Cosmic rays are very high energy particles, mainly originating in outer space, outside the Solar system. They may produce showers of secondary particles that penetrate the Earth's atmosphere and sometimes even Earth's surface.
The term ray is an historical accident as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. Now, the common usage is to call particles with non-zero rest mass “cosmic” rays, and particles (photons) which are quanta of electromagnetic radiation (and thus with zero rest-mass) by their common names of gamma rays or x-rays.
Of primary cosmic rays, about 99% are the nuclei (stripped of their electron shells) of well-known atoms, and about 1% are solitary electrons (similar to beta particles). Of the nuclei, about 90% are simple protons, i. e. hydrogen nuclei; 9% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. A very small fraction are stable particles of antimatter, such as positrons or antiprotons, and the precise nature of this remaining fraction is an area of active research.
Read more about Cosmic Rays: Introduction, Primary Cosmic Rays, Secondary Cosmic Ray Particles, Detection By Particle Track-etch Technique, Research and Experiments, History
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