Mon 21 Aug 2017

Synchrotron radiation

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Synchrotron radiation is electromagnetic radiation, similar to cyclotron radiation, but generated by the acceleration of ultrarelativistic (i.e., moving near the speed of light) electrons through magnetic fields. This may be achieved artificially by storage rings in a synchrotron, or naturally by fast moving electrons moving through magnetic fields in space. The radiation typically includes infrared, optical, ultraviolet, x-rays.

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]

Synchrotron radiation is also called "Non-thermal emission" by radio astronomers.[3]

Synchrotron radiation characterization

  • High brightness and high intensity, many orders of magnitude more than with X-rays produced in conventional X-ray tubes
  • High brilliance, exceeding other natural and artificial light sources by many orders of magnitude: 3rd generation sources typically have a brilliance larger than 1018 photons/s/mm2/mrad2/0.1%BW, where 0.1%BW denotes a bandwidth 10-3w centered around the frequency w.
  • High collimation, i.e. small angular divergence of the beam
  • Low emittance, i.e. the product of source cross section and solid angle of emission is small
  • Widely tunable in energy/wavelength by monochromatization (sub eV up to the MeV range)
  • High level of polarization (linear or elliptical)
  • Pulsed light emission (pulse durations at or below one nanosecond, or a billionth of a second);

Synchrotron radiation in astronomy

M87's Energetic Jet. The glow is caused by synchrotron radiation, high-energy electrons spiraling along magnetic field lines, and was first detected in 1956 by Geoffrey R. Burbidge in M87 confirming a prediction by Hannes Alfvén and Nicolai Herlofson in 1950, and Iosif S. Shklovskii in 1953.

Synchrotron radiation is also generated by astronomical structures and motions, typically where relativistic electrons spiral (and hence change velocity) through magnetic fields. Two of its characteristics include (1) Non-thermal radiation (2) Polarization.[4]

History

It was first detected, in a jet emitted by Galaxy M87, in 1956 by Geoffrey R. Burbidge [5], who saw it as confirmation of a prediction by Iosif S. Shklovskii in 1953,[6] 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 gravitationally accelerating ions through magnetic fields.

Footnotes

  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. Gerrit L. Verschuur, The Invisible Universe: The Story of Radio Astronomy, (2007), Springer, ISBN: 0-387-30816-4
  4. Vladimir A. Bordovitsyn, "Synchrotron Radiation in Astrophysics" (1999) Synchrotron Radiation Theory and Its Development, ISBN 981-02-3156-3
  5. Burbidge, G. R. "On Synchrotron Radiation from Messier 87. Astrophysical Journal, vol. 124, p.416"
  6. I.S. Shklovskii (Iosif Shklovsky), "On the nature of the luminescence of the Crab Nebula", Doklady Akad. Nauk SSSR, 1953. English translation in: K.R. Lang & O. Gingerich (eds), Source Book in astronomy and astrophysics 1900-1975, Harvard University Press, 1979, ISBN 0674822005
  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)

References

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