Ultrafast Scintillation in Co2+-Codoped GAGG:Ce Crystals via Excited-State Lifetime Engineering

Abstract The temporal performance of Ce3+-activated oxide scintillators is fundamentally bottlenecked by the intrinsic radiative lifetime of the 5d → 4f transition. While conventional defect engineering can minimize trap-related slow decay components, it fails to accelerate the primary de-excitation pathway. Herein, we demonstrate an active excited-state lifetime engineering strategy to transcend this physical limit by codoping GAGG:Ce with Co2+. This codoping establishes a highly efficient Ce3+ → Co2+ nonradiative energy transfer channel that kinetically outcompetes the intrinsic radiative decay. Simultaneously, the Co2+ substitution drives spontaneous charge compensation, converting a specific fraction of Ce3+ to Ce4+. This valence conversion facilitates electron capture and is accompanied by a substantial reduction in trap-mediated delayed recombination. Operating synergistically, these mechanisms drastically reduce the fast scintillation decay component from 121.2 ns to an ultrafast 16.7 ns, and effectively clear the delayed afterglow. Although this kinetic competition inherently trades a fraction of the light yield for sub-20 ns decay, the engineered scintillator maintains a light yield of 14,000 photons MeV–1, while its ultrafast response helps mitigate signal pile-up at high photon fluxes. We validate its practical utility in high-resolution X-ray imaging, providing a predictive design framework for next-generation ultrafast scintillators tailored for high-count-rate and timing applications.