Neutron damage in MgO

Abstract

Single crystals of MgO irradiated to a dose of 4 × 10¹⁹ nvt at < 200°c become very brittle, and the length of the slip bands spreading from a microhardness indent becomes very small. Damage is visible under the electron microscope, but is too small to identify positively. From the density of this damage, and the smallness in the increase of the lattice parameter, 0•09%, it is concluded that most of the ions which had been displaced subsequently returned to normal lattice sites during irradiation. Upon annealing, the damage coarsens and can be identified as prismatic dislocation loops of Burgers vector lying on {110}. A crystal, irradiated to a dose of 5•4 × 10¹⁹ nvt at 650°c, contained similar dislocation loops, the majority, if not all, of which were of interstitial type. The damage in this crystal was homogeneous except for a narrow surface zone ≲1 μ deep, but when exactly similar crystals were annealed in air at 1200°c and 1350°c, the loops disappeared from a surface zone many microns deep whilst coarsening in the centre of the crystal. The length of the slip bands spreading from an indent varied with depth accordingly. The implications of this result are discussed.