A Theoretical Analysis of the Weak-beam Method of Electron Microscopy

In electron microscope studies of lattice defects, two of the principle features of interest are the geometry and character of the individual defects, or defect elements. Many of these defect systems require image analysis at high resolution (e.g. the dissociation of dislocations in pure metals; the study of small point defect clusters) and the weakbeam method of electron microscopy1 enables systematic analyses to be carried out at a resolution approaching the resolution capabilities of the electron microscope. In this paper, the theoretical basis for the method is discussed, and some approximate methods for interpreting the images are deduced. The experimental conditions which are necessary to obtain weak-beam images, and to use these methods of interpretation, are considered. Other than studies using the weak-beam method, high resolution electron microscope investigations of defect structures have followed two courses. One of these is to form lattice fringe images from crystals containing defects and to deduce the defect structure from the perturbations of the fringes. However, it has been shown recently2 that the interpretation of such images is more complicated than has generally been assumed. The second method is to form darkor bright-field images with the lattice oriented near to the Bragg condition for the dark-field reflection in order to optimise the image contrast while keeping the image intensity appreciable (so-called "strongbeam" conditions). Where such images do not enable the defect structure to be determined unambiguously, or in sufficient detail, or where some