synchrotron radiation

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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 also 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 also 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.

Layout of the 1.5-GeV Advanced Light Source at Lawrence Berkeley National Laboratory, a low-energy, third-generation synchrotron radiation source. Applications of experimental stations on beam lines are indicated.

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.

This article concerns the physical phenomenon of synchrotron radiation. For details on the production of this radiation and applications in laboratories, see Synchrotron light source.

Synchrotron radiation is electromagnetic radiation, similar to cyclotron radiation, but generated by the acceleration of ultrarelativistic (i.e., moving near the speed of light) charged particles through magnetic fields. This may be achieved artificially in synchrotrons or storage rings, or naturally by fast electrons moving through magnetic fields in space. The radiation produced may range over the entire electromagnetic spectrum, from radio waves to infrared light, visible light, ultraviolet light, X-rays, and gamma rays. It is distinguished by its characteristic polarization and spectrum.

Contents 1 History 2 Emission mechanism 3 Synchrotron radiation from accelerators 4 Synchrotron radiation in astronomy 4.1 History 4.2 Pulsar wind nebulae 5 See also 6 Notes 7 External links

History

General Electric synchrotron accelerator built in 1946, the origin of the discovery of synchrotron radiation. The arrow indicates the evidence of arcing.

The radiation was named after its discovery in a General Electric synchrotron accelerator built in 1946 and announced in May 1947 by Frank Elder, Anatole Gurewitsch, Robert Langmuir, and Herb Pollock in a letter entitled "Radiation from Electrons in a Synchrotron"^[1]. Pollock recounts:

"On April 24, Langmuir and I were running the machine and as usual were trying to push the electron gun and its associated pulse transformer to the limit. Some intermittent sparking had occurred and we asked the technician to observe with a mirror around the protective concrete wall. He immediately signaled to turn off the synchrotron as "he saw an arc in the tube." The vacuum was still excellent, so Langmuir and I came to the end of the wall and observed. At first we thought it might be due to Cherenkov radiation, but it soon became clearer that we were seeing Ivanenko and Pomeranchuk radiation."^[2]

Emission mechanism

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When high-energy relativistic electrons are forced to travel in a curved path by a magnetic field, synchrotron radiation is produced, similar to a radio antenna, but with the difference that the relativistic speed changes the observed frequency due to the Doppler effect by a factor γ. Relativistic Lorentz contraction bumps the frequency by another factor of γ, thus multiplying the GeV frequency of the resonant cavity that accelerates the electrons into the X-ray range. Another dramatic effect of relativity is that the radiation pattern is distorted from the isotropic dipole pattern expected from non-relativistic theory into an extremely forward-pointing cone of radiation. This makes artificial synchrotron radiation the brightest known source of X-rays. The planar acceleration geometry makes the radiation linearly polarized when observed in the orbital plane, and circularly polarized when observed at a small angle to that plane.

Synchrotron radiation from accelerators

Main article: synchrotron light source

Synchrotron radiation may occur in accelerators either as a nuisance, causing undesired energy loss in particle physics contexts, or as a deliberately produced radiation source for numerous laboratory applications. Electrons are accelerated to high speeds in several stages to achieve a final energy that is typically in the GeV range.

Synchrotron radiation in astronomy

M87's Energetic Jet., HST image. The blue light from the jet emerging from the bright AGN core, towards the lower right, is due to synchrotron radiation.

Synchrotron radiation is also generated by astronomical objects, typically where relativistic electrons spiral (and hence change velocity) through magnetic fields. Two of its characteristics include non-thermal power-law spectra, and polarization.^[3]

History

It was first detected in a jet emitted by M87 in 1956 by Geoffrey R. Burbidge ^[4], who saw it as confirmation of a prediction by Iosif S. Shklovskii in 1953, but it had been predicted several years earlier by Hannes Alfvén and Nicolai Herlofson ^[5] in 1950.

T. K. Breus noted that questions of priority on the history of astrophysical synchrotron radiation is quite complicated, writing:

"In particular, the Russian physicist V.L. Ginsburg broke his relationships with I.S. Shklovsky and did not speak with him for 18 years. In the West, Thomas Gold and Sir Fred Hoyle were in dispute with H. Alfven and N. Herlofson, while K.O. Kiepenheuer and G. Hutchinson were ignored by them."^[6]

Supermassive black holes have been suggested for producing synchrotron radiation, by relativistic beaming of jets produced by gravitationally accelerating ions through magnetic fields.

Pulsar wind nebulae

Crab Nebula

The bluish glow from the central region of the nebula is due to synchrotron radiation.

A class of astronomical sources where synchrotron emission is important is the pulsar wind nebulas, or plerions, of which the Crab nebula and its associated pulsar are archetypal. Pulsed emission gamma-ray radiation from the Crab has recently been observed up to ≥25 GeV^[7], probably due to synchrotron emission by electrons trapped in the strong magnetic field around the pulsar. Polarization in the Crab^[8] at energies from 0.1 to 1.0 MeV illustrates a typical synchrotron radiation.

See also

• Synchrotron for this type of particle accelerator
• Synchrotron light source for laboratory generation and applications of synchrotron radiation
• Cyclotron Radiation
• Relativistic beaming
• Radiation reaction
• Sokolov–Ternov effect

Notes

1. ^ Elder, F. R.; Gurewitsch, A. M.; Langmuir, R. V.; Pollock, H. C., "Radiation from Electrons in a Synchrotron" (1947) Physical Review, vol. 71, Issue 11, pp. 829-830
2. ^ Handbook on Synchrotron Radiation, Volume 1a, Ernst-Eckhard Koch, Ed., North Holland, 1983, reprinted at "Synchrotron Radiation Turns the Big Five-O"
3. ^ Vladimir A. Bordovitsyn, "Synchrotron Radiation in Astrophysics" (1999) Synchrotron Radiation Theory and Its Development, ISBN 981-02-3156-3
4. ^ Burbidge, G. R. "On Synchrotron Radiation from Messier 87. Astrophysical Journal, vol. 124, p.416"
5. ^ Alfvén, H.; Herlofson, N. "Cosmic Radiation and Radio Stars" Physical Review (1950), vol. 78, Issue 5, pp. 616-616
6. ^ Breus, T. K., "Istoriya prioritetov sinkhrotronnoj kontseptsii v astronomii %t (Historical problems of the priority questions of the synchrotron concept in astrophysics)" (2001) in Istoriko-Astronomicheskie Issledovaniya, Vyp. 26, p. 88 - 97, 262 (2001)
7. ^ "Observation of Pulsed {gamma}-Rays Above 25 GeV from the Crab Pulsar with MAGIC", Science 21 November 2008: Vol. 322. no. 5905, pp. 1221 - 1224"
8. ^ Dean et al.,"Polarized Gamma-Ray Emission from the Crab", Science 29 August 2008: Vol. 321. no. 5893, pp. 1183 - 1185

External links

• Cosmic Magnetobremsstrahlung (synchrotron Radiation), by Ginzburg, V. L., Syrovatskii, S. I., ARAA, 1965
• Developments in the Theory of Synchrotron Radiation and its Reabsorption, by Ginzburg, V. L., Syrovatskii, S. I., ARAA, 1969
• Lightsources.org
• X-Ray Data Booklet

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