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The half-life of 39 Ar is measured using the DEAP-3600 detector located 2 km underground at SNOLAB. Between 2016 and 2020, DEAP-3600 used a target mass of (3269 ± 24) kg of liquid argon distilled from the atmosphere in a direct-detection dark matter search. Such an argon mass also enables direct measurements of argon isotope properties. The decay of 39 Ar in DEAP-3600 is the dominant source of triggers by two orders of magnitude, ensuring high statistics and making DEAP-3600 well-suited for measuring this isotope’s half-life. Use of the pulse-shape discrimination technique in DEAP-3600 allows powerful discrimination between nuclear recoils and electron recoils, resulting in the selection of a clean sample of 39 Ar decays. Observing over a period of 3.4 years, the 39 Ar half-life is measured to be ( 302 ± 8 stat ± 6 sys ) years. This new direct measurement suggests that the half-life of 39 Ar is significantly longer than the accepted value, with potential implications for measurements using this isotope’s half-life as input.

The knowledge of scintillation quenching of α -particles plays a paramount role in understanding α -induced backgrounds and improving the sensitivity of liquid argon-based direct detection of dark matter experiments. We performed a relative measurement of scintillation quenching in the MeV energy region using radioactive isotopes ( 222 Rn, 218 Po and 214 Po isotopes) present in trace amounts in the DEAP-3600 detector and quantified the uncertainty of extrapolating the quenching factor to the low-energy region.

The half-life of$$^{39}$$39 Ar is measured using the DEAP-3600 detector located 2 km underground at SNOLAB. Between 2016 and 2020, DEAP-3600 used a target mass of (3269 ± 24) kg of liquid argon distilled from the atmosphere in a direct-detection dark matter search. Such an argon mass also enables direct measurements of argon isotope properties. The decay of$$^{39}$$39 Ar in DEAP-3600 is the dominant source of triggers by two orders of magnitude, ensuring high statistics and making DEAP-3600 well-suited for measuring this isotope’s half-life. Use of the pulse-shape discrimination technique in DEAP-3600 allows powerful discrimination between nuclear recoils and electron recoils, resulting in the selection of a clean sample of$$^{39}$$39 Ar decays. Observing over a period of 3.4 years, the$$^{39}$$39 Ar half-life is measured to be$$(302 \\pm 8_\\textrm{stat} \\pm 6_\\textrm{sys})$$( 302 ± 8 stat ± 6 sys ) years. This new direct measurement suggests that the half-life of$$^{39}$$39 Ar is significantly longer than the accepted value, with potential implications for measurements using this isotope’s half-life as input.

Abstract The half-life of$$^{39}$$39 Ar is measured using the DEAP-3600 detector located 2 km underground at SNOLAB. Between 2016 and 2020, DEAP-3600 used a target mass of (3269 ± 24) kg of liquid argon distilled from the atmosphere in a direct-detection dark matter search. Such an argon mass also enables direct measurements of argon isotope properties. The decay of$$^{39}$$39 Ar in DEAP-3600 is the dominant source of triggers by two orders of magnitude, ensuring high statistics and making DEAP-3600 well-suited for measuring this isotope’s half-life. Use of the pulse-shape discrimination technique in DEAP-3600 allows powerful discrimination between nuclear recoils and electron recoils, resulting in the selection of a clean sample of$$^{39}$$39 Ar decays. Observing over a period of 3.4 years, the$$^{39}$$39 Ar half-life is measured to be$$(302 \\pm 8_\\textrm{stat} \\pm 6_\\textrm{sys})$$( 302 ± 8 stat ± 6 sys ) years. This new direct measurement suggests that the half-life of$$^{39}$$39 Ar is significantly longer than the accepted value, with potential implications for measurements using this isotope’s half-life as input.

Abstract The specific activity of theβ β decay of³⁹39 Ar in atmospheric argon is measured using the DEAP-3600 detector. DEAP-3600, located 2 km underground at SNOLAB, uses a total of (3269 ± 24) kg of liquid argon distilled from the atmosphere to search for dark matter. This detector is well-suited to measure the decay of³⁹39 Ar owing to its very low background levels. This is achieved in two ways: it uses low background construction materials; and it uses pulse-shape discrimination to differentiate between nuclear recoils and electron recoils. With 167 live-days of data, the measured specific activity at the time of atmospheric extraction is (0.964 ± 0.001_(\\textrm{stat}{}{})stat ± 0.024_(\\textrm{sys}{}{})sys ) Bq/kg_(\\textrm{atmAr}{}{})atmAr , which is consistent with results from other experiments. A cross-check analysis using different event selection criteria and a different statistical method confirms the result.

