cosmic ray

noun Physics.
a radiation of high penetrating power that originates in outer space and consists partly of high-energy atomic nuclei.

Origin:
1920–25, Americanism

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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American Heritage
Science Dictionary
cosmic ray  
A ray of radiation of extraterrestrial origin, consisting of one or more charged particles such as protons, alpha particles, and larger atomic nuclei. Cosmic rays entering the atmosphere collide with atoms, producing secondary radiation, such as pions, muons, electrons, and gamma rays. Cosmic rays (and secondary radiation) can be easily seen in a cloud chamber.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
We have produced a simple cosmic ray detector that can be built by high school
  teachers.
When a high-energy cosmic ray enters the atmosphere it loses its energy via
  interactions with the nuclei that make up the air.
Then cosmic ray experiments revealed the existence of other fundamental
  particles--pions and muons.
If you want, you may see the sketches of my design of the cosmic ray detector
  box.
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