Cavity growth in neutron-irradiated magnesium oxide

Abstract

The growth of cuboidal cavities in neutron-irradiated magnesium oxide after annealing in the temperature range 1475 to 1775° C has been followed using the techniques of transmission electron microscopy and electron spin resonance. Microscopic examination has shown that, while cavities are nucleated on annealing at about 1500° C, most of their growth does not occur until a temperature approaching 1625° C is exceeded. Electron spin resonance spectra from the same samples annealed in the temperature range 1475 to 1575° C indicate that some of the vacancies, which are released to the lattice when cavities are nucleated, are used in the formation, from iron present at impurity level in the crystals, of Fe³⁺ in octahedral symmetry at magnesium sites. This corresponds to the region of negligible cavity growth. For annealing treatments at and above 1625° C however, the fractional volume of crystal occupied by cavities increases by a factor of ten and simultaneously the octahedral symmetry Fe³⁺ transitions disappear. The role of iron in controlling vacancy movement and cavity growth is discussed.