Abstract
LaF 3 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 LaF 3:RE 3+ (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 LaF 3: Pr 3+, LaF 3: Tb 3+, LaF 3: Ho 3+ and LaF 3: Tm 3+ the emissions from the higher excited states are weaker or absent. LaF 3: Ce 3+ and LaF 3: Gd 3+ do not exhibit thermally stimulated emission. It is proposed that upon recombination an exciton is formed, which is able to transfer its energy to Pr 3+, Tb 3+, Dy 3+, Ho 3+ and Tm 3+ at about 22000 cm -1. The lowest excited levels of Ce 3+ and Gd 3+ in this lattice are at too high energy to allow this transfer.
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