Growth of magnesium oxide during neutron irradiation

Abstract

Abstract The macroscopic and x-ray growth of single crystal and polycrystalline magnesium oxide after neutron irradiation at 75–100°C has been investigated. At doses below 1020 nvt the x-ray growth was found to exceed the macroscopic growth by a small but significant amount. At doses above 1020nvt the macroscopic growth continued to increase at a reduced rate but the x-ray growth decreased and rapidly became much less than the macroscopic growth. Annealing studies show that the recovery was dependent on dose and that the lattice parameter changes became negative before returning to the pre-irradiation value. An interpretation of the data is presented involving the formation of interstitial loops at doses above 1020 nvt and during annealing. Mean values of the distortion per defect are derived which correspond to the distortion averaged over all unannealed defects, whether isolated or clustered. For vacancies the distortion is a contraction having a maximum value of 0·4 atomic volumes and decreasing with increasing dose, whereas interstitials produce a dilatation of around 3 atomic volumes. The defect production rate is calculated to be 6–8 vacancies per fast neutron collision.