*2.3.2. β-irradiated fibers*

Consider in more details the results of β-irradiation of Ce-doped and Ce/Au-codoped fibers (**Figures 2**–**4**). The processes, involved at irradiating the fibers with the result being rise followed by saturation of IA, described by the "stretched-exponent" law [14, 24, 26]), comprise: (*i*) creating of secondary carriers (holes h+ and electrons e<sup>−</sup> ) in the core-glass matrix by β (primary) electrons and their trapping on such glass imperfections as Ce ions (Ce3+/Ce4+), nonbridging oxygen centers, other centers associated with Al and P, and oxygen vacancies; (*ii*) direct h+ …e<sup>−</sup> recombination (annihilation); (*iii*) thermally or/and radiatively activated recom‐ bination between the centers or defects that have arisen during and after β-irradiation. Concerning the role of Ce-doping, we assume that IA is produced via irradiation-induced reactions Ce3++h+ →Ce3+h+ (→?Ce4+) and Ce4++e<sup>−</sup> →(?Ce4+e<sup>−</sup> )→Ce3+ [26–28], implying Ce was in valences 3+/4+ in pristine fibers or/and being generated via irradiation. Note that determina‐ tion of relative contents of Ce3+/Ce4+ ions in the pristine state is hard and that at low Ce doping, mainly fluorescing Ce3+ are formed in the core-glass, while at higher overall Ce concentration both Ce3+ and Ce4+ (nonfluorescing) ions can be present. Unfortunately, the absorption spectra of glasses containing both Ce3+/Ce4+ ions have the featuring bands within a 200–400 nm (UV) range (not detectable by our spectral equipment); so any arguing about Ce3+↔Ce4+ transfor‐ mations for this range is impossible. In the meantime, the absorption bands of Ce3+h+ and Ce4+e − defect centers are expectedly located in VIS (see above), on one hand, and, on the other hand, the detected spectral changes at β-irradiation occur in VIS, too (band 2); thus, formation of metastable centers Ce3+h+ /Ce4+e<sup>−</sup> as its result is a worthy proposal.

Furthermore, IA bands 1 (~3.0 eV) and 2 (~2.4 eV) (see **Figure 4**) have been undoubtedly separated; see above. The first of them, in Ce-doped and Ce/Au-codoped fibers, has seemingly the same origin as the one in the Ce-free fiber (see **Figure 2**), that is, it most probably belongs to one, most simply organized, type of the two NBOHCs centers, inherent to silica. The other would stem from Ce doping: it is seen from **Figure 2** that such band does not exist in the reference Ce-free fiber. However, the irradiated Ce-free fiber demonstrates ~600 nm band, probably attributing the other type of NBOHCs [26], absent in both Ce-doped fibers subjected to irradiation: compare spectra 1–3 shown in **Figure 2**. The fact that the dose dependences of IA (Figure 4) have different characters for the fibers points on different nature of the centers represented by bands 1 and 2. Therefore, our hypothesis that ~3.0 eV band stems from NBOHCs and that ~2.4 eV one is associated with a Ce-related center (Ce3+h+ /Ce3+e<sup>−</sup> ) seems to be relevant. In our case (alumino-silicate core glass of Ce-doped and Ce/Au-codoped fibers), such "point" defects as Al-E' and Al-oxygen-deficient centers can be also created at trapping secondary electrons and holes born at β-irradiation (phosphorous (P) presented in small amount plays a little effect).

Thus, the processes roughly schematized as Ce3+↔Ce4+ seem to be a sole way to address the spectral transformations seen from Figures 2–4 to happen via the formation in the Ce- and Ce/ Au-doped fibers of metastable states Ce3+h+/Ce4+e−. Furthermore, it deserves mentioning that overall susceptibility to β-irradiation (overall IA loss) of Ce-doped fiber is higher than of Ce/Au-codoped one. This may signify that the core glass containing gold is more stable than that solely doped with Ce. On the other hand, deviations in the experimental data for kinetics of IA versus β-irradiation dose for the fibers (refer to IA spectra in **Figure 3** and to dose dependences in **Figure 4**) are more pronounced in the former than in the latter fiber. A possible explanation for this can be that codoping with Au gives rise to the core-glass system more ordered. This property seems to have impact for establishing almost the same path kinetics of defect centers' formation as compared with other factors involved in such of type fibers.
