Gamma-Ray Induced 2 eV Optical Absorption Band in Pure-Silica Core Fibers

The mechanism of optical absorption near 2 eV induced by γ-irradiation in pure silica glass has been studied by many workers1–6. It appears that non-bridging oxygen hole centers (NBOHC), which were revealed by Friebele et al. through the study of electron spin resonance (ESR) spectra1, are the most probable factor causing the optical absorption2–5. However, it is also clear that the 2 eV band cannot be ascribed to only one kind of defect center, because, as the OH-group content of the sample increases, the peak wavelength shifts from 630 nm to 600 nm3,5 and the oscillator strength decreases3. Friebele et al. thus assumed that the 2 eV band is caused mainly by NBOHC in high-OH silica (i.e., silica which contains a high amount of OH groups) and by a non-paramagnetic center such as ≡ Si:- in low-OH silica3. Contrary to the above assumption, the present authors thought that the difference in the 2 eV band induced in high-OH silica and in low-OH silica is caused by hydrogen bonds between the NBOHC and an OH group near the NBOHC, and that the 2 eV band in both low- and high-OH silica is caused only by NBOHC2. In this paper, experimental results are presented followed by discussion of the model derived from these results.