In the DEAP-3600 dark matter search experiment, precise reconstruction of the positions of scattering events in liquid argon is key for background rejection and defining a fiducial volume that enhances dark matter candidate events identification. This paper describes three distinct position reconstruction algorithms employed by DEAP-3600, leveraging the spatial and temporal information provided by photomultipliers surrounding a spherical liquid argon vessel. Two of these methods are maximum-likelihood algorithms: the first uses the spatial distribution of detected photoelectrons, while the second incorporates timing information from the detected scintillation light. Additionally, a machine learning approach based on the pattern of photoelectron counts across the photomultipliers is explored.

We report experimental evidence for electron neutrino charged-current interactions (neutrino absorption, CC \\(_e\\)) from \\(^8\\)B solar neutrinos on \\(^40\\)Ar using an exposure of (\\(7.29 0.05\\)) tonne\\(\\)years in the DEAP-3600 detector. A region of interest (ROI) of 10.5-13.0 MeV reconstructed energy calibrated on single-peak events, corresponding to incident neutrino energy in 12.0-14.5 MeV, is used for this measurement. We observe 5 single-peak and 1 double-peak neutrino-like events consistent with the \\(^8\\)B solar neutrino energy spectrum in the ROI after correcting for nonlinearities in the detector response at high energies. With an expected background of \\(0.48~^+0.16_-0.15\\) events, the data correspond to a significance of \\(4.0\\,\\) with respect to the background-only hypothesis. We report an energy-averaged cross section of \\((4.0~^+2.0_-1.6~(stat)~^+0.8_-0.7~(sys)) 10^-41\\,cm^2\\) in the ROI for the CC \\(_e\\) signal, a factor \\((2.4~^+1.3_-1.0)\\) higher than predicted by Bhattacharya, Goodman and García (2009).

We present here a search for WIMP dark matter using 790.8 live-days of data collected with 3269 kg of liquid argon (1266 kg fiducial) by the DEAP-3600 detector at SNOLAB, using the Profile Likelihood Ratio method. The likelihood model is based on three parameters: estimated energy, pulse-shape discrimination parameter, and reconstructed position within the detector. Using this method, the expected signal sensitivity of DEAP-3600 benefits from an increased fiducial volume and improved event selection acceptance. Alpha-decays from a small number of dust particulates circulating within the liquid argon target are the dominant source of background events and limit the sensitivity of this search. This result provides improved exclusion upper limits on the WIMP-nucleon spin-independent cross section on liquid argon for WIMP masses between 20 GeV/\\(c^{2}\\) and 100 GeV/\\(c^{2}\\). At 100 GeV/\\(c^{2}\\) the observed limit is 3.4 \\(\\times\\) 10\\(^{-45}\\) cm\\(^2\\) at 90% confidence level.

The knowledge of scintillation quenching of \\(\\)-particles plays a paramount role in understanding \\(\\)-induced backgrounds and improving the sensitivity of liquid argon-based direct detection of dark matter experiments. We performed a relative measurement of scintillation quenching in the MeV energy region using radioactive isotopes (\\(^222\\)Rn, \\(^218\\)Po and \\(^214\\)Po isotopes) present in trace amounts in the DEAP-3600 detector and quantified the uncertainty of extrapolating the quenching factor to the low-energy region.

The half-life of \\(^{39}\\)Ar is measured using the DEAP-3600 detector located 2 km underground at SNOLAB. Between 2016 and 2020, DEAP-3600 used a target mass of (3269 \\(\\pm\\) 24) kg of liquid argon distilled from the atmosphere in a direct-detection dark matter search. Such an argon mass also enables direct measurements of argon isotope properties. The decay of \\(^{39}\\)Ar in DEAP-3600 is the dominant source of triggers by two orders of magnitude, ensuring high statistics and making DEAP-3600 well-suited for measuring this isotope's half-life. Use of the pulse-shape discrimination technique in DEAP-3600 allows powerful discrimination between nuclear recoils and electron recoils, resulting in the selection of a clean sample of \\(^{39}\\)Ar decays. Observing over a period of 3.4 years, the \\(^{39}\\)Ar half-life is measured to be \\((302 \\pm 8_{\\rm stat} \\pm 6_{\\rm sys})\\) years. This new direct measurement suggests that the half-life of \\(^{39}\\)Ar is significantly longer than the accepted value, with potential implications for measurements using this isotope's half-life as input.