## 3 Results
### Thermoluminescence (TL) Characteristics of Irradiated Glasses
#### 3.2.1 Electron Accelerator, Pulse Duration 4 \(\upmu\)s
From literature it is known that matrix distortions due to presence of large dopant ions such as Tb\({}^{3+}\) may form complex defect sites, which can be stabilized by Na\({}^{+}\) and intrinsic oxygen vacancies [41]. The possible mechanism of TL emission from terbium-doped phosphors includes few steps. It is considered that trivalent terbium ions Tb\({}^{3+}\) may be effectively reduced by free electrons formed in the matrix after \(\beta\)- and \(\gamma\) irradiation. Further, the formed Tb\({}^{2+}\) ion is efficient hole scavenger, forming excited Tb\({}^{3+}\) ions in this process, and TL emission is considered to follow the reaction expressed by Equation (1). \[\text{Tb}^{2+}+\text{hole}\rightarrow\text{Tb}^{3+*}\rightarrow\text{Tb}^{3 }+\lambda\upsilon \tag{1}\]  
The holes are supplied by traps via thermal excitation [42]. An elegant explanation for these mechanisms based on the simultaneous participation of rare earth metals ions and interstitial oxygen atoms is presented in the [PERSON] paper [43]. The complex  
Figure 2: TL curves for samples irradiated using beta radiation (4 \(\upmu\)s pulse duration), PPG (**a**), PPG_Tb01 (**b**), PPG_Tb05, (**c**) and PPG_Tb30 (**d**). PPG samples were read without shutter. nature of energy transitions between excited states and ground level of Tb3+ ions involves different transitions between D and F levels (especially \({}^{5}\)D4\(\rightarrow\)\({}^{7}\)F6, \({}^{5}\)D4\(\rightarrow\)\({}^{7}\)F5, \({}^{5}\)D4\(\rightarrow\)\({}^{7}\)F4 and \({}^{5}\)D4\(\rightarrow\)\({}^{7}\)F3), which are then responsible for enhanced photon emission from terbium-doped materials [44]. As one can see in Figure 3, dependencies of signal integral vs. dose cannot be considered as linear and are clearly saturation-type curves, which resemble functions of exponential type. The integrated luminescence intensity (N\({}_{\text{int}}\)) may be approximated by a simple summation, according to Equation (2) [39]:  
\[\text{N}_{\text{int}}=\int_{\text{T}_{0}}^{\text{T}}\text{N}_{\text{p}}(\text{T })\text{d}\text{T}=\sum_{\text{T}_{0}}^{\text{T}}\text{N}_{\text{p}}(\text{T}) \tag{2}\]  
where Np is the photon count intensity at point temperature T. For the approximation of the experimental data presented in Figure 2, the function expressed by Equation (3) can be used:  
\[\text{N}_{\text{int}}=\text{N}_{\text{inf}}\Big{(}1-\text{e}^{-\text{kD}}\Big{)} \tag{3}\]  
where N\({}_{\text{inf}}\) is the pre-exponential factor dependent on the material properties and related to the saturation intensity, k is the thermoluminescence yield per unit dose factor or material sensitivity coefficient, in kGy-1, and D is the radiation dose in kGy. It is obvious that increasing the Tb\({}^{3+}\) ions concentration has the most pronounced influence on the light-emission efficiency, what is also confirmed by the data provided in the further parts of this work. The sensitizing effect of rare earth elements was already discussed in previous papers, where other glass samples containing such types of dopants were investigated [27; 45; 46; 47]. It seems that terbium is one of the most efficient sensitizing agent among rare earth metals. However, for the terbium ions the strongest enhancement of the light-emission efficiency is observed for the low dopant concentration range to upmost few percent and decrease above this value. Above relatively low concentrations of ca. 0.3%, one observed the quenching effects due to simultaneous effects of multipolar transitions or these related to the so called inner effect filter, which were observed already for terbium concentration starting from 6 (mole) % [46; 47]. The data obtained and presented in this study correlate well with those provided in literature, which is demonstrated by the results provided in Figure 3, such as the significant increase of TL intensity when shifting from PPG_Tb01 sample to PPG_Tb05 and lower efficiency for further transition to PPG_Tb30 glass.