PNNL

Abstract

We have experimentally determined the production rate of 39Ar and 37Ar from cosmic ray neutron interactions in argon at sea level. Understanding these production rates is important for argon-based dark matter experiments that plan to utilize argon extracted from deep underground because it is imperative to know what the ingrowth of 39Ar will be during the production, transport, and storage of the underground argon. These measurements also allow for the prediction of 39Ar and 37Ar concentrations in the atmosphere which can be used to determine the presence of other sources of these isotopes. Through controlled irradiation with a neutron beam that mimics the cosmic ray neutron spectrum, followed by direct counting of 39Ar and 37Ar decays with sensitive ultra-low background proportional counters, we determined that the production rate from cosmic ray neutrons at sea-level is expected to be (759 ± 128) atoms/kg/day for 39Ar, and (51.0 ± 7.4) atoms/kg/day for 37Ar. We also performed a survey of the alternate production mechanisms based on the state-of-knowledge of the associated cross-sections to obtain a total sea-level cosmic ray production rate of (1048 ± 133) atoms/kg/day for 39Ar, (56.7 ± 7.5) atoms/kg/day for 37Ar in underground argon, and (92 ± 13) atoms/kg/day for 37Ar in atmospheric argon.