## 3 Results and Discussion
### Scintillation Characteristics  
Figure 6 presents the radioluminescence (RL) spectra of LuYAGG:Tb single crystals under X-ray excitation. The emission spans approximately 350 to 700 nm, corresponding to the characteristic 4f-4f transitions of Tb\({}^{3+}\): \({}^{5}\)D\({}_{3}\rightarrow\)\({}^{7}\)F\({}_{6,5,4}\) (-375-450 nm) and \({}^{5}\)D\({}_{4}\rightarrow\)\({}^{7}\)F\({}_{6}\) (-490 nm), \({}^{7}\)F\({}_{5}\) (-545 nm), \({}^{7}\)F\({}_{4}\) (-585 nm), \({}^{7}\)F\({}_{3}\) (-620 nm), consistent with typical Tb\({}^{3+}\) scintillation behavior. The RL spectra of LuYAGG doped with 0.5, 1, 5, and 10 mol\(\%\) Tb\({}^{3+}\) revealed a clear trend in luminescence efficiency. The sample doped with 1 mol\(\%\) Tb\({}^{3+}\) exhibited the highest RL intensity, particularly at the \({}^{5}\)D\({}_{4}\rightarrow\)\({}^{7}\)F\({}_{5}\) transition (-545 nm), indicating this as the optimal activator concentration. In contrast, the 0.5 mol\(\%\) doped sample showed noticeably weaker emissions, likely due to an insufficient number of activator centers. At higher doping levels (5 mol\(\%\) and 10 mol\(\%\)), the RL intensity decreased significantly, attributed to concentration quenching, a result of enhanced cross-relaxation and non-radiative energy transfer to defect sites [12; 15; 24; 27]. These findings suggest that 1 mol\(\%\) Tb\({}^{3+}\) offers the best compromise between adequate activator density and minimal non-radiative losses among all the samples. Figure 7 shows the pulse height spectra of LuYAGG:Tb single-crystal samples compared with a commercial Bi\({}_{4}\)Ge\({}_{3}\)O\({}_{12}\) (BGO) crystal, used as a reference, under 662 keV \(\gamma\)-ray excitation from a \({}^{137}\)Cs source. The corresponding light yield (\(L\gamma\)) values are summarized in Table 2. Among all the samples, LuYAGG:Tb\({}^{3+}\) with 5 mol\(\%\) doping exhibited the highest absolute light yield of 60,000 ph/MeV, which is 732\(\%\) of the BGO output. After correcting for the lower quantum efficiency of the PMT used with LuYAGG:Tb\({}^{3+}\) (10.7\(\%\) compared to 24.7\(\%\) for BGO), the actual light output was estimated to exceed 138,000 ph/MeV, underscoring its strong potential as a next-generation scintillator. Figure 5: Photoluminescence decay curves of LuYAGG:Tb single crystals recorded at \(\lambda_{\rm ex}=340\) nm and \(\lambda_{\rm em}=550\) nm. The highest \(LY\) was observed for the LuYAGG:Tb\({}^{3+}\) crystal doped with 5 mol% Tb\({}^{3+}\), outperforming all other compositions when compared against the BGO reference. This suggests that 5 mol% represents the optimal balance between sufficient activator concentration for efficient radiative transitions and minimal non-radiative quenching. Although 10 mol% still exhibited a characteristic of \({}^{5}\)D\({}_{4}\)\(\rightarrow\)\({}^{7}\)F\({}_{\text{j}}\) transitions of Tb\({}^{3+}\), its slightly reduced light yield implies the onset of concentration quenching mechanisms, such as cross-relaxation or energy migration to non-luminescent sites. Conversely, samples with 0.5 and 1 mol% \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Sample** & \(LY_{\gamma}\) & \(LY_{\gamma}\) & **PMT’s Quantum** \\  & **(ph/MeV)** & **(\% of BGO)** & **Efficiency (\%)** \\ \hline BGO & 8200 & 100 & 24.7 \\ LuYAGG:Tb 0.5 mol\% & 30,000 & 366 & 10.7 \\ LuYAGG:Tb 1 mol\% & 42,000 & 512 & 10.7 \\ LuYAGG:Tb 5 mol\% & 60,000 & 732 & 10.7 \\ LuYAGG:Tb 10 mol\% & 50,000 & 610 & 10.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: \(LY_{\gamma}\) at 662 keV \(\gamma\) rays for the LuYAGG:Tb single-crystal samples. Figure 6: Radioluminescence spectra of Tb-doped LuYAGG single-crystal samples. Figure 7: Pulse height spectra obtained from 662 keV \(\gamma\)-ray excitation for LuYAGG:Tb single-crystal samples compared with a BGO reference crystal. Tb\({}^{3+}\) showed weaker emission due to limited radiative centers and inefficient energy transfer [7; 12; 15; 24].