Dislocations, Point-Defect Clusters, and Cavities in Neutron Irradiated LiF Crystals

Micrographic techniques have been used to study dislocations, point defect clusters, and cavities in neutron-irradiated LiF crystals. No evidence of inhomogeneous damage caused by ``thermal or displacement spikes'' was found. However, a uniform etching effect was found for doses greater than ∼1015 nvt. The etching is believed to be caused by clusters of point defects greater than 2 A but less than 50 A in size. This agrees with previous x-ray results. The defect clusters decrease in number and increase in size with increasing annealing temperatures. At temperatures greater than ∼400°C only (110) dislocation loops remain and these disappear above ∼600°C. Electron bombardment causes a similar effect. The as-grown dislocation structure of LiF is not affected by neutron bombardment. However, it becomes increasingly difficult to move fresh dislocations in crystals given neutron doses greater than ∼1012 nvt. The hardening is independent of dislocation velocity and temperature and hence cannot be caused by Cottrellpinning of dislocations. The color changes that accompany hardening suggest that it is caused by defect clusters rather than single defects. At annealing temperatures above ∼600°C crystallographic cavities appear in LiF crystals. They are rectangular parallelepipeds with (100) sides. Evidence is presented to show that the cavities are not caused by fission gases but probably by loss of fluorine from the crystals.