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Technological factors and processes contributing to the scintillation mechanism have been considered in quaternary garnet ceramics doped with Ce(Gd,Lu)3Al2Ga3O12. The super-stoichiometric additive of gadolinium in the material composition or its co-doping with a low concentration of Mg were found to be effective tools to suppress phosphorescence in the quaternary garnet, confirming that it is not an intrinsic property of the material. The Monte-Carlo simulation of electronic excitation transfer demonstrates that the hopping migration along the gadolinium sublattice plays an essential role in forming the scintillation kinetic parameters. Breaking the integrity of the gadolinium sublattice by substitution with heavier lutetium ions increases the role of self-trapped states in the excitation of Ce3+ ions, which ensures both an increase in the fraction of short ~20 ns and very long ~600 ns components in the scintillation kinetics.

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A mechanism of firefly high‐scintillation light yield (LY) of Tb‐doped quaternary (Gd, Y)3Al2Ga3O12 garnet ceramics is reported. Through measurements with the synchrotron source, the high efficiency of the luminescence excitation, providing a quantum yield Q > 1 below the photon multiplication energy range, is defined. The excitation efficiency reaches two at the excitation energy slightly above 2Eg. The cascade of photons is explained by combining three factors: first, the high quantum yield Q ≈ 1 of the luminescence at excitation in lower mixed states 4f75d1 with high spin (HS) and low spin (LS); second, the cross‐relaxation 4f7(6P)5d1(HS) → 4f7(8S)5d1(HS) provides the excitation of 4f8(7F0) →4f8(5D1,4) transition of the 4f8 configuration of the same or neighboring Tb ion, which is followed by the luminescence from 5D1,4; and finally, the relaxation of 4f7(8S)5d1‐5(HS) configuration into the excited 4f8 configuration occurs with future emission from 4f8(5D1,4) states. This cascade forms the final stage of the scintillation in the compound being studied and provides a LY twice as high compared with the material when doped with Ce.