**1. Introduction**

In this chapter, a few examples are demonstrated of the impact of high-energy (β) electrons irradiation on the absorptive and fluorescence properties of silica-based optical fibers doped with rare earths and metals. The results presented hereafter seem to be useful for understand‐ ing the processes standing behind the highlighted phenomena and for possible applications of the fibers, say, in dosimetry and space technology.

In each case, we used for irradiating fiber samples a controllable linear accelerator of the LU type that emits β-electrons with a narrow-band energy spectrum (~6 MeV) in a shortpulse (~5 μs) mode. The samples with lengths of around 1–2 m were placed into the

accelerator's chamber for various time intervals, which provided growing irradiation doses. The irradiated fibers were then left for 2 weeks prior to the main-course spectral measure‐ ments to avoid the role of short-living components in the decay of induced absorption (IA). The measurements were done during a limited time (viz., the following 2…3 weeks) for diminishing the effect of spontaneous IA recovering. Note that ionization, that is, the production of β-induced carriers by an electron beam (i.e., of secondary free holes and electrons), is the main cause of the spectral transformations in the fibers. This happens because high-energy primary β-electrons are virtually nondissipating at the propagation through a fiber sample; on the other hand, certain contribution in ionization of the fibers' core-glasses arising from *γ*-quanta born at inelastic scattering of the high-energy electrons cannot be disregarded.

We demonstrate below first a study of the resistance of a couple of cerium (Ce)-doped alumino-phospho-silicate fibers (one of them being codoped with gold (Au)), to β-electrons. The experimental data reveal a severe effect of β-irradiation upon the fibers' absorptive properties, given by noticeable susceptibility of Ce ions being in Ce3+/Ce4+ states to the treatment, arising as growth followed by saturation of IA. We also report the essentials of posterior bleaching of β-darkened fibers, also in terms of attenuation spectra' transforma‐ tions, at exposing them to low-power green (a He-Ne laser) and ultra violet (UV, a mercury lamp) light. It is shown that both phenomena are less expressed in Ce fiber codoped with Au than in Au-free one and that the spectral changes in the former are more regular versus dose and bleaching time.

Then, we provide a comparative experimental analysis of IA, induced by β-electrons, for a series of ytterbium (Yb)-doped alumino-germano-silicate fibers with different concentrations of Yb3+ ions and compare this effect with the photodarkening (PD) phenomenon in the same fibers, arising at resonant (into 977 nm absorption peak of Yb3+ ions) optical pumping. The experimental data obtained reveals that, in these two circumstances, substantial and complex but different in appearance changes affecting the resonant absorption band of Yb3+ ions and the off-resonance background loss are produced in the fibers.

Finally, we report a study of attenuation spectra' transformations in a set of bismuth (Bi)-doped silica fibers with various contents of emission-active Bi centers, which occur as the result of β-irradiation. Among the data obtained, notice a substantial decrease of concentration of Bi centers, associated with the presence of Germanium (Ge) in core-glass, with increasing irradiation dose (the "bleaching" effect), while, on the contrary, an opposite trend, that is, dosedependent growth of resonant-absorption ascribed to Bi active centers, associated with the presence in core-glass of Aluminium (Al). These results are worth noticing for understanding the nature of Bi-related centers in silica fibers, yet uncovered.
