent estimate of the size of the fracture created by injection. When combined with a simple mass balance calculation, the SIMFIP stress measurements yield predictions of fracture size that are reasonably consistent with the estimates from microseismic. The calculations imply an apparent fracture toughness 2-3x higher than typical laboratory-derived values.

Predictive and Inverse Modeling of a Radionuclide Diffusion Experiment in Crystalline Rock at ONKALO (Finland)

The REPRO-TDE test was performed at a depth of about 400 m in the ONKALO underground research facility in Finland. Synthetic groundwater containing radionuclide tracers [tritiated water tracer (HTO), 36Cl, 22Na, 133Ba, and 134Cs] was circulated for about 4 years in a packed-off interval of the injection borehole. Tracer activities were additionally monitored in two observation boreholes. The test was the subject of a modeling exercise by the SKB GroundWater Flow and Transport of Solutes Task Force. Eleven teams participated in the exercise, using different model concepts and approaches. Predictive model calculations were based on laboratory-based information concerning porosities, diffusion coefficients, and sorption partition coefficients. After the experimental results were made available, the teams were able to revise their models to reproduce the observations. General conclusions from these back-analysis calculations include the need for reduced effective diffusion coefficients for 36Cl compared to those applicable to HTO (anion exclusion), the need to implement weaker sorption for 22Na compared to results from laboratory batch sorption experiments, and the observation of large differences between the theoretical initial concentrations for the strongly sorbing 133Ba and 134Cs, and the first measured values a few hours after tracer injection. Different teams applied different concepts, concerning mainly the implementation of isotropic versus anisotropic diffusion, or the possible existence of borehole disturbed zones around the different boreholes. The role of microstructure was also addressed in two of the models.

Energy resolution of the LZ detector for high-energy electronic recoils

The LUX-ZEPLIN (LZ) detector is a dual-phase liquid xenon time projection chamber (TPC) installed at the Sanford Underground Research Facility (Lead, South Dakota) at a depth of 1478 meters. Although the main objective of LZ is the direct detection of dark matter, its low background environment allows for the search of other rare processes, such as the neutrinoless double beta decay of xenon isotopes 134Xe and 136Xe with the respective Q-values of 826 keV and 2458 keV. The sensitivity of the detector to these decays is directly determined by the energy resolution, which, in turn, is degraded by non-uniformities in detector response. In this work, we present a novel method to correct, in the data, the non-uniformity of the light collected by an array of photosensors in a scintillation detector. This method is based on the knowledge of the light response functions of individual photosensors. With these techniques, we report, at a very early phase of the detector operations, a state-of-the-art energy resolution (σ/μ) of (0.67 ± 0.01)% at 2614 keV for the fiducial volume of 5.6 tonnes of liquid xenon.