PNNL

Abstract

The production of radioactive isotopes by interactions of cosmic-ray particles with sodium iodide (NaI) crystals can produce radioactive backgrounds in detectors used to search for rare events. Through controlled irradiation of NaI crystals with a neutron beam that matches the cosmic-ray neutron spectrum, followed by direct counting and fitting the resulting spectrum across a broad range of energies, we determined the integrated production rate of several long-lived radioisotopes. The measurements were then extrapolated to determine the sea-level cosmogenic neutron activation rate, including the first experimental determination of the tritium activation rate ((80 ± 21) atoms/kg/day). These results will help constrain background estimates and determine the maximum time that NaIbased detectors can remain unshielded above ground before cosmogenic backgrounds impact the sensitivity of next-generation detectors.