Thermoluminescence, photoluminescence and optically stimulated luminescence characteristics of CaSO₄:Eu phosphor: Experimental and density functional theory (DFT) investigations

The CaSO₄:Eu phosphor in nanocrystalline form was obtained by chemical method. The sample was annealed at various temperatures and quenched. The structural, electronic and optical properties are studied using various experimental techniques. As synthesized CaSO₄:Eu particles have nanorod shapes with diameter of ~15 nm and length of ~250 nm. After annealing (at around 900 °C) a significant increase in their size (~2–4 μm) with phase transformation from hexagonal to orthorhombic was observed. Thermoluminescence (TL) and optically stimulated luminescence (OSL) intensities were found to increase with temperature up to 900 °C and decrease thereafter for 1 Gy of test dose of β-rays from ⁹⁰Sr-⁹⁰Yr source. However, the maximum OSL sensitivity was found to be more than that of CaSO₄:Eu microcrystalline phosphor (prepared by acid recrystallization method) contrary to the usually found in the literature but much less than that of commercially available α-Al₂O₃:C phosphor. The activation energy for thermally assisted OSL process was found to be 0.0572 ± 0.0028 eV. The dose ranges of TL and OSL response was found from 0.04 Gy to 100 Gy and 0.02 Gy–100 Gy, respectively. The experimental results are also correlated with computational calculations based on density functional theory (DFT). The crystal structures and electronic structures of both hexagonal and orthorhombic CaSO₄ and CaSO₄:Eu materials show that they are direct band gap (5.67–5.86 eV) insulators, with Ca²⁺ substitution by Eu²⁺ found to introduce donor states in the band gap near Fermi level and the valence band edge of CaSO₄ on doping with Eu²⁺ impurity ions.