Fission fragment damage to crystal lattices: track contrast

Abstract

In previous papers (Bowden & Chadderton 1962; Chadderton & Montagu-Pollock 1963) some account has been given of the type of damage sustained by both stable and unstable crystals, and its relation to the principle mechanisms of energy loss. In particular it has been demonstrated that fission fragments can create so much damage that ‘tracks’ may be observed in the electron microscope, and that these may be black or white, depending upon the thickness of the bombarded crystal. This paper considers the nature of black tracks and describes their formation as images in the electron microscope in terms of ‘diffraction contrast’. A model is established for the damage produced by a fission fragment in an initially perfect crystal. This essentially consists of a central cylinder of disturbed material sur­rounded by a radial strain field, and is mathematically very similar to the model invoked by Ashby & Brown (1963 a ) to describe the spherically symmetrical coherency strain. The model is used in conjunction with the two-beam dynamical theory of electron diffraction (Howie & Whelan 1961) and machine calculations are made of contrast profiles for tracks at different positions in the crystal and for various values of the parameters involved. A good measure of agreement between experiment and theory is demonstrated, and, in particular, it is shown how anomalous images and oscillatory contrast effects may be explained. Asymmetric images on dark-field micrographs can be used in a determination of the direction of radial strain, and anomalous images in both bright and dark field yield information about the direction of particle travel. A method is suggested for obtaining information concerning the pattern of damage in continuous and discontinuous tracks and for testing the validity of the depleted zone concept.