Création de nouveaux champs d'ondes généralisés dus à la présence d'un objet diffractant à l'intérieur d'un cristal parfait. II. Cas d'un défaut isolé

Takagi's equations of X-ray diffraction by an imperfect crystal are solved in the most general case by the method used in a previous paper in the particular case of a perfect crystal. It is shown that, for small strain gradients, the amplitude inside the crystal is the sum of two generalized wavefields obtained by convolution of the Hankel functions H¹₀ and H²₀ with a `source distribution' depending on local deformations. This is in agreement with the results given by other authors (Kato, Penning, Malgrange). This corresponds to the approximation of geometrical optics for X-ray propagation in crystals. A quantitative criterion of validity for this approximation is given. When this criterion is not fulfilled in some part of the crystal, the generalized wavefields are diffracted. It is shown that this implies creation of new generalized wavefields. This provides a physical interpretation of the occurrence of this phenomenon in the vicinity of a dislocation line and properties of the contrast of images in X-ray topography.