High energy x-ray diffraction

The fundamental scattering process of x-rays of energies around 10 keV is governed by absorption for most atomic species. This has led to the development of several experimental techniques based on the discontinuous character of x-ray absorption, such as multi-wavelength anomalous diffraction, anomalous diffuse scattering including small-angle scattering, and absorption spectroscopy (EXAFS, SEXAFS, XANES). This paper shows the various contributions to the total scattering cross-section as a function of photon energy for two examples: carbon and lead. For carbon there is a cross-over of the coherent and the photoelectric cross-section at about 40 keV whereas for lead photoabsorption is always much stronger than coherent scattering, even in the range from 100 keV to 1 MeV. In both cases the level of Compton scattering is comparable. Fortunately, Bragg-scattering or diffraction gives rise to an enhancement of the elastic coherent scattering such that diffraction or extinction effects can be much stronger than absorption. High energy x-ray diffraction aims at minimizing absorption effects so that the true scattering effects of x-rays by crystalline matter are predominant. One consequence is that the bulk of materials rather than the surface is studied thus overcoming what is often considered to be a disadvantage of x-ray techniques as compared to e.g., neutron scattering: the strongly limited penetration depth. But there are also other interesting advantages which will be pointed out in this paper. Scattering theories for perfect and non-perfect (mosaic) crystals can be found in standard textbooks, for instance by Zachariasen. In the following sections only some essential, basic features will be recalled