synchrotron radiation - Synchrotron radiation from storage rings, Synchrotron radiation in astronomy

X-ray radiation emitted by charged particles travelling around a synchrotron. Accelerating electrical charges emit electromagnetic radiation, so by the same mechanism charged particles moving in a circle emit radiation. Thought at one time merely a radiation hazard associated with accelerators, synchrotron radiation has become one of the most powerful research tools available for the study of condensed matter and biological systems, using X-ray diffraction, microscopy, and spectroscopy; custom-built synchrotrons produce very bright, well collimated beams no more than a few millimetres across. Important synchrotron radiation sources include the European Synchrotron Source in Grenoble, France, the Advanced Photon Source at Argonne National Laboratory, Illinois, and SPring-8 in Nishi-Harima, Japan.

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". At first we thought it might be due to Cherenkov radiation, but it soon became clearer that we were seeing Ivanenko and Pomeranchuk radiation."

Synchrotron radiation from storage rings

Synchrotron radiation is characterized by:

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 10/mradw centered around the frequency w.

Electrons are accelerated to high speeds in several stages to achieve a final energy that is typically in the GeV range. The change of direction is a form of acceleration and thus the electrons emit radiation at GeV frequencies. Another dramatic effect of relativity is that the radiation pattern is also distorted from the isotropic dipole pattern expected from non-relativistic theory into an extremely forward-pointing cone of radiation. This makes synchrotron radiation sources the brightest known sources 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.

The advantages of using synchrotron radiation for spectroscopy and diffraction have been realized by an ever-growing scientific community, beginning in the 1960s and 1970s. In the beginning, storage rings were built for particle physics and synchrotron radiation was used in "parasitic mode" when bending magnet radiation had to be extracted by drilling extra holes.

As the application of synchrotron radiation became more intense and promising, devices that enhanced the intensity of synchrotron radiation were built into existing rings. Third-generation synchrotron radiation sources were conceived and optimized from the outset to produce bright X-rays.

As mentioned above, bending electromagnets are usually used to generate the radiation, but to generate stronger radiation, another kind of device, called an insertion device, is sometimes employed. Current third-generation synchrotron radiation sources are typically heavily based upon these insertion devices, when straight sections in the storage ring are used for inserting periodic magnetic structures (composed of many magnets that have a special repeating row of N and S poles) that force the electrons into a sinusoidal path or helical path.

There are openings in the storage ring to let the radiation exit and follow a beam line into the experimenters' vacuum chamber. A great number of such beamlines can emerge from modern third-generation synchrotron radiation sources.

Synchrotron radiation is used in particle accelerators in radiation damping, a method of reducing beam emittance.

Synchrotron radiation in astronomy

Synchrotron radiation is also generated by astronomical structures and motions, typically where relativistic electrons spiral (and hence change velocity) through magnetic fields.

Supermassive black holes have been suggested for producing synchrotron radiation, by gravitationally accelerating ions through magnetic fields.