LAUSR

There are a number of operational environments where the performance of materials is likely to be affected significantly by fast-particle irradiation. These include amongst others: fission and fusion reactors, nuclear waste storage containers, particle accelerators, and spacecraft [1]. Thus, understanding the basic processes involved when materials are exposed to energetic particle irradiation is a key point in the choice of materials for use in such environments. During irradiation when energetic particles interact with the material, the target atoms are displaced from their original position in the crystal resulting in the production of a high density of defects, namely vacancies and interstitial atoms. The condensation of these supersaturated vacancies into cluster defects, and further growth by absorption of defects of the same type results in the formation of cavities. The term cavities refers to voids as well as bubbles. Cavities can be imaged using Fresnel contrast [2] which arises from the phase shift generated by the variation of the mean inner potential of the matrix owing to atom deficiency caused by cavities [2]. The effect depends sensitively on the amplitude value of underor overfocus of the objective lens. In an under-focused image, cavities appear as dark Fresnel fringes surrounding a bright dot, while in an over-focused image, cavities appear as bright fringes surrounding a dark dot. In order to go further in this phase contrast approach, the phase shift between electrons which traverse the cavity and those which pass through adjacent perfect crystal may be retrieved by using in-line electron holography.