Precision measurement of the specific activity of ³⁹ Ar in atmospheric argon with the DEAP-3600 detector

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.