Paramagnetic defects induced by electron irradiation in barium hollandite ceramics for caesium storage

We have studied by electron paramagnetic resonance the mechanism of defect production by electron irradiation in barium hollandite, a material used for immobilization of radioactive caesium. The irradiation conditions were the closest possible to those occurring in Cs storage waste forms. Three paramagnetic defects were observed, independently of the irradiation conditions. A hole centre (H centre) is attributed to a superoxide ion O-2(-) originating from hole trapping by interstitial oxygen produced by electron irradiation. An electron centre (Et centre) is attributed to a Ti3+ ion adjacent to the resulting oxygen vacancy. Another electron centre (E-2 centre) is attributed to a Ti3+ ion in a cation site adjacent to an extra Ba2+ ion in a neighbouring tunnel, originating from barium displacement by elastic collisions. Comparison of the effects of external irradiations by electrons with the beta-decay of Cs in storage waste forms is discussed. It is concluded that the latter would be dominated by E-1 and H centres rather than E-2 centres.