## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Point Defect** & **Ec [eV]** & **FWHM [eV]** & _Low-OH 1_ & _Medium_-OH 1_ & _High_-OH 1_ \\ \hline STH\({}_{1}\) & 2.6 & 1.2 & 1 & 0.15 & 0.2 \\ STH\({}_{2}\) & 2.16 & 0.6 & 0.77 & 0.36 & 0.46 \\ NBOHC & 1.95, 2.04, 2.21 & 0.16, 0.27, 0.46 & - & 0.05, 0.08, 0.06 & 0.36, 0.55, 0.42 \\ Cl\({}^{0}\) & 3.26 & 1.17 & 19.66 & 2.1 & 4.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Best-fit parameters for the RIA at 300 kGy of the three tested fibers. Figure 5: (**a**) Decomposition of the RIA spectrum measured for the _low_-OH sample at 300 kGy dose. Open circles represent the experimental data while the red line represents the best fitting function obtained by summing the different absorption bands related to single point defects. (**b**) Normalized RIA spectra, acquired at 300 kGy dose, with respect to the RIA measured at 2.8 eV (450 nm) for the _low-_, _medium-_ and _high_-OH samples. NBOHC contribution around 2 eV as a sum of three different Gaussian bands with a fixed relative amplitude ratio. Figure 6a shows the fit related to the \(\Delta\)RIA between the _high_-OH and _low_-OH samples. Here it is possible to observe that the contribution of the NBOHC absorption band stands out and that the selected model well suits the profile. However, to achieve a good quality of the fit, we were obliged to introduce the Gaussian absorption band related to the \(\text{STH}_{2}\) point defect. The introduction of this band is well supported by the literature, where it has been found that a higher OH concentration corresponds to a higher \(\text{STH}_{2}\) relative contribution [34]. In addition, the inset of Figure 6a reports the same fitting routine but with the NBOHC considered as a single Gaussian absorption band centered around 2 eV. In this case, it is evident that the result of the fit does not match the experimental data so, to complete the fit, new absorption bands associated with one or more yet unknown points defects should be introduced. Figure 6b reports the fitting function obtained on the overall RIA profile of the _high_-OH sample at 300 kGy as well as the individual bands of the point defects used for the decomposition. The same approach was used for the analysis of the irradiated _medium_-OH sample. Figure 7a displays the obtained fitting function for the \(\Delta\)RIA at 300 kGy with respect to the _low_-OH. In accordance to Equation (3), in the _medium_-OH sample we observe a lower content of the NBOHC contribution. Moreover, the inset of Figure 6 shows again that, with the classical 1-Gaussian model of the NBOHC, the central region still needs to be completed by some other unknown absorption bands. Figure 7b reports the match between the experimental RIA and the fitting function in the _medium_-OH sample. As in the case of the _high_-OH fiber, in Figure 7a the absorption bands related to NBOHC and \(\text{STH}_{2}\) have been used. In this case, the relative contribution provided by the \(\text{STH}_{2}\) increases in comparison to the _high_-OH case. The greater contribution of the STH-related bands could also justify the non-monotonic behavior observed in the RIA kinetics shown in Figure 3b, considering their strained assisted variants (s-a STHs) [14; 40; 43]. Indeed, it is possible to notice that the _medium_-OH RIA has a broader band covering also energies lower than 1.5 eV compared to the one of the same sample at 300 kGy (Figure 5b), in agreement with the s-a \(\text{STH}_{1}\) and s-a \(\text{STH}_{2}\) spectral characteristics, centered respectively at 1.88 eV (660 nm) and 1.63 eV (760 nm). For this reason, an in-depth analysis of the RIA at 10 kGy (the dose which corresponds at the maximum in the kinetics at 650 nm reported in Figure 3b) has been carried out. At this dose, Figure 8a reports together the normalized  
Figure 6: (**a**) Fitting function of the difference between the normalized RIA (ARIA) of the _high_- and the _low_-OH samples, taking in account the model of the NBOHC of [39].