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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) 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.
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".
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Encyclopedia
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.
History
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". 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."
Emission mechanism
Whenever 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 .
Relativistic Lorentz contraction bumps the frequency by another factor of , 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
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
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 (1) Non-thermal power-law spectrum, and (2) Polarization.
History
It was first detected in a jet emitted by M87 in 1956 by Geoffrey R. Burbidge , 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 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."
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
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, probably due to synchrotron emission by electrons trapped in the strong magnetic field around the pulsar.
Observation of polarization in the Crab at energies from 0.1 to 1.0 MeV is the signature of synchrotron radiation.
See also
External links
- , by Ginzburg, V. L., Syrovatskii, S. I., ARAA, 1965
- , by Ginzburg, V. L., Syrovatskii, S. I., ARAA, 1969
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