EPR and luminescence studies on high-valency uranium centres in CaF2

Results of the combined EPR and luminescence measurements on white cloudy samples of fluorite, containing hexavalent and pentavalent uranium, are reported. The prevailing U5+ EPR spectrum, having very low symmetry, is analysed. Studies of the changes of the luminescence and EPR spectra in terms of gamma -dose give a direct proof that the main luminescent U6+ centre in fluorite has a trigonally distorted octahedral (UO66-) structure. A discussion of the vibronic structure for two luminescent centres is given. The dependence of the luminescence amplitude on the frequency of the chopped pump light in a chronospectroscopic experiment is also interpreted.