cosmic-ray

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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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.
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