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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

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

When high-energy particles are in rapid motion, including electrons 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, in theory, the relativistic speed will change the observed frequency due to the Doppler effect by the Lorentz factor, γ. Relativistic time contraction then bumps the frequency observed in the lab by another factor of γ, thus multiplying the GeV frequency of the resonant cavity that accelerates the electrons into the X-ray range. The radiated power is given by the relativistic Larmor formula while the force on the emitting electron is given by the Abraham-Lorentz-Dirac force. The radiation pattern can be distorted from an isotropic dipole pattern into an extremely forward-pointing cone of radiation. Synchrotron radiation is the brightest artificial source of X-rays. The planar acceleration geometry appears to make the radiation linearly polarized when observed in the orbital plane, and circularly polarized when observed at a small angle to that plane. Amplitude and frequency are however focussed to the polar ecliptic.

Synchrotron radiation from accelerators

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. In the LHC proton bunches also produce the radiation at increasing amplitude and frequency as they accelerate with respect to the vacuum field, propagating photoelectrons, which in turn propagate secondary electrons from the pipe walls with increasing frequency and desnsity up to 7x1010 Each proton may loose 6.7keV per turn due to this phenomina. [3] As it takes exponentially more energy to accelerate a particle towards 'c' this allows the system to comply with the Law of Conservation of Energy.

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.[4] The radiation is also detected from the particle shocks of planets and groupings of mass such as the heliosphere. It is used by the ESA to calibrate the Planck CMB mission, where the planetary shock on the ecliptic polar (orbital path) produces a peak of foreground emmision[5].

History

It was first detected in a jet emitted by M87 in 1956 by Geoffrey R. Burbidge [6], 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 [7] 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."[8]

Supermassive black holes have been suggested for producing synchrotron radiation, by ejection of jets produced by gravitationally accelerating ions through the super contorted 'tubular' polar areas of magnetic fields. Such jets, the nearest being in Messier 87, have been confirmed by the Hubble telescope as apparently Superluminal, travelling at 6xC from our planetary frame. A number of inconsistent solutions have been offered but it is suggested by both the DFM and 'Einstein Aether theory' to be the columnar effect of charged particles following and travelling within previous 'tubes' of particles, all travelling at 'c' locally with respect to the previous particles. The observations are of change of position, with light from the observations arriving at 'c', so the 2nd postulate of SR (invarience of speed of light) would not be violated.

Pulsar wind nebulae

Crab Nebula
Crab Nebula.jpg

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[9], probably due to synchrotron emission by electrons trapped in the strong magnetic field around the pulsar. Polarization in the Crab[10] at energies from 0.1 to 1.0 MeV illustrates a typical synchrotron radiation.

See also

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. ^ [1] Synchrotron Radiation Damping in the LHC 2005 Joachim Tuckmantel.
  4. ^ Vladimir A. Bordovitsyn, "Synchrotron Radiation in Astrophysics" (1999) Synchrotron Radiation Theory and Its Development, ISBN 981-02-3156-3
  5. ^ [2] ESA Planck mission, foreground radiation and dust. Images.
  6. ^ Burbidge, G. R. "On Synchrotron Radiation from Messier 87. Astrophysical Journal, vol. 124, p.416"
  7. ^ Alfvén, H.; Herlofson, N. "Cosmic Radiation and Radio Stars" Physical Review (1950), vol. 78, Issue 5, pp. 616-616
  8. ^ 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)
  9. ^ "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"
  10. ^ Dean et al.,"Polarized Gamma-Ray Emission from the Crab", Science 29 August 2008: Vol. 321. no. 5893, pp. 1183 - 1185

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