Radiation Effects in LiF Crystals

Recovery behaviors of lattice expansion and optical absorption of neutron-irradiated LiF crystals have been measured as a function of annealing temperature. Remarkable correspondence between the two properties leads an interpretation of steps or humps and reduction in recovery curves of lattice constant respectively by formation of colloidal centers which are probably coagulations of vacancies and recombination of vacancies and interstitials. Optical microscopic observations reveal the fact that the square cavities, considered to be coagulations of vacancies remaining after displacement of atoms by neutron irradiation, preferentially formed on dislocation lines, along which F 2 gas escapes from the crystals. It is suggested that the colloidal centers are formed by clustering of point defects and some of them, which probably consist of vacancies, grow to square cavities during heat treatment.