gamma ray

noun Physics.
1.
a photon of penetrating electromagnetic radiation (gamma radiation) emitted from an atomic nucleus.
2.
a photon emitted by an electron as a result of internal conversion.
3.
electromagnetic radiation with wavelengths shorter than approximately one tenth of a nanometer.

Origin:
1900–05

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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American Heritage
Medical Dictionary

gamma ray n.
Electromagnetic radiation emitted from the nucleus of an atom by radioactive decay and having energies in a range from ten thousand (104) to ten million (107) electron volts.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
gamma ray  
A stream of high-energy electromagnetic radiation given off by an atomic nucleus undergoing radioactive decay. Because the wavelengths of gamma rays are shorter than those of x-rays, gamma rays have greater energy and penetrating power than x-rays. Gamma rays are emitted by pulsars, quasars, and radio galaxies but cannot penetrate the Earth's atmosphere. See more at radioactive decay.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
They can't find it and they need the best gamma ray scientist for the job.
OK, so there's no such thing as a gamma ray machine that zaps scientists and
  turns them into giant green monsters.
Certain gamma ray frequencies correspond to the chemical signature of
  explosives.
Those gamma ray bursts from the early universe could have been anywhere in the
  universe.
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