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synchrotron radiation,in physics, electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
..... Click the link for more information. emitted by high-speed electrons spiraling along the lines of force of a magnetic field (see magnetismmagnetism,
force of attraction or repulsion between various substances, especially those made of iron and certain other metals; ultimately it is due to the motion of electric charges.
..... Click the link for more information. ). Depending on the electron's energy and the strength of the magnetic field, the maximum intensity will occur as radio waves, visible light, or X rays. The emission is a consequence of the constant acceleration experienced by the electrons as they move in nearly circular orbits; according to Maxwell's equations, all accelerated charged particles emit electromagnetic radiation. Although predicted much earlier, synchrotron radiation was first observed as a glow associated with protons orbiting in high-energy particle acceleratorsparticle accelerator,
apparatus used in nuclear physics to produce beams of energetic charged particles and to direct them against various targets. Such machines, popularly called atom smashers, are needed to observe objects as small as the atomic nucleus in studies of its
..... Click the link for more information. , such as the synchrotron. In astronomy, synchrotron radiation has been suggested as the mechanism for producing strong celestial radio sources like the Crab NebulaCrab Nebula,
diffuse gaseous nebula in the constellation Taurus; cataloged as NGC 1952 and M1, the first object recorded in Charles Messier's catalog of nonstellar objects (see Messier catalog).
..... Click the link for more information. (see radio astronomyradio astronomy,
study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally. Radio Telescopes
Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes,
..... Click the link for more information. ). Synchrotron radiation is employed in a host of applications, ranging from solid-state physicssolid-state physics,
study of the properties of bulk matter rather than those of the individual particles that compose it. Solid-state physics is concerned with the properties exhibited by atoms and molecules because of their association and regular, periodic arrangement in
..... Click the link for more information. to medicine. As excellent producers of X rays, synchrotron sources offer unique probes of the semiconductors that lie at the heart of the electronics industry. Both ultraviolet radiation and X rays generated by synchrotrons are also employed in the treatment of diseases, especially certain forms of skin cancer.
Electromagnetic radiation emitted by relativistic charged particles curving in magnetic or electric fields. With the development of electron storage rings, radiation with increasingly high flux, brightness, and coherent power levels has become available for a wide variety of basic and applied research in biology, chemistry, and physics, as well as for applications in medicine and technology. See Electromagnetic radiation, Particle accelerator, Relativistic electrodynamics
Electron storage rings provide radiation from the infrared through the visible, near-ultraviolet, vacuum-ultraviolet, soft-x-ray, and hard-x-ray parts of the electromagnetic spectrum extending to 100 keV and beyond. The flux [photons/(second, unit bandwidth)], brightness (or brilliance) [flux/(unit source size, unit solid angle)], and coherent power (important for imaging applications and proportional to brightness) available for experiments, particularly in the vacuum-ultraviolet, soft-x-ray, and hard-x-ray parts of the spectrum, are many orders of magnitude higher than is available from other sources.
The radiation has many features (natural collimation, high intensity and brightness, broad spectral bandwidth, high polarization, pulsed time structure, small source size, and high-vacuum environment) that make it ideal for a wide variety of applications in experimental science and technology. Very powerful sources of synchrotron radiation in the ultraviolet and x-ray parts of the spectrum became available when high-energy physicists began operating electron synchrotrons in the 1950s. Although synchrotrons produce large amounts of radiation, their cyclic nature results in pulse-to-pulse intensity changes and variations in spectrum and source shape during each cycle. By contrast, the electron-positron storage rings developed for colliding-beam experiments starting in the 1960s offered a constant spectrum and much better stability. Beam lines were constructed on both synchrotrons and storage rings to allow the radiation produced in the bending magnets of these machines to leave the ring vacuum system and reach experimental stations. In most cases the research programs were pursued on a parasitic basis, secondary to the high-energy physics programs.
Since about 1980, fully dedicated storage ring sources have been completed in several countries. They are called second-generation facilities to distinguish them from the first-generation rings that were built for research in high-energy physics.
Special magnets may be inserted into the straight sections between ring bending magnets to produce beams with extended spectral range or with higher flux and brightness than is possible with the ring bending magnets. These devices, called wiggler and undulator magnets, utilize periodic transverse magnetic fields to produce transverse oscillations of the electron beam with no net deflection or displacement. They provide another order-of-magnitude or more improvement in flux and brightness over ring bending magnets, again opening up new research opportunities. However, their potential goes well beyond their performance levels, in first- and second-generation sources.
Third-generation sources are storage rings with many straight sections for wiggler and undulator insertion device sources and with a smaller transverse size and angular divergence of the circulating electron beam. The product of the transverse size and divergence is called the emittance. The lower the electron-beam emittance, the higher the photon-beam brightness and coherent power level. With smaller horizontal emittances and with straight sections that can accommodate longer undulators, third-generation rings provide two or more orders of magnitude higher brightness and coherent power level than earlier sources.
