## 3 Results  
The comparison of the PL spectrum measured at room temperature \(\lambda\)ex =273 nm, corresponding to the excitation in the 4\({}^{7}\)-5d\({}^{16}\) interconfigurational transition of Tb\({}^{3+}\) ions and the scintillation spectrum, is shown in Figure 2. For both types of excitations, the spectrum is typical for the luminescence of Tb\({}^{3+}\) ions from two radiative states: \({}^{5}\)D\({}_{3}\rightarrow{}^{7}\)F\({}_{\rm J}\), and \({}^{5}\)D\({}_{4}\rightarrow{}^{7}\)F\({}_{\rm J}\). The luminescence spectra of GYAGG:Ce completely coincide with data published elsewhere [27; 28]. The PL intensity increases in proportion to the concentration of the activator for transitions from the \({}^{5}\)D\({}_{4}\) state but has the opposite trend for transitions from the \({}^{5}\)D\({}_{3}\) state. The cross-relaxation process between neighboring Tb\({}^{3+}\) ions depends on the concentration of the activator as shown in [29; 30] and causes observed trends. The decay kinetics of PL in the region of the \({}^{5}\)D\({}_{4}\rightarrow{}^{7}\)F\({}_{\rm J}\) transition are shown in Figure 3. The kinetics of luminescence from the lower radiative state \({}^{5}\)D\({}_{4}\) practically do not change, remaining close to the single exponential with a decay time of 3 ms. This indicates that no concentration quenching of \({}^{5}\)D\({}_{4}\rightarrow{}^{7}\)F\({}_{\rm J}\) luminescence is observed in the concentration range studied. Similar results have been observed for the samples with a different ratio Gd/Y (samples GYAGG:Tb-2, Tb = 0.2 and GYAGG:Tb-3, Tb = 0.2). Figures 4 and 5 show the luminescence spectra and luminescence kinetics of the corresponding samples. The PL intensity for sample GYAGG:Tb-2, Tb = 0.2 is approximately 20% lower than for sample GYAGG:Tb-3. Therefore, the ratio of components close to the latter composition was used in the concentration series. The kinetics of scintillation at excitation by an electron-beam pulse were measured in the region of two radiative transitions \({}^{5}\)D\({}_{3}\rightarrow{}^{7}\)F\({}_{\rm J}\) (380 nm) and \({}^{5}\)D\({}_{4}\rightarrow{}^{7}\)F\({}_{\rm J}\), (546 nm). Figure 6 shows scintillation kinetics measured with GYAGG:Tb, Tb = 0.15. The scintillation, which is caused by the \({}^{5}\)D\({}_{3}\rightarrow{}^{7}\)F\({}_{\rm J}\) transition, is strongly quenched by the nonradiative transfer from the \({}^{5}\)D\({}_{3}\) to the \({}^{5}\)D\({}_{4}\) state. As a result, there is a rising part in the kinetics of the scintillation in the spectral range of the \({}^{5}\)D\({}_{4}\rightarrow{}^{7}\)F\({}_{\rm J}\) transition with a constant of 150 \(\upmu\). At a low excitation intensity, \(\sim\)0.1 A/cm\({}^{2}\), the scintillation kinetics is close to that of photoluminescence; however, upon highest excitation, the decay constant decreases by \(\sim\)20%, which is due to additional quenching. Figure 7 shows the dependences of the scintillation yield on the excitation intensity of GYAGG:Tb = 0.15 and GYAGG:Ce measured at room temperature. Figure 2: Room temperature PL spectra at excitation in Tb\({}^{3+}\) interconfiguration transition (\(\lambda\)ex = 273 nm) of GYAGG:Tb samples versus the activator concentration (**a**); scintillation spectrum of the GYAGG:Tb, Tb = 0.15 at excitation by the electron beam at the maximum intensity (**b**). Figure 4: Room temperature PL spectra at excitation in Tb\({}^{3+}\) interconfiguration transition (\(\lambda\)ex = 273 nm) of GYAGG:Tb-2 and GYAGG:Tb-3 samples. Inset includes the Gd/Y ratio, assuming a homogeneous distribution of the Tb ions between positions occupied by Gd and Y ions. Therefore, the Gd and Y indexes are each increased by 0.1. Figure 3: Room temperature PL kinetics \({}^{5}\)D\({}_{4}\)\(\rightarrow\)\({}^{7}\)F\({}_{\text{J}}\) (Arge = 546 nm) in GYAGG:Tb ceramic samples versus Tb content. Figure 5: Room temperature PL kinetics \({}^{5}\)D\({}_{4}\)\(\rightarrow\)\({}^{7}\)F\({}_{\rm J}\) (\(\lambda\)reg = 546 nm) in GYAGG:Tb-2 ii GYAGG:Tb-3, samples.