Cosmogenic activation of sodium iodide

TL;DR

This study measures the production rates of cosmogenic isotopes in sodium iodide crystals caused by cosmic-ray neutrons, providing crucial data to improve background modeling in rare event detection experiments.

Contribution

It presents the first experimental determination of tritium production rate in NaI and offers new measurements of cosmogenic activation rates relevant for detector background estimates.

Findings

First measurement of tritium production rate: (80 ± 21) atoms/kg/day

Determined cosmogenic activation rates for multiple isotopes

Results aid in background modeling for NaI-based detectors

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 production rate: $(80 \pm 21)$ atoms/kg/day. These results will help constrain background estimates and determine the maximum time that NaI-based detectors can remain unshielded above ground before cosmogenic backgrounds impact the sensitivity of next-generation experiments.