Abstract

The discovery of argon from deep underground sources with significantly less ³⁹Ar than atmospheric argon was an important step in the development of direct dark matter detection experiments using argon as the active target. We report on the design and operation of a low-background single-phase liquid argon detector that was built to study the ³⁹Ar content of this underground argon. Underground argon from the Kinder Morgan CO₂ plant in Cortez, Colorado was determined to have less than 0.65% of the ³⁹Ar activity in atmospheric argon, or 6.6 mBq/kg specific ³⁹Ar activity.