First observation of the scintillation cascade in Tb${}^{3+}$-doped quaternary garnet ceramics

A mechanism of firefly high-scintillation light yield (LY) of Tb-doped quaternary (Gd, Y)${}_3$Al${}_2$Ga${}_3$O${}_{12}$ 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 2E_g. 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 4f${}^7$5d${}^1$ with high spin (HS) and low spin (LS); second, the cross-relaxation 4f${}^7$(${}^6$P)5d${}_1$(HS) → 4f${}^7$(${}^8$S)5d${}_1$(HS) provides the excitation of 4f${}^8$(${}^7$F${}_0$) → 4f${}^8$(${}^5$D${}_{1,4}$) transition of the 4f${}^8$ configuration of the same or neighboring Tb ion, which is followed by the luminescence from ${}^5$D${}_{1,4}$; and finally, the relaxation of 4f${}^7$(${}^8$S)5 d${}_{1-5}$(HS) configuration into the excited 4f${}^8$ configuration occurs with future emission from 4f${}^8$(${}^5$D${}_{1,4}$) 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. (© 2022 Wiley‐VCH GmbH)