Session J10: Minisymposium: Low Radioactivity Background Techniques in Underground Experiments - I

Low Radioactivity Background Techniques

Ultra-low-background experiments address some of the most important open questions in particle physics, astrophysics and cosmology today: the nature of dark matter, whether the neutrino is its own antiparticle and does the proton decay. To address these questions, rare event searches require well-understood and minimized backgrounds. Experimentalist rely on a variety of assay methods and techniques to characterize the radioactivity and impurities in their detectors, shielding and constructions materials which produce backgrounds. In this talk, I will review the current techniques and methods used to assay materials used in the construction of these experiments and highlight the assay challenges the experimental community will face as we continue to push towards the next generation of experiments that will address these important questions.   

Improving Capabilities of Low-Background Assay at SNOLAB

SNOLAB is a deep-underground clean laboratory for particle, nuclear, and astro physics. as well as extreme biology and other underground science. SNOLAB has recently, and is continuing to, expanded capabilities for low-background measurements on support of the SNOLAB and world-wide program. The capabilities include gamma counting, surface alpha counting, ultra-low-background radon emanation, and x-ray fluorescence. Chemical assay is expanding with ICP - Atomic Emission Spectroscopy, particulate microscopy, and (soon) Flame Atomic Absorption spectroscopy. For plastics and organic liquids, concentration by vaporization is available. We are looking to expand our capabilities and are looking for input from the global community to help guide further efforts.   

Measurement of the Davis Cavern Gamma-Ray Background at the Sanford Underground Research Facility

The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a liquid xenon TPC located within the Davis cavern at the Sanford Underground Research Laboratory, Lead, South Dakota, 4850 feet below the surface. Whilst the underground environment is ideal for low background searches due to the attenuation of cosmic rays, it introduces a new background from gamma-rays emitted from the decays of $^{\mathrm{40}}$K and the $^{\mathrm{238}}$U and $^{\mathrm{232}}$Th chains within the cavern rock. I will report on a series of gamma-ray measurements performed inside the cavern with a sodium iodide detector, the determination of rock isotope concentrations using comparison to a simulated model and the expected consequences for the LZ experiment.   

Simulations of external backgrounds at SURF for the LUX and LZ experiments

As underground experiments searching for rare physics events continue to achieve unprecedented sensitivities, it is important to establish whether or not detector performance will be limited by external backgrounds. Monte-Carlo simulations are an important tool in these efforts, and in this talk we will present a full characterization of cosmic-ray muons, gammas and neutrons coming from the cavern walls of the Davis Laboratory at SURF (Sanford Underground Research Facility). This laboratory hosted the LUX (Large Underground Xenon) direct dark matter experiment and will also host its successor, the LZ (LUX-ZEPLIN) experiment. The implications of these external backgrounds on the sensitivity of both detectors will be discussed.   

The Active Veto System for the Large Underground Xenon (LUX) Experiment and Underground Muon Signals

The Large Underground Xenon (LUX) Experiment successfully finished its operation searching for dark matter at the Sanford Underground Research Facility (SURF) in 2016. The 350 kg liquid xenon time projection chamber (TPC) operated in a large instrumented water shield during its lifespan. Besides passively shielding low energy electromagnetic particles and absorbing thermal neutrons, the water shield worked as an active water Cherenkov detector that registered events associated with high energy muons. Since the same water tank will house the LUX-ZEPLIN (LZ) detector, understanding the expected external background is of special importance. This talk describes the pulse shape discrimination techniques used in LUX to isolate the muon events from in-situ background signals, as well as comparisons to the simulated results in an attempt to understand its accuracy for use in LZ. The overall muon rate agrees within 15\% with the simulation. Studies of seasonal variation and events in coincidence with the LUX detector are also presented.