Electromagnetic Backgrounds and Potassium-42 Activity in the DEAP-3600 Dark Matter Detector

Abstract

The DEAP-3600 experiment is searching for WIMP dark matter with a 3.3 tonne single phase liquid argon (LAr) target, located 2.1 km underground at SNOLAB. The experimental signature of dark matter interactions is keV-scale ${}^{40}$Ar nuclear recoils (NR) producing 128 nm LAr scintillation photons observed by PMTs. The largest backgrounds in DEAP-3600 are electronic recoils (ER) induced by $\beta$ and $\gamma$-rays originating from internal and external radioactivity in the detector material. A background model of the ER interactions in DEAP-3600 was developed and is described in this work. The model is based on several components which are expected from radioisotopes in the LAr, from ex-situ material assay measurements, and from dedicated independent in-situ analyses. This prior information is used in a Bayesian fit of the ER components to a 247.2 d dataset to model the radioactivity in the surrounding detector materials. While excellent discrimination between ERs and NRs is reached with pulse shape discrimination, utilizing the large difference between fast and slow components of LAr scintillation light, detailed knowledge of the ER background and activity of detector components, sets valuable constraints on other key types of backgrounds in the detector: neutrons and alphas. In addition, the activity of ${}^{42}$Ar in LAr in DEAP-3600 is determined by measuring the daughter decay of ${}^{42}$K. This cosmogenically activated trace isotope is a relevant background at higher energies for other rare event searches using atmospheric argon e.g. DarkSide-20k, GERDA or LEGEND. The specific activity of ${}^{42}$Ar in the atmosphere is found to be $40.4\pm 5.9$ $\mu$Bq/kg of argon.

References in corpus (15)

• Planck 2015 results. XIII. Cosmological parameters
• Luminosity determination in pp collisions at $\sqrt{s}$ = 8 TeV using the ATLAS detector at the LHC
• BAT - The Bayesian Analysis Toolkit
• DarkSide-50 532-day Dark Matter Search with Low-Radioactivity Argon
• The event generator DECAY4 for simulation of double beta processes and decay of radioactive nuclei
• Search for dark matter with a 231-day exposure of liquid argon using DEAP-3600 at SNOLAB
• Measurement of the specific activity of Ar-39 in natural argon
• Design and Construction of the DEAP-3600 Dark Matter Detector
• First results from the DEAP-3600 dark matter search with argon at SNOLAB
• The background in the neutrinoless double beta decay experiment GERDA
• Measurement of the scintillation time spectra and pulse-shape discrimination of low-energy beta and nuclear recoils in liquid argon with DEAP-1
• In-situ characterization of the Hamamatsu R5912-HQE photomultiplier tubes used in the DEAP-3600 experiment
• Backgrounds and pulse shape discrimination in the ArDM liquid argon TPC
• On concentration of ${}^{42}$Ar in the Earth's atmosphere
• Liquid Argon Ionization Detector for Double Beta Decay Studies

Cited by in corpus (17)

• First direct detection constraints on Planck-scale mass dark matter with multiple-scatter signatures using the DEAP-3600 detector
• Constraints on dark matter-nucleon effective couplings in the presence of kinematically distinct halo substructures using the DEAP-3600 detector
• Physics reach of a low threshold scintillating argon bubble chamber in coherent elastic neutrino-nucleus scattering reactor experiments
• Direct Detection of Dark Matter -- APPEC Committee Report
• Pulseshape discrimination against low-energy Ar-39 beta decays in liquid argon with 4.5 tonne-years of DEAP-3600 data
• The liquid-argon scintillation pulseshape in DEAP-3600
• Sensitivity projections for a dual-phase argon TPC optimized for light dark matter searches through the ionization channel
• Large Low Background kTon-Scale Liquid Argon Time Projection Chambers
• Cosmogenic activation in double beta decay experiments
• Precision Measurement of the Specific Activity of ${}^{39}$Ar in Atmospheric Argon with the DEAP-3600 Detector
• The science and technology of liquid argon detectors
• Recent results from DEAP-3600
• Study of cosmogenic activation above ground for the DarkSide-20k experiment
• Ambient Dose and Dose Rate Measurement in SNOLAB Underground Laboratory at Sudbury, Ontario, Canada
• Direct Measurement of the ${}^{39}$Ar Half-life from 3.4 Years of Data with the DEAP-3600 Detector
• Mitigation Strategies for ${}^{42}$Ar/${}^{42}$K Background Reduction using Encapsulation with Ultra-Pure Plastic for the LEGEND Experiment
• Study of cosmogenic activation above ground of Ar for DarkSide-20k