Laboratory X-ray sources
X-rays of a suitable wavelength range for protein crystallography (~0.8 - 2.3 Å) are generated by three commonly used devices: X-ray tubes, rotating anodes and synchrotrons. In-house or laboratory sources will produce X-rays using either an evacuated tube or a rotating anode. X-ray tubes consist of a filament that acts as a cathode. Electrons are emitted by the glowing cathode and accelerated by several tens of kVs across the vacuum towards the anode, which consists of a metal target made of a characteristic material, usually copper or chromium, for protein crystallography. As the electron beam impacts the anode, the high kinetic energy of the electrons is converted during deceleration into X-rays producing a) a continuous spectrum consisting of bremsstrahlung ("braking radiation") and b) emission lines characteristic for electronic transitions caused in the anode material. The characteristic X-ray emissions, which are important for crystallography, have an intensity that is several orders of magnitude higher than the bremsstrahlung. The Ka1 and Ka2 components of the X-rays emission are cut out from the bremsstrahlung and other emission lines by filters, monochromators or X-ray mirrors, and the resulting monochromatic X-rays are collimated and focused onto the crystals. When X-rays are produced by a rotating anode, the cathode and anode are housed under vacuum, in which the anode target rotates at high speed to efficiently distribute and dissipate heat. The wavelength of an in-house source such as a tube or rotating anode generator is fixed by the choice of anode target material and not tunable, as is the case at a synchrotron, and the intensity of the source is less than that of a synchrotron.
Synchrotron X-ray sources
At a synchrotron facility, bunches of electrons, several GeV in energy, move in a large, carefully steered, closed electron beam loop containing bending elements and linear segments, collectively called the storage ring. In each section, magnetic devices are inserted - bending magnets in the curved sections, insertion devices called wigglers and undulators in the straight sections - to bend, wiggle or undulate the path of the electrons while they pass around the ring (Figure 6). Due to the acceleration experienced in the bending magnets or insertion devices, the electrons emit a narrow fan of intense white (polychromatic) radiation ranging from soft UV to hard X-rays over a very tightly defined angle tangential to the ring. The radiation is 'tunable' by cutting out fine bands (few eV or 10-5 Å wide) of wavelengths appropriate for particular experiments with monochromator crystals that selectively pass the wavelength of choice. The intensity of X-rays generated by modern 3rd generation synchrotron sources is so high that radiation damage to crystals has become a major concern, and this has given rise to the near-exclusive use of cryo-crystallographic techniques, in which crystals are kept at near-liquid nitrogen temperatures to minimize radiation damage. Synchrotron radiation has additional features that make it attractive for advanced applications. Because it is pulsed, it can be exploited for examining time-dependent phenomena, and because it is highly polarized, it can be used to examine polarization-dependent and angle-dependent effects.