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.

Fast and high-resolution x-ray imaging demands scintillator films with negligible afterglow, high scintillation yield, and minimized cross-talk. However, grain boundaries (GBs) are abundant in polycrystalline scintillator film, and, for current inorganic scintillators, detrimental dangling bonds at GBs inevitably extend radioluminescence lifetime and increase nonradiative recombination loss, deteriorating afterglow and scintillation yield. Here, we demonstrate that scintillators with one-dimensional (1D) crystal structure, Cs ₅ Cu ₃ Cl ₆ I ₂ explored here, possess benign GBs without dangling bonds, yielding nearly identical afterglow and scintillation yield for single crystals and polycrystalline films. Because of its 1D crystal structure, Cs ₅ Cu ₃ Cl ₆ I ₂ films with desired columnar morphology are easily obtained via close space sublimation, exhibit negligible afterglow (0.1% at 10 ms) and high scintillation yield (1.2 times of CsI:Tl). We have also demonstrated fast x-ray imaging with 27 line pairs mm ⁻¹ resolution and frame rate up to 33 fps, surpassing most existing scintillators. We believe that the 1D scintillators can greatly boost x-ray imaging performance.