Fission fragment damage to crystal lattices: dislocation formation

Abstract

The disorder in the molecular array of a crystal lattice caused by a fission fragment has been studied by high-resolution electron microscopy. Disturbances in the (20ī) fringes of platinum phthalocyanine have been observed and a limited theoretical interpretation of these disturbances suggests that complex dislocation arrays are formed over an area which is easily determined. In particular there is evidence that dislocation dipoles can remain behind in a crystal after fission fragment bombardment. Molecular planes are shown to curve inwards towards the localized regions of damage. Observations made on the optical absorption spectra of copper and platinum phthalocyanine, both irradiated and unirradiated, are also described. The technique of irradiating thin crystals with fission fragments and observing the resultant damage in the electron microscope has been extended to other materials. In moiré patterns on overlapping crystals of molybdenum trioxide there is further evidence of a curvature of the molecular planes near regions of damage and in molybdenum disulphide dislocations resulting from irradiation are revealed by diffraction contrast. There is evidence that the radiation damage bothe crystal lattice can be due to both thermal and displacement spikes and their relative importance is dependent on the nature of the crystal.