Cosmogenic Isotope Production in Argon

Ar-39, a primarily cosmogenic isotope produced by neutron interactions with argon in the atmosphere, is the dominant background for argon-based dark matter detectors. It has been demonstrated that argon gas derived from underground sources has significantly reduced concentrations of 39Ar compared to atmospheric argon. To be used in dark matter detectors, gas extracted from underground must undergo substantial processing and purification in order to reach the purity required. Cosmogenic activation of the underground argon during this processing is a potential concern for dark matter experiments. In this talk we will review the current best estimates of the cosmogenic activation rates of 39Ar in argon, highlighting the lack of experimental measurements and the wide range of predicted cross sections. We will describe our experimental effort (currently underway) to measure the activation rate using a high energy neutron beam at the Los Alamos Neutron Science Center (LANSCE) and subsequent measurement of the 39Ar in ultra-low background proportional counters at PNNL. We will also briefly discuss other cosmogenic isotopes that are of interest to low radioactivity argon-based detectors. This presentation was used for the Low-Radioactivity Underground Argon Workshop held at Pacific Northwest National Laboratory in Richland, Washington on March 19 - 20, 2018