SCINTILLATION MECHANISMS IN CERIUM FLUORIDE

Ultraviolet photoelectron spectroscopy, optical transmission, fluorescence excitation spectroscopy and time-resolved fluorescence spectroscopy are used to investigate the scintillation mechanisms of cerium fluoride (CeF3) and of lanthanum fluoride doped with cerium in concentrations between 0.01 and 50mol% cerium. In LaF3:Ce, the absorption of either optical or ionizing radiation directly or indirectly results in excitation of the Ce3+ 4f electron to the lowest 5d level followed by 5d --> 4f fluorescence at 284-300 nm. Whereas for optical excitation the fluorescence has a 20 ns decay time, for ionizing radiation there is an additional faster (2-10 ns) initial decay component. As the cerium concentration increases, another band appears that partially absorbs the 284 300 nm emission and re-radiates it in a broad band peaking at 340 nm and having a longer (approximately 30 ns) decay time. In the limit of 100% CeF3, radiation trapping is very pronounced. The additional absorption and emission bands present at large Ce concentrations are attributed to Ce3+ ions in perturbed sites. The relative efficiency for excitation of unperturbed and perturbed cerium sites via the Ce3+ 5d and 6s bands, the F- 2p valence band, and the Ce3+ or La3+ 5p core levels are determined from fluorescence excitation spectra.