Research on Efficient Fast Scintillators: Evidence and X‐Ray Absorption Near Edge Spectroscopy Characterization of Ce⁴⁺ in Ce³⁺, Mg²⁺‐Co‐Doped Gd₃Al₂Ga₃O₁₂ Garnet Crystal

Herein, the most advanced oxide scintillator Ce 3+ , Mg 2+ ‐co‐doped Gd 3 Al 2 Ga 3 O 12 (GAGG) garnet host, grown using the Czochralski method is reported. The charge transfer absorption band in UV, such as in other Ce 3+ , Mg 2+ ‐co‐doped oxides, reveals the creation of stable Ce 4+ oxidation state, in addition to the usual Ce 3+ one, admitted as responsible of the scintillation mechanism improvement displayed by this garnet. Strangely, the confirmation of quantitative values of Ce 4+ concentration has never been done, and the goal is mainly focused on the presence of Ce 4+ and the evaluation of the Ce 3+ /Ce 4+ ratio measured by X‐ray absorption near edge spectroscopy (XANES) spectroscopy at the Ce L III threshold of the European Synchrotron Radiation Facility (ESRF). This result is compared with those obtained for Ce 3+ , Mg 2+ ‐co‐doped Lu 3 Al 5 O 12 (LuAG) and, also, in garnets without any Mg 2+ ions such as Ce 3+ ‐doped GAGG, Ce 3+ ‐doped LuAG, and Ce 3+ ‐doped YAG. The role of Li + ion in place of Mg 2+ one is also considered in Ce 3+ , Li + ‐co‐doped LuAG single crystal.