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The DEAP-3600 detector searches for the scintillation signal from dark matter particles scattering on a 3.3 tonne liquid argon target. The largest background comes from 39Ar beta decays and is suppressed using pulse-shape discrimination (PSD). We use two types of PSD estimator: the prompt-fraction, which considers the fraction of the scintillation signal in a narrow and a wide time window around the event peak, and the log-likelihood-ratio, which compares the observed photon arrival times to a signal and a background model. We furthermore use two algorithms to determine the number of photons detected at a given time: (1) simply dividing the charge of each PMT pulse by the mean single-photoelectron charge, and (2) a likelihood analysis that considers the probability to detect a certain number of photons at a given time, based on a model for the scintillation pulse shape and for afterpulsing in the light detectors. The prompt-fraction performs approximately as well as the log-likelihood-ratio PSD algorithm if the photon detection times are not biased by detector effects. We explain this result using a model for the information carried by scintillation photons as a function of the time when they are detected.

The amount of energy released by a nuclear recoil ionizing the atoms of the active volume of detection appears “quenched” compared to an electron of the same kinetic energy. This different behavior in ionization between electrons and nuclei is described by the Ionization Quenching Factor (IQF) and it plays a crucial role in direct dark matter searches. For low kinetic energies (below 50keV), IQF measurements deviate significantly from common models used for theoretical predictions and simulations. We report measurements of the IQF for proton, an appropriate target for searches of Dark Matter candidates with a mass of approximately 1GeV, with kinetic energies in between 2keV and 13keV in 100mbar of methane. We used the Comimac facility in order to produce the motion of nuclei and electrons of controlled kinetic energy in the active volume, and a NEWS-G SPC to measure the deposited energy. The Comimac electrons are used as a reference to calibrate the detector with 7 energy points. A detailed study of systematic effects led to the final results well fitted by IQF(EK)=EKα/(β+EKα) with α=0.70±0.08 and β=1.32±0.17. In agreement with some previous works in other gas mixtures, we measured less ionization energy than predicted from SRIM simulations, the difference reaching 33% at 2keV.

SNO+ is a multipurpose detector built to investigate neutrinoless double-beta decay and neutrino oscillations. The Scattering Module is part of the SNO+ calibration program, and is designed to measure its scattering properties using laser pulses sent into the detector at different wavelengths. We describe the modules hardware and outline the general strategy for measuring the scattering length using a cut based analysis selecting direct in-beam photons and Rayleigh scattered photons. The cut verification process for a water-filled detector is presented. The efficiency of selecting scattered photons is found to be > 60% with a purity in excess of 95%.

SNO+ aims to conduct a world leading search for neutrino-less double beta decay of 130Te with a 5 year half-life sensitivity of 1.9×1026 years using 3.9 tonnes of natural Tellurium isotropically loaded in 780 tonnes of liquid scintillator. The total background budget within 0.5σ to 1.5σ of the 0νββ energy is 13.4 events per year, dominated by 8B solar neutrinos. We discuss SNO+ analysis strategies to measure the residual Uranium and Thorium chain backgrounds through the timing coincidence of short half-life components 214Bi - 214Po and 212Bi - 212Po. We show that these so-called Bi-Po events can be rejected from the 0νββ region of interest (ROI) with 99.995% efficiency and minimal (<2%) signal sacrifice. Pure samples of Bi-Pos can also be created to accurately measure the rates of Uranium and Thorium decays in-situ. In a fraction of decays, the Polonium alpha decay will occur within the same trigger window as the beta, resulting in a higher energy 'pile-up' event. Separate classifications based on the hit timing structure efficiency reject these events with minimal (<1%) signal loss. A final class of background events results from the pile up of higher frequency low energy backgrounds with events such as 2νββ, which would not otherwise contribute to the ROI. We present a final set of of event classifiers developed specifically to reject these events and show that their contribution to the double beta analysis can be reduced to negligible levels.

SNO+ is a neutrinoless double beta decay and low energy neutrino experiment located in Sudbury, Canada. To improve our understanding of the detector energy resolution and systematics, calibration systems have been developed to continuously monitor the optical properties of the detector, such as absorption, reemission and scattering. This poster provides an overview of the scattering calibration system: the Scattering Module of the Embedded LED/Laser Light Injection Entity (SMELLIE), designed to measure the scattering length in situ, over a wavelength range of 375nm - 700nm. We present analyses for both water and scintillator filled detector states.

DEAP-3600 is a liquid-argon scintillation detector looking for dark matter. Scintillation events in the liquid argon (LAr) are registered by 255 photomultiplier tubes (PMTs), and pulseshape discrimination (PSD) is used to suppress electromagnetic background events. The excellent PSD performance of LAr makes it a viable target for dark matter searches, and the LAr scintillation pulseshape discussed here is the basis of PSD. The observed pulseshape is a combination of LAr scintillation physics with detector effects. We present a model for the pulseshape of electromagnetic background events in the energy region of interest for dark matter searches. The model is composed of (a) LAr scintillation physics, including the so-called intermediate component, (b) the time response of the TPB wavelength shifter, including delayed TPB emission at O (ms) time-scales, and c) PMT response. TPB is the wavelength shifter of choice in most LAr detectors. We find that approximately 10% of the intensity of the wavelength-shifted light is in a long-lived state of TPB. This causes light from an event to spill into subsequent events to an extent not usually accounted for in the design and data analysis of LAr-based detectors.

Aria is a plant hosting a 350m cryogenic isotopic distillation column, the tallest ever built, which is being installed in a mine shaft at Carbosulcis S.p.A., Nuraxi-Figus (SU), Italy. Aria is one of the pillars of the argon dark-matter search experimental program, lead by the Global Argon Dark Matter Collaboration. It was designed to reduce the isotopic abundance of 39Ar in argon extracted from underground sources, called Underground Argon (UAr), which is used for dark-matter searches. Indeed, 39Ar is a β-emitter of cosmogenic origin, whose activity poses background and pile-up concerns in the detectors. In this paper, we discuss the requirements, design, construction, tests, and projected performance of the plant for the isotopic cryogenic distillation of argon. We also present the successful results of the isotopic cryogenic distillation of nitrogen with a prototype plant.

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 specific activity of the β decay of 39 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 of 39 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 stat ± 0.024 sys ) Bq/kg 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.

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.