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

Abstract The specific activity of the $$\beta $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>β</mml:mi> </mml:math> decay of $$^{39}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>39</mml:mn> </mml:msup> </mml:math> 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 $$^{39}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mn>39</mml:mn> </mml:msup> </mml:math> 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 $$_\textrm{stat}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow/> <mml:mtext>stat</mml:mtext> </mml:msub> </mml:math> ± 0.024 $$_\textrm{sys}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow/> <mml:mtext>sys</mml:mtext> </mml:msub> </mml:math> ) Bq/kg $$_\textrm{atmAr}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow/> <mml:mtext>atmAr</mml:mtext> </mml:msub> </mml:math> , 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.