Comparison of Ce$^{3+}$ and Pr$^{3+}$ activators in alkaline-earth   fluoride crystals
  The emission spectra of Ce$^{3+}$ or Pr$^{3+}$ doped CaF$_2$, SrF$_2$, BaF$_2$ excited by vacuum ultraviolet photons or by x-ray as well as excitation and absorption spectra in vacuum ultraviolet region (6-13 eV) were studied. The transfer of exciton energy is the main channel for Ce$^{3+}$ excitation in alkaline-earth fluorides. Three different stages of energy transfer were observed. Pr$^{3+}$ excited by two processes, slow f-f luminescence excited by excitons, fast d-f luminescence excited by some fast process. 
E.Radzhabov and A.Nepomnyaschikh
cond-mat.mtrl-sci