Radiation, Synchrotron
RADIATION, SYNCHROTRON
Electromagnetic radiation can be generated by accelerating electrically charged particles. This phenomenon is well known from the electrons moving up and down in an antenna or from the electrons that are decelerated at the surface of the anode in an X-ray tube, which results in the emission of Bremsstrahlung. The term synchrotron radiation is employed when an accelerated charge moves with a velocity close to the speed of light relative to an observer. This occurs in storage rings where highly relativistic free electrons or positrons, moving in a closed orbit at a constant energy, are deflected by strong magnetic fields. The centripetal acceleration stimulates the emission of synchrotron radiation. Due to the relativistic velocity the radiation is emitted within a small cone tangential to the direction of the particle, which results in a high degree of collimation of synchrotron radiation.
The power emitted by a relativistic particle with rest mass m and energy E per revolution in the storage ring is proportional to (E/m )4/R , with R being the radius of curvature. Since a proton is 1,836 times heavier than an electron, the total radiated power per revolution is much larger for lighter electrons or positrons than for heavier protons. The unique properties of synchrotron radiation, which distinguish it in particular from conventional X-ray tubes, are its continuous spectrum, extending from the farinfrared to the hard X-ray region; its high intensities; its small source size, determined by the electron beam; its high degree of natural collimation; as well as its linear polarization in the orbit plane and its elliptical, nearly circular polarization above and below the orbit plane. Furthermore, synchrotron radiation provides a well-defined time structure, and its intensity distribution can be calculated quantitatively with high accuracy.
The first experimental observation of synchrotron radiation emitted by centripetally accelerated relativistic electrons was made in 1947 at the General Electric 70 MeV synchrotron in Schenectady, New York. Since then, dedicated synchrotron radiation facilities with large user communities have developed from the early synchrotrons with low electron currents to today's third generation storage rings dedicated exclusively to the production of synchrotron radiation.
Several terms characterize the emitted X-ray beam: flux describes the number of emitted photons per second, brightness refers to how much the beam diverges as it propagates, while brilliance includes additionally the source size as defined by the electron beam.
The properties of synchrotron radiation emitted from a bending magnet is fully determined by the energy E of the electrons in the storage ring and the radius R of the orbit of the bending magnet. For an actual storage ring, R is fixed and E can only be varied over a limited range. More flexibility is offered by straight sections into which arrays of magnets can be installed. These insertion devices force the electron beam to wiggle, and by adjusting the magnetic field and thereby the amplitude of the oscillations, one can adjust the X-ray properties to the needs of a particular experiment. Depending on the strength of the electron deflection, one distinguishes wigglers and undulators. In a wiggler the electron beam is deflected by strong magnetic fields in a sinusoidal transverse motion. At each oscillation the electrons emit synchrotron radiation, and the radiation emitted in the different poles is incoherently superimposed. The wiggler radiation is therefore a superposition of the radiation fans from N individual bending magnets, and the intensity is on the order of N times that of a corresponding bending magnet source.
An undulator is a similar arrangement of successive small bending magnets like a wiggler, but the field strength is smaller such that the electron-beam deflection is small compared to the natural opening angle of the emitted synchrotron radiation. Therefore, the properties of undulator radiation are based on a coherent superposition of the X-rays emitted from each individual electron in the poles of the device with itself, and the spectral and spatial distributions are characterized by these interference effects. The superior properties of an undulator are its high brightness proportional to N2. Undulators with N = 100 magnetic poles are common and can theoretically achieve an increase in brightness by a factor of 104over that of a bending magnet.
These extremely bright X rays can be used to investigate objects of atomic and molecular size and a large variety of experimental challenges in physics, chemistry, geology, biology and applied sciences benefit from the possibility of fine-tuning the required properties of synchrotron radiation. With the use of appropriate X-ray monochromators, one can choose a particular photon energy, which is best suited for the experiment. This makes photon and electron based spectroscopic techniques, such as X-ray absorption spectroscopy, photoemission, and X-ray scattering as well as X-ray microscopy and X-ray microtomography, powerful techniques for the investigation of the electronic and geometric structure of materials.
See also:Radiation, Cherenkov; Relativity
Bibliography
Koch, E. E., ed. Handbook on Synchrotron Radiation (North-Holland Publishing Company, New York, 1983).
Winick, H., and Doniach, S. Synchrotron Radiation Research (Plenum, New York, 1980).
Katharina Baur