Theory of imaging a perfect crystal under the conditions of X-ray spherical wave dynamical diffraction

A theory of the formation of interference patterns due to X-ray spherical w ave two-beam dynamical diffraction in a perfect crystal is presented. An as ymmetrical Laue case is analyzed in detail, when a polychromatic focus is r ealized with different distances in front of and behind the crystal. Such a property is essential for high-energy X-rays produced by synchrotron radia tion sources of the third generation because of the long distance between s ource and object. It is shown that a monochromatic X-ray spherical wave is focused due to dynamical diffraction when a definite relation between dista nces and crystal thickness is held. An X-ray beam of less than 10 mum width may be obtained. A two-dimensional intensity distribution (topograph) may be registered with a wedge-shaped crystal. It shows interference fringes of different kinds depending on crystal thickness and asymmetry rate. It is a lso discussed how a slit in front of the crystal influences the interferenc e pattern. An example of an interference pattern is presented under the con dition of highly asymmetrical diffraction which was obtained by a computer simulation technique. Fringes of a new kind are observed and their physical nature is discussed.