When a crystalline sample is illuminated with x-rays, the x-rays are scattered (diffracted) into very specific directions with various intensities. Detectors are used to measure this “diffraction pattern,” which is then processed by computers to deduce the arrangement of atoms within the crystal.
Hard x-rays have wavelengths comparable to the distance between atoms. Essentially everything we know about the atomic structure of materials is based on results from x-ray and neutron diffraction. From advanced ceramics to catalysts, from semiconductor technology to the frontiers of medicine, and from new magnetic materials and devices to framework compounds used to sequester radioactive waste, crystallography using hard x-ray diffraction techniques at synchrotron radiation facilities plays a crucial role in our ability to understand and control the world in which we live.
The scattering of x-rays from protein crystals is the most powerful method of determining the three-dimensional structure of large biological molecules (macromolecules). Because macromolecules are large and flexible, their crystals tend to be small, imperfect, and weakly diffracting. In many cases, the intensity, small beam size, and collimation of a synchrotron beam is vital for successful results.
Soft x-ray scattering techniques employ the excitation of electrons in relatively shallow core energy levels (100–2000 eV) to probe the electronic structure and other properties of various kinds of matter. The sample is illuminated with monochromatic soft x-rays, and the scattered photons are detected over a small angular range. In the elastic scattering mode, one measures the speckle diffraction pattern. In the inelastic mode, the scattered photons are passed through a spectrometer and analyzed.