The specific activity of the $$\beta $$
β
decay of $$^{39}$$
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 $$^{39}$$
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$$_\textrm{stat}$$
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.
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