Cosmogenic background is increasingly recognized as being important for dark matter direct detection experiments, especially as other sources of background have been well-understood and massively reduced, to increase the sensitivity for detecting rare dark matter events. SABRE is a local dark matter experiment that will use ultrapure NaI(Tl) scintillators at the Stawell Underground Physics Laboratory. While high purity growth of the NaI(Tl) crystal for SABRE minimises the amount of intrinsic radioactivity, the key background arises from the cosmogenic radioisotopes generated when exposed to cosmic rays above ground can persist in the NaI(Tl) crystal for years. 

Despite its importance, there have been no reliable measurements that can quantify the cosmogenic radioisotope yields. Therefore, we conducted an experiment to mimic cosmic activation of a NaI(Tl) crystal at the LANSCE neutron beam facility at Los Alamos, U.S.A. in 2019. The NaI(Tl) crystals were irradiated for several days using a cosmic-like neutron beam with a flux ~108 times larger than the natural flux at sea level, which allowed an amount of the cosmogenic isotopes to be created in the crystals that is equivalent to around 2,000,000 years of natural exposure.

Two measurements and one simulation have been conducted to analyse the cosmic activation. One measurement was done shortly after the irradiation to evaluate the short-lived cosmogenic isotopes. Another measurement is being performed at Yale University to directly count the long-lived cosmogenic isotopes in the NaI(Tl) crystal.  Combined with the dedicated Geant4 simulation that allows a dual-validation of the measurement, a model describing the cosmically activated products and how they change with time in the NaI(Tl) crystal will be accurately mapped.

Ultimately, the cross-sections of the cosmogenic isotopes evaluated from the two measurements and the enhanced Geant4 simulation models will enable development of suitable transport shielding for the SABRE NaI(Tl) crystals and hence a decreased and better understood background. This thorough study will improve the SABRE sensitivity and also inform the development and analysis of other NaI(Tl) based experiments.

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