Observation on long afterglow of Tb in CaWO4

The Tb³⁺ doped CaWO₄ phosphors are synthesized via high temperature solid state reaction. The X-ray diffraction shows that small amount of Tb³⁺ does not have a significant influence on the structure of CaWO₄. A broad absorption band of the WO₄²⁻ group is observed from photoluminescence and the energy transfer from WO₄²⁻ group to Tb³⁺ ions induces the f-f transition. The cross-relaxation between two adjacent Tb³⁺ ions weakens ⁵D₃-⁷F_j transitions and enhances the ⁵D₄-⁷F_j transitions, leading to a green long afterglow of the phosphors. The thermoluminescence curves centered around 75 °C reveal the trap depth for afterglow generation is about 0.74-0.77 eV. The optimum Tb³⁺ concentration for afterglow properties is about 1%. A deep hole trap is induced when Tb³⁺ concentration exceeds 1% and it suppresses the thermoluminescence and the decay properties.