Scintillation mechanism in RE-activated fluorides

In this Communication we present and discuss results of VUV spectroscopic experiments on selected fluorides activated by rare-earth ions. Excitation spectra of luminescence in the vicinity of band-gap energies as well as scintillation rise and decay time measurements are used to identify and evaluate various processes likely to contribute to the host-to-ion energy transfer. We argue that the dominant process in those fluorides that scintillate efficiently (e.g. CaF2:Eu, BaF2:Ce), involves sequential trapping of charge carriers by a rare-earth activator followed by a radiative transition within the ion itself. We analyze our results in the frame-work of a simple model, which assumes that there are two possible routes of radiative recombination via activating ions. These routes are characterized by the sequence in which charge trapping occurs which, in turn, depends strongly on the ion and the host. We conclude that the quasi-stability of alternative charge states of the activating ion provides a useful criterion to evaluate scintillation performance of RE-activated fluorides.