Cosmogenic production of Ar-39 and Ar-37 in argon

摘要

We have experimentally determined the production rate of Ar-39 and Ar-37 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 Ar-39 will be during the production, transport, and storage of the underground argon. These measurements also allow for the prediction of Ar-39 and Ar-37 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 Ar-39 and Ar-37 decays with sensitive ultralow background proportional counters, we determined that the production rate from cosmic ray neutrons at sea level is expected to be (759 +/- 128) atoms/(kg(Ar)day) for Ar-39, and (51.0 +/- 7.4) atoms/(kg(Ar)day) for Ar-37. 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(Ar)day) for Ar-39, (56.7 +/- 7.5) atoms/(kg(Ar)day) for Ar-37 in underground argon, and (92 +/- 13) atoms/(kg(Ar)day) for Ar-37 in atmospheric argon.