Cerium oxidation state in LSO:Ce scintillators

Trivalent cerium ions form the luminescence centers in several important families of scintillation materials including the rare earth oxyorthosilicates, pyrosilicates, and aluminates. When comparing the experimentally determined scintillation properties of cerium-doped scintillators to theoretical models of scintillation mechanisms, there is often speculation regarding the fraction of the total cerium that exists in the radiative trivalent charge state (Ce³⁺) rather than the non-radiative tetravalent state (Ce⁴⁺). Until now, however, no technique has been developed to quantitatively measure both Ce³⁺ and Ce⁴⁺. We report here for the first time direct measurements of Ce³⁺ and Ce⁴⁺ in Lu₂SiO₅:Ce scintillators. Synchrotron radiation was used to measure the X-ray absorption on the M₄ and M₅ edges of Ce, and the results were compared to model samples of Ce³⁺ (Ce₂O₃) and Ce⁴⁺ (CeO₂) which provided clear signatures of the two charge states. The spectra were obtained with a high-resolution superconducting tunnel junction spectrometer on beamline 4.0.2 at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The results clearly show 100% Ce³⁺, independent of light yield and sample coloration. Therefore, energy migration to the luminescence centers appears to be the determining factor in the scintillation efficiency of these samples, rather than variations in the Ce³⁺/Ce⁴⁺ ratio.