Eu²⁺ concentration effects in KCa_0.8Sr_0.2I₃:Eu²⁺: A novel high-performance scintillator

We report here the effect of Eu²⁺ concentration in KCa_0.8Sr_0.2I₃:Eu²⁺ single crystal scintillators. KCa_0.8Sr_0.2I₃:Eu²⁺ single crystals doped with 0.5, 1, 3, 5, and 7 mol% Eu²⁺ were grown by the Bridgman method. The effects of varying Eu²⁺concentration and crystal volume on the scintillation properties, including light yield, energy resolution, nonproportionality, scintillation decay time and afterglow level, were systematically investigated. For 5 mm×5 mm×5 mm samples, the best light yield of 86,000±4000 photons/MeV was achieved with a content of 5 mol% Eu²⁺; its energy resolution of 2.5% at 662 keV was comparable to that of LaBr₃:Ce³⁺ and SrI₂:Eu²⁺. With larger samples of about 2.2 cm³, the best performances achieved were for 3 mol% Eu²⁺ concentration, i.e. a light yield of 76,000±4000 photons/MeV and an energy resolution of 3% at 662 keV. A direct correlation between nonproportionality and Eu²⁺ concentration was found. A continuous lengthening of scintillation decay time and x-ray induced afterglow level with increasing Eu²⁺ concentration was observed. The self-absorption effect was evaluated by using the Stokes shift and the temperature dependence of the photoluminescence decay (PL) of the Eu²⁺ centers. The sample with the highest dopant concentration had more severe temperature quenching of the Eu²⁺5d-4f emission than the sample with the lowest dopant concentration, which could be ascribed to the thermally activated concentration quenching.