Examination of radioargon production by cosmic neutron interactions

Radioargon isotopes, particularly ³⁷Ar, are currently being considered for use as an On-Site Inspection (OSI) relevant radionuclide within the context of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). In order to understand any soil air measurements taken during an OSI, the radioargon background due to cosmic ray induced activation along with other sources must be understood. An MCNP6 model was developed using the cosmic ray source feature within the code to examine the neutron flux at ground level as a function of various conditions: date during the solar magnetic activity cycle, latitude of sampling location, geology of the sampling location, and sampling depth. Once the cosmic neutron flux was obtained, calculations were performed to determine the rate of radioargon production for the main interactions. Radioargon production was shown to be highly dependent on the soil composition, and a range of ³⁷Ar production values at 1 m depth was found with a maximum production rate of 4.012 atoms/sec/m³ in carbonate geologies and a minimum production rate of 0.070 atoms/sec/m³ in low calcium granite. The sampling location latitude was also shown to have a measurable effect on the radioargon production rate, where the production of ³⁷Ar in an average continental crust is shown to vary by a factor of two between the equator and the poles. The sampling date's position within the solar magnetic activity cycle was also shown to cause a smaller change, less than a factor of 1.2, in activation between solar maxima and solar minima. - Highlights: • Cosmic neutron flux modeled in various geologic materials using MCNP6. • Radioargon production rate calculated in various geologic materials. • Variations in production considered for latitude, date, material, and depth. • Geology and depth have greatest impact, some latitude effect, smaller date effect