GNU Collaborative International Dictionary of English

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1.       
   Gamma ray.  n. (Physics) A very penetrating electromagnetic ray not appreciably deflected by a magnetic or electric field, emitted by radioactive substances.  Gamma rays are photons of electromagnetic radiation having a wavelength shorter than that of X-rays, (i. e. shorter than 0.1 nanometer) and are correspondingly more penetrating than X-rays.  In addition to being given off in certain types of radioactive decay, they may be found in cosmic radiation, though they are largely absorbed by the earth's atmosphere.  Gamma-ray detectors orbited above the atmosphere have found bursts of gamma radiation, in some cases associated with visually observed supernova explosions, but in most cases from unidentified sources.
   
   [Webster 1913 Suppl. +PJC]