Determination of the visible range optical absorption spectrum of peroxy radicals in gamma-irradiated fused silica

Abstract

Two silicas (Russian KS-4V and an F-doped silica) with <1 ppm cationic impurities, OH contents <5 ppm, and Cl contents <20 ppm have been investigated in optical-fiber forms by differential-optical spectroscopy following 14-MGy(Si) 60Co γ irradiation at 27°C and isochronal anneals to 600°C. Bulk samples of the F-doped silica and of two other high-purity silicas (Suprasil F300 and F310) were studied by electron spin resonance (ESR) and isochronal annealing following exposure to (i) a γ-ray dose of 12 MGy(Si) and (ii) the same 12 MGy γ irradiation plus a subsequent fission-reactor irradiation. The strength of an optical band near 2 eV is shown to correlate with the intensity of the ESR spectrum of peroxy radicals (PORs) as a function of anneal temperature. The peak energy, band width, and oscillator strength of this band are determined to be E 0=1.97 ± 0.01 eV, W=0.17 ± 0.01 eV, and f POR=0.00057 ± 0.00015, respectively. This POR band overlaps with, and has previously defied separation from, those of the non-bridging-oxygen hole centers (NBOHCs), here fitted by two bands ( E 0=2.19 ± 0.01 eV, W=0.51 ± 0.01 eV and E 0=2.08 ± 0.01 eV, W=0.32 ± 0.01 eV) with an aggregate oscillator strength f NBOHC=0.00054 ± 0.00015. The annealing processes of the PORs and NBOHCs displayed a dose-rate dependence, tentatively linked to the effect of dose rate (1 Gy/s vis-à-vis 5.3 Gy/s) on the steady-state self-trapped-hole populations, which in turn determine the equilibrium of radiolytic interstitial oxygen species: O 0 + O 0 ⇌ O 2.