Diffusion of cesium in sodium borosilicate glasses for nuclear waste immobilisation, studied by low-energy ion scattering

Low-energy ion scattering (LEIS) is shown to be a convenient technique for measuring the diffusion coefficient of cesium in sodium borosilicate glass. A 3 keV ⁴He⁺ ion beam is first used to create an alkali depletion layer in the outermost 60 nm of the glass. After annealing, to remove the vacancies from the glass matrix, the return of cesium to the surface is monitored. Diffusion coefficients at 815 and 842 K are determined. Both are found to agree, within experimental error, with the values from the concentration couple method. The latter analyzes the cesium profiles by SEM/EDS. The present technique, however, can be applied at lower temperatures, where the concentration couple method would require too long measuring times. This gives the possibility to predict the diffusion coefficient of cesium at actual temperatures in nuclear waste glass cylinders.