Cosmogenic Activation of Radioisotopes in NaI(Tl) Detectors*

    A challenge in rare event searches that use thallium-doped sodium iodide, NaI(Tl), detectors is maintaining low background levels. A prominent application of these detectors is the dark matter direct detection field. COSINE-100, SABRE, and ANAIS-112 are NaI(Tl)-based dark matter experiments seeking improved detector sensitivity to test the DAMA/LIBRA experiment’s long-standing claim of a dark matter discovery. A primary low energy background component, below 10 keV, is tritium, one of the radioisotopes produced in NaI(Tl) by cosmic ray activation.

    We present the results of a measurement for cosmogenic isotope production rates in NaI(Tl) detectors activated at the Los Alamos Neutron Science Center which has a neutron beam energy spectrum similar to cosmic ray neutrons. After irradiation, isotope production rates are measured and extrapolated to determine production rates by cosmic rays. This enables determination of acceptable above-ground residency times for NaI(Tl) detectors, allowing future NaI(Tl)-based experiments to meet background activity goals.