Licensed from Columbia University Press
Licensed from Columbia University Press
Licensed from Columbia University Press
Licensed from Columbia University Press
Licensed from Columbia University Press
Electromagnetic radiation emitted by charged particles that are moving at speeds close to that of light when their paths are altered. It is so called because it is produced by high-speed particles in a synchrotron. Such radiation is highly polarized (see polarization) and continuous. Its intensity and frequency depend on the strength of the magnetic field that alters the path of the particles, as well as on the energy of those particles. Synchrotron radiation at radio frequencies is emitted by high-energy electrons as they spiral through magnetic fields in space, such as those around Jupiter. Synchrotron radiation is emitted by a variety of astronomical objects, from planets to supernova remnants to quasars.
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Imaging technique used in diagnosis and biomedical research. A chemical compound labeled with a radioactive isotope (see radioactivity) that emits positrons is injected into the body, and detectors measure their activity in the tissues as they combine with electrons and are annihilated. Computers analyze, integrate, and reconstruct the data to produce images of the organs scanned. PET is particularly useful for studying brain and heart functions.
Learn more about positron emission tomography (PET) with a free trial on Britannica.com.
Device that produces and amplifies electromagnetic radiation in the microwave range of the spectrum. The first maser was built in 1951 by Charles H. Townes. Its name is an acronym for “microwave amplification by stimulated emission of radiation.” The wavelength produced by a maser is so constant and reproducible that it can be used to control a clock that will gain or lose no more than a second over hundreds of years. Masers have been used to amplify faint signals returned from radar and communications satellites, and have made it possible to measure faint radio waves emitted by Venus, giving an indication of the planet's temperature. The maser was the principal precursor of the laser.
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Device that produces an intense beam of coherent light (light composed of waves having a constant difference in phase). Its name, an acronym derived from “light amplification by stimulated emission of radiation,” describes how its beam is produced. The first laser, constructed in 1960 by Theodore Maiman (born 1927) based on earlier work by Charles H. Townes, used a rod of ruby. Light of a suitable wavelength from a flashlight excited (see excitation) the ruby atoms to higher energy levels. The excited atoms decayed swiftly to slightly lower energies (through phonon reactions) and then fell more slowly to the ground state, emitting light at a specific wavelength. The light tended to bounce back and forth between the polished ends of the rod, stimulating further emission. The laser has found valuable applications in microsurgery, compact-disc players, communications, and holography, as well as for drilling holes in hard materials, alignment in tunnel drilling, long-distance measurement, and mapping fine details.
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Any of three processes of radioactive disintegration in which a beta particle is spontaneously emitted by an unstable atomic nucleus in order to dissipate excess energy. Beta particles are either electrons or positrons. The three beta-decay processes are electron emission, positron emission, and electron capture. The process of beta decay increases or decreases the positive charge of the original nucleus by one unit without changing the mass number. Though beta decay is in general a slower process than gamma or alpha decay, beta particles can penetrate hundreds of times farther than alpha particles. Beta decay half-lives are a few milliseconds or more. Seealso radioactivity.
Learn more about beta decay with a free trial on Britannica.com.
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