Line patterns in wide-angle convergent beam electron diffraction

Wide-angle convergent-beam electron diffraction patterns from thin single crystals show sets of parallel lines somewhat similar to Kikuchi or Kossel lines. In some cases, each line separates into a black and white line pair when the electron beam is defocused. It is shown that this separation depends on the curvature of the crystal. A method for analysis of the line geometry is proposed which takes into account the aberrations of the electron lens. Using a high-voltage transmission scanning electron microscope, through-focus series of patterns from graphite crystals have been obtained and analyzed to give values for the radius of curvature of the crystal. In defocussed patterns the distortions of the lattice by defects are clearly visible.