The SABRE (Sodium-iodide with Active Background Rejection) experiment consists of 50 kg of ultrapure NaI(Tl) crystal contained within a 10.5 ton liquid scintillator (LS) veto detector, and will search for dark matter interactions in the inner NaI(Tl) detector. The relative scintillation light yield in NaI(Tl) scintillator for different incident particle energies is not constant and is important for characterizing the detector response. The relative scintillation light yield in two different NaI(Tl) scintillators was measured with a 10 µCi 137Cs radioactive source using the Compton coincidence technique (CCT) for scattering angles 30? - 135? using electron energies ranging from 60 to 500 keVee, and these measurements are compared to the previously published results. Light yield was proportional within 3.5% at energies between 60 and 500 keVee, but non-proportionality increases drastically below 60 keVee which might be due to the non-uniform ionization density and multiple Compton scattering background events in the scintillator. An improved experimental setup with ultrapure NaI(Tl) scintillator and proper coincidence timing of radioactive events could allow scintillation light yield measurement at lower electron recoil energy. The obtained light yield non-proportionality results will be useful for the SABRE dark matter detector experiment.

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