Luminescence of New Storage Phosphors: Alkaline Earth Fluoro-Halides Doped with Divalent Ytterbium

Abstract

A storage phosphor is a material that is able to store energy from incident radiation. When a crystalline insulator is irradiated with X-rays, electrons may be promoted to the conduction band. These mobile electrons may be trapped at e.g. anion vacancies. An analog process takes place for the hole which is left in the valence band. When an impurity, such as Eu²⁺, is present, the hole may be trapped at or near it. If the energy difference between the trap and the band is large enough the electron and hole will remain separated at room temperature for a considerable time. When the electron is released from its trap by either thermal stimulation or irradiation with an appropriate wavelength, recomination on the hole trap occurs, giving rise to characteristic emission in the case of Eu²⁺. The role of Eu²⁺ in the trapping process is still under discussion; in the present study a comparison is made between Eu²⁺ and Yb²⁺ who both may act as a hole trap in alkaline earth fluorohalides (MFX; M=Ca,Sr,Ba; X=C1,Br). It is shown that Yb²⁺ has a 5d 4 4f emission in the Ca and Sr compounds whereas there is no luminescence in BaFX:Yb²⁺. The thermoluminescence behavior of the Yb-doped Ca and Sr fluorohalides is the same as that of the Eu-doped fluorohalides; hence it is concluded that the role of the Yb²⁺ and Eu²⁺ in the trapping process is the same.