One consequence of the extraordinary brilliance of these sources is that the x-ray beam is partially coherent. By aperturing the beam, a fully coherent beam can be obtained, but at the expense of flux. Nonetheless, there is still sufficient flux remaining to explore the use and application of coherent x-ray beams. See Coherence
Several third-generation rings are in operation. Low-energy (typically 1–2-GeV) third-generation rings (see illustration) are optimized to produce high-brightness radiation in the vacuum ultraviolet (VUV) and soft x-ray spectral range, up to photon energies of about 2–3 keV. High-energy rings (typically 6–8 GeV) aim at harder x-rays with energies of 10–20 keV and above.
The radiation produced by an electron in circular motion at low energy (speed much less than the speed of light) is weak and rather nondirectional. At relativistic energies (speed close to the speed of light) the radiated power increases markedly, and the emission pattern is folded forward into a cone with a half-opening angle in radians given approximately by γ - 1 = mc2/E, where mc2 is the rest-mass energy of the electron (0.51 MeV) and E is the total energy. Thus, at electron energies of the order of 1 GeV, much of the very strong radiation produced is confined to a forward cone with an instantaneous opening angle of about 1 mrad (0.06°). At higher electron energies this cone is even smaller. The large amount of radiation produced combined with the natural collimation gives synchrotron radiation its intrinsic high brightness. Brightness is further enhanced by the small cross-sectional area of the electron beam, which is as low as 0.01 mm2 in the third-generation rings.
the emission of electromagnetic waves by charged particles moving at relativistic speeds in a magnetic field. The radiation is due to the acceleration associated with the bending of the trajectories of the particles in the magnetic field.
The analogous radiation of nonrelativistic particles moving in circular or spiral trajectories is called cyclotron radiation. Such radiation occurs at the Larmor frequency and its first harmonics. As the speed of the particle increases, the role of higher harmonics is enhanced. When the relativistic limit is approached, the radiation in the region of the most intense higher harmonics has a practically continuous spectrum and is concentrated in the direction of the instantaneous velocity in a narrow cone with an opening angle ψ ~ mc2E, where m and E are the mass and energy of the particle and c is the speed of light in a vacuum.
The total power radiated by a particle of energy E ≫ mc2 is equal to
where e is the charge of the particle and H ⊥ is the magnetic field component perpendicular to the particle’s velocity. Because the power radiated is strongly dependent on the mass of the particle, significant synchrotron radiation is produced only by light particles—electrons and positrons. The spectral distribution of the radiated power— that is, the distribution with respect to the frequency v— is determined by the expression
where Vc = (3eH⊥/4 Πmc) (E/mc2)2 and K5/3 (η) is a cylindrical function of the second kind of the imaginary independent variable. The graph of the function
is shown in Figure 1. The frequency at which the maximum in the particle’s emission spectrum occurs is, in hertz,
v ≈ 0.29vc = 1.8 × 1018H⊥ E2
where E is in ergs; for E in electron volts, v ≈ 4.6 × 10-6H ⊥ E2.
In the general case, the radiation of an individual particle is elliptically polarized with the major axis of the ellipse of polarization lying perpendicular to the apparent projection of the magnetic field. The degree of ellipticity and the direction of rotation of the electric vector depend on the direction of observation with respect to the cone described by the vector of the particle’s velocity about the magnetic field direction. The polarization is linear for directions of observation lying on the cone.
Synchrotron radiation was originally observed from electrons in cyclic accelerators—in particular, synchrotrons (hence its name). The energy losses due to synchrotron radiation and also the quantum effects in particle motion that are associated with synchrotron radiation must be taken into account in designing cyclic high-energy electron accelerators. The synchrotron radiation of cyclic electron accelerators is used to produce intense beams of polarized electromagnetic radiation in the ultraviolet region of the spectrum and in the region of soft X rays. Beams of X-ray synchrotron radiation are used, in particular, in X-ray diffraction analysis.
The synchrotron radiation of cosmic objects is of great interest. Particularly important are the nonthermal radio-frequency background radiation of the Galaxy and the nonthermal radio-frequency and optical radiation of such discrete sources as supernovas, pulsars, quasars, and radio galaxies. The synchrotron nature of these radiations is confirmed by the characteristics of their spectrum and polarization. According to current notions, the relativistic electrons found in cosmic rays produce synchrotron radiation in cosmic magnetic fields in the radio-frequency, optical, and, perhaps, X-ray regions. Measurements of the spectral intensity and polariation of cosmic synchrotron radiation permit information to be obtained on the concentration and energy spectrum of relativistic electrons and on the strength and orientation of the magnetic field in remote parts of the universe.
C. I. SYROVATSKII