ABSTRACT

In crystallography, diffraction is the scattering of electromagnetic radiation by ordered arrays of molecules. X-ray crystallography measures diffraction patterns of crystals to obtain structural information. The wavelength of X-rays is on the scale of atomic distances (100–250 pm), and X-ray diffraction thus provides structural information with atomic resolution. There are two widely used methods to produce high-brilliance X-rays, emission during electron transfer from a high-energy state to a low-energy state between the inner shells of atoms and emission from particles accelerated to relativistic velocities. The intensity and quality of an X-ray beam is described by several parameters, among them the number of photons in the beam, the beam angular divergence, the cross-section of the beam, and its monochromaticity. When an electron bunch enters an undulator, its constituent electrons have a uniform spatial distribution, so the radiation they emit is incoherent, i.e. there is no phase relation between the X-rays.