ON THE RECOMBINATION MECHANISM IN X-RAY STORAGE PHOSPHORS BASED ON LANTHANUM FLUORIDE

LaF3 doped with trivalent rare earths can act as an X-ray storage phosphor. Trivalent luminescent rare earth ions are used as a probe to elucidate the recombination mechanism. The photoluminescence emission spectra of LaF3:RE3+ (RE = Ce, Pr, Gd, Tb, Dy, Ho, Tm) are compared to those of the thermally stimulated luminescence (TSL) after X-ray irradiation. In the TSL spectra of LaF3: Pr3+, LaF3: Tb3+, LaF3: Ho3+ and LaF3: Tm3+ the emissions from the higher excited states are weaker or absent. LaF3: Ce3+ and LaF3: Gd3+ do not exhibit thermally stimulated emission. It is proposed that upon recombination an exciton is formed, which is able to transfer its energy to Pr3+, Tb3+, Dy3+, Ho3+ and Tm3+ at about 22 000 cm-1. The lowest excited levels of Ce3+ and Gd3+ in this lattice are at too high energy to allow this transfer.