Direct measurement of the $$^{39}$$Ar half-life from 3.4 years of data with the DEAP-3600 detector

Abstract The half-life of $$^{39}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>39</mml:mn> </mml:mmultiscripts> </mml:math> 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 $$^{39}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>39</mml:mn> </mml:mmultiscripts> </mml:math> 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 $$^{39}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>39</mml:mn> </mml:mmultiscripts> </mml:math> Ar decays. Observing over a period of 3.4 years, the $$^{39}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>39</mml:mn> </mml:mmultiscripts> </mml:math> Ar half-life is measured to be $$(302 \pm 8_\textrm{stat} \pm 6_\textrm{sys})$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mn>302</mml:mn> <mml:mo>±</mml:mo> <mml:msub> <mml:mn>8</mml:mn> <mml:mtext>stat</mml:mtext> </mml:msub> <mml:mo>±</mml:mo> <mml:msub> <mml:mn>6</mml:mn> <mml:mtext>sys</mml:mtext> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> years. This new direct measurement suggests that the half-life of $$^{39}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>39</mml:mn> </mml:mmultiscripts> </mml:math> Ar is significantly longer than the accepted value, with potential implications for measurements using this isotope’s half-life as input.