Positron annihilation and X-ray nano-CT to investigate nuclear materials

Positrons preferentially trap at vacancies, if present, prior to annihilation. Doppler broadening of the annihilation line carries information about the annihilation site. A variable energy beam enables the identification and assessment of vacancies as a function of depth. X-ray Nano-Computed Tomography (CT) provides 3D images of density variations such as pores. Nuclear materials such as fuel elements or vitrified waste forms are exposed to energetic particle irradiation from fission products or alpha decay or are altered by environmental water during long term storage. Defects including vacancies and porosity up to micrometer scales are generated. Herein, we will discuss the benefits of positron annihilation spectroscopies and X-ray nano-CT for understanding nuclear materials. Examples include recent work combining both techniques on the leaching of ISG-2 glass used for waste vitrification as well as positron annihilation applied to defects generated by ion implantation into materials.

The figure shows the composition of segments from 4 Nano-CT X-ray images of ISG-2 glass fibers after various time in deionized water at 150 °C. One segment stems from a fiber with a crack. The formation and growth of an alteration layer can be seen as a change in color from orange-red to yellow-green. The change represents about 5% reduction in absorption of X-rays, i.e., a reduction in density. Positron data from matching samples show the formation of vacancies and pores (not shown here, please refer to the text).