To search for rare events like dark matter, it is critical to minimize the rate of background events in the detector. Crystals of NaI(Tl) have been widely exploited by dark matter experiments, and one of these experiments called DAMA (DArk MAtter) has claimed to observe a signal due to dark matter. However, other dark matter experiments using either the same or different target materials have not observed a signal consistent with DAMA. The DAMA signal is an annual modulation in their measurement rate in the low energy region that is dominated by background events caused by both naturally occurring radioimpurities in NaI(Tl) and the radioisotopes activated in the NaI(Tl) crystal through the interaction with energetic cosmic rays, primarily neutrons. Measurement using the same target material of NaI(Tl), but with lower background than DAMA, is required to either verify or refute DAMA’s claim.

While high-purity growth of the NaI(Tl) crystal minimizes the amount of intrinsic radioimpurities, the key background arises from the cosmogenic radioisotopes such as 3H, with a long half-life of 12.3 years, that can persist in the NaI(Tl) crystal for throughout the multi-year underground operation of a dark matter detector. 

This work reports on the activation of NaI(Tl) at the Los Alamos Neutron Science Center (LANSCE) facility in November 2019. The LANSCE neutron beam has a cosmic ray--like spectrum, and was used to activate NaI(Tl) crystals with the equivalent of approximately half a million years’ worth of sea-level cosmic neutron exposure. The activation rates of short-lived and long-lived isotopes have been measured using gamma ray spectroscopy of the irradiated crystal, and interpreted with the help of a simulation model that accounts for the NaI(Tl) scintillation nonlinearity. 

The deduced radioisotope production rates will be used to help improve models that predict cosmogenic activation of NaI(Tl), as well as by dark matter experiments such as SABRE to better plan for and understand experimental cosmogenic backgrounds in the NaI(Tl). The importance of optimising crystal production pathways at sea level, as well as shielding of the crystal during storage and transport can now be quantified and optimised. The potential for using shielding made of concrete, iron and polyethylene and an innovative method for tracking the cosmic-ray exposure of the crystals by monitoring the cosmic-ray muon flux have been investigated for the transport and storage of crystals for the SABRE South experiment that is being installed in the Stawell Underground Physics Laboratory.