X-ray sources

An Introductory Course by Bernhard Rupp

Laboratory X-ray sources

X-rays of a suitable wavelength range for protein crystallography (~0.8 - 2.3 ) 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.

You can read the first pages 1, 2, 3 of Chapter 8 (Data Collection)  of my book Biomolecular Crystallography or buy the book from Amazon.

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. 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 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.

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