On the production of characteristic X-rays in thin metal crystals

Abstract

According to the two beam approximation of the dynamical theory of electron diffraction an electron propagating through a crystal is represented by a combination of two Bloch waves, one of which (the type 1 wave) tends to channel the electron between the planes of atoms while the other (the type 2 wave) locates the electron on average closer to the centres of the atoms. It has been suggested that as X-ray production might be expected to occur in regions close to the nuclei of the atoms, the type 2 wave should be more efficient at producing X-rays than the type 1 wave, and that this should result in a variation of X-ray production with orientation (Hirsch, Howie & Whelan 1962). This paper describes an experimental investigation of this effect. A more complete theoretical treatment of the process is given followed by a description of the apparatus used in the experiments. It is shown that the results are in qualitative agreement with the simple theory, and can be explained quantitatively by extending the theory to take into consideration the effect of the other scattering processes which act upon the electrons which produce the X-rays.