## 4 Discussion
The inset shows the attempt to fit the \(\Delta\)RIA with the NBOHC considered as a single Gaussian band. (**b**) Fitting function of the RIA at 300 kGy of the _high_-OH sample. RIA at 2.8 eV of the _medium_- and _low_-OH samples. Figure 8b illustrates the fitting of the \(\Delta\)RIA, where it becomes eVident that the NBOHC contribution at lower doses is negligible and that the \(\text{STH}_{2}\) centers are still present. As already mentioned in Section 2.2, the pristine _medium_-OH sample has been also loaded with H\({}_{2}\) and thermally treated in order to eliminate the absorption band centered around 600 nm mainly due to DID. Figure 9a shows the normalized spectra of the _low_-OH and the treated _medium_-OH sample at the dose of 10 kGy. By comparing this figure with Figure 8a, we show that the treatment has no significant impact on the shape of the radiation-induced absorption but it slightly influences the relative magnitudes of the absorption bands used in the decomposition (Figure 9b). Particularly, the \(\text{STH}_{2}\) band area slightly decreases compared to the case of the non-treated sample and an increase in the NBOHC band is noted. The best-fit parameters for the treated and non-treated _medium_-OH sample are listed in Table 3.  
Figure 8: (**a**) Normalized spectra with respect to the RIA measured at 2.8 eV (450 nm) for the _low_- and _medium_-OH samples at 10 kGy dose. (**b**) Fitting function obtained for the difference of the normalized RIA of the _medium_- and _low_-OH samples measured at a dose of 10 kGy. Figure 7: (**a**) Fitting function of the difference between the normalized RIA of the _medium_- and the _low_-OH samples, taking into account the NBOHC model of [39]. The inset shows the attempt to fit the \(\Delta\)RIA with the NBOHC considered as a single Gaussian band. (**b**) Fitting function of the RIA at 300 kGy of the _medium_-OH sample. Finally, the two RIAs at 300 kGy for the treated and non-treated _medium_-OH sample are displayed in Figure 10 for a direct comparison. It can be seen that in the treated sample that the magnitude of the RIA increases significantly with respect to the untreated sample for energies above 2.4 eV. Simultaneously, it is reduced by approximately 10% in the 1.9-2.4 eV range. This last feature could be explained by the slight decrease of the \(\mathrm{STH}_{2}\) magnitude already observed in Figure 9b. It is known from the literature [23; 34] that STH annealing could be responsible for the creation of the \(\mathrm{Cl}^{0}\) defect. \begin{table}
\begin{tabular}{c c c c c} \hline
**Point Defect** & **Ec [eV]** & **FWHM [eV]** & _Medium_-OH 1 ** & _Medium_-OH+H\({}_{2}\)+TT 1 ** \\ \hline \(\mathrm{STH}_{1}\) & 2.6 & 1.2 & 0.24 & 0.24 \\ \(\mathrm{STH}_{2}\) & 2.16 & 0.6 & 0.17 & 0.09 \\ s-a \(\mathrm{STH}_{1}\) & 1.88 & 0.2 & 0.18 & 0.13 \\ s-a \(\mathrm{STH}_{2}\) & 1.63 & 0.3 & 0.07 & 0.06 \\ NBOHC & 1.95, 2.04, 2.21 & 0.16, 0.27, 0.46 & \(4\times 10^{-4}\), \(6\times 10^{-4}\), \(5\times 10^{-3}\), \(10^{-4}\) & \(9.5\times 10^{-3}\), \(14.5\times 10^{-3}\), \\  & & & \(10^{-4}\) & \(11.2\times 10^{-3}\) \\ \(\mathrm{Cl}^{0}\) & 3.26 & 1.17 & 1.74 & 1.89 \\ \hline \end{tabular}
\end{table} 
Table 3: Best-fit parameters for the RIA at 10 kGy of the treated and not-treated _medium_-OH sample. Figure 10: Measured RIA of the _medium_-OH and the treated _medium_-OH samples at a dose of 300 kGy. Figure 9: (**a**) Normalized spectra with respect to the RIA measured at 2.8 eV (450 nm) for the _low_- and the treated _medium_-OH samples at 10 kGy dose. (**b**) Fitting function obtained for the difference of the normalized RIA of the treated _medium_- and _low_-OH samples measured at a dose of 10 kGy.