New Measurement of $^{39}$Ar in Underground Argon with a Low Background Liquid Argon Detector

A low background liquid argon detector has been developed for sensitive measurements of the beta radioactive $^{39}$Ar in argon from underground sources. The measurement is motivated by the need to improve on earlier studies that showed no sign of $^{39}$Ar in certain sources of underground argon, but with a limited sensitivity of $\sim$ 5{\%} relative to $^{39}$Ar in atmospheric argon[1]. We will report preliminary measurements taken with the low background detector that was commissioned and operated at the Kimballton Underground Research Facility (KURF) in Virginia. A combination of passive and active background reduction techniques resulted in a very low background and a null result with sensitivity to~$^{39}$Ar less than 1{\%} of atmospheric. The results confirm that underground argon is well suited for direct detection of dark matter WIMPs. \\[4pt] [1] D. Acosta-Kane et al., Nucl. Instr. Meth. A \textbf{587}:46 (2008)