Electron spin resonance studies of defect centers induced in a high-level nuclear waste glass simulant by gamma-irradiation and ion-implantation

Abstract

Samples of an iron-free soda-lime boro-aluminosilicate high-level nuclear waste (HLW) glass simulant were implantated with 160-keV protons or He + ions or irradiated by 60Co γ-rays. Electron spin resonance (ESR) spectra were recorded before and after implantation or irradiation and after postirradiation isochronal anneals to 550°C. The most numerous paramagnetic states in the unannealed samples following γ-irradiation were boron–oxygen hole centers and Ti 3+ trapped-electron centers. The most numerous paramagnetic states in the as-implanted samples were peroxy radicals (PORs). Annealing of the γ-irradiated samples at temperatures >300°C caused recombination of the trapped electrons and holes, revealing an underlying POR spectrum which annealed in stages at ∼400°C and 550°C, similar to the annealing of PORs in the He +-implanted sample. For equivalent depositions of ionizing energy, the initial POR concentration in the He +-implanted sample was ∼100 times that in the γ-irradiated sample and at least 10 times larger than the (unmeasurable) number of PORs in the proton-implanted sample. Since TRIM calculations indicate that 160-keV protons deliver twice the ionizing dose of He + ions of the same energy but displace only 1/10 as many atoms, it is evident that the PORs induced by He + implantation result from displacements of oxygens in elastic collision cascades. The absence of trapped electrons and holes in the implanted samples is ascribed to annealing during irradiation associated with the fraction of the implantation energy deposited as heat. One of the fundamental defects to be expected in HLW glasses containing α-particle emitters is therefore the POR. Several distinct families of radiation-induced PORs are unambiguously identified in our samples by computer line-shape simulations constrained by the Känzig–Cohen g-value formulae. Possible reasons for the non-observation of PORs in similarly-irradiated glasses containing several wt% Fe 2O 3 are proposed.