To control the time-response performance of widely used cerium-activated scintillators in cutting-edge medical-imaging devices, such as time-of-flight positron-emission tomography, a comprehensive understanding of the role of Ce valence states, especially stable ${\mathrm{Ce}}^{4+}$, in the scintillation mechanism is essential. However, despite some progress made recently, an understanding of the physical processes involving ${\mathrm{Ce}}^{4+}$ is still lacking. The aim of this work is to clarify the role of ${\mathrm{Ce}}^{4+}$ in scintillators by studying ${\mathrm{Ca}}^{2+}$ codoped ${\mathrm{Gd}}_3{\mathrm{Ga}}_3{\mathrm{Al}}_2{\mathrm{O}}_{12}:\mathrm{Ce}(\mathrm{GGAG}:\mathrm{Ce})$. By using a combination of optical absorption spectra and x-ray absorption near-edge spectroscopies, the correlation between ${\mathrm{Ca}}^{2+}$ codoping content and the ${\mathrm{Ce}}^{4+}$ fraction is seen. The energy-level diagrams of ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Ce}}^{4+}$ in the ${\mathrm{Gd}}_3{\mathrm{Ga}}_3{\mathrm{Al}}_2{\mathrm{O}}_{12}$ host are established by using theoretical and experimental methods, which indicate a higher position of the $5d_1$ state of ${\mathrm{Ce}}^{4+}$ in the forbidden gap in comparison to that of ${\mathrm{Ce}}^{3+}$. Underlying reasons for the decay-time acceleration resulting from ${\mathrm{Ca}}^{2+}$ codoping are revealed, and the physical processes of the ${\mathrm{Ce}}^{4+}$-emission model are proposed and further demonstrated by temperature-dependent radioluminescence spectra under x-ray excitation.

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