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Cosmic Ray

The Nature And Origin Of Cosmic Rays

An atom of a particular element consists of a nucleus surrounded by a cloud of electrons, which are negatively charged particles. The nucleus is made up of protons, which have a positive charge, and neutrons, which have no charge. These particles can be further broken down into smaller constituents; all of these particles are known as subatomic particles. Cosmic rays consist of nuclei and of various subatomic particles. Almost all of the primary cosmic rays are nuclei of various atoms. The great majority of these are single protons, which are nuclei of hydrogen atoms. The next most common primary cosmic ray is the nucleus of the helium atom, made up of a proton and a neutron. Hydrogen and helium nuclei make up about 99% of the primary cosmic radiation. The rest consists of nuclei of other elements and of electrons.

When primary cosmic rays enter Earth's atmosphere, they collide with molecules of gases present there. These collisions result in the production of more high-energy subatomic particles of different types; these are the secondary cosmic rays. These include photons, neutrinos, electrons, positrons, and other particles. These particles may in turn collide with other particles, producing still more secondary radiation. If the energy of the primary particle that initiates this process is very high, this cascade of collisions and particle production can become quite extensive. This is known as a shower, air shower, or cascade shower.

The energy of cosmic rays is measured in units called electron volts (abbreviated eV). Primary cosmic rays typically have energies on the order of billions of electron volts. Some are vastly more energetic than this; a few particles have been measured at energies in excess of 1019 eV. This is in the neighborhood of the amount of energy required to lift a weight of 2.2 lb (1 kg) to a height of 3.3 ft (1 m). Energy is lost in collisions with other particles, so secondary cosmic rays are typically less energetic than primary ones. The showers of particles described above diminish as the energies of the particles produced decrease. The energy of cosmic rays was first determined by measuring their ability to penetrate substances such as gold or lead.

Since cosmic rays are mostly charged particles (some secondary rays such as photons have no charge), they are affected by magnetic fields. The paths of incoming primary cosmic rays are deflected by the earth's magnetic field, somewhat in the way that iron filings will arrange themselves along the lines of force emitted by a magnet. More energetic particles are deflected less than those having less energy. In the 1930s it was discovered that more particles come to Earth from the West than from the East. Because of the nature of Earth's magnetic field, this led scientists to the conclusion that most of the incoming cosmic radiation consists of positively charged particles. This was an important step towards the discovery that the primary cosmic rays are mostly bare atomic nuclei, since atomic nuclei carry a positive charge.

The ultimate origin of cosmic radiation is still not completely understood. Some of the radiation is believed to have been produced in the "big bang" at the origin of the universe. Other cosmic rays are produced by our Sun, particularly during solar disturbances such as solar flares. Exploding stars, called supernovas, are also a source of cosmic rays.

The fact that cosmic ray collisions produce smaller subatomic particles has provided a great deal of insight into the fundamental structure of matter. The construction of experimental equipment such as particle accelerators has been inspired by a desire to reproduce the conditions under which high-energy radiation is produced, in order to gain better experimental control of collisions and the production of particles.

See also Particle detectors.



Friedlander, Michael. Cosmic Rays. Cambridge: Harvard University Press, 1989.

Longair, M. S. High Energy Astrophysics. Cambridge: Cambridge University Press, 1981.

Millikan, Robert Andrews. Electrons (+ and -), Protons, Photons, Neutrons, and Cosmic Rays. Chicago: University of Chicago Press, 1935.


"Cosmic Rays: Are Air Crews At Risk?" Occupational and Environmental Medicine 59, no. 7 (2002): 428-432.

"Radiation Risk During Long-Term Spaceflight." Advaces in Space Research 30, no. 4 (2002): 989-994.

John Bishop


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—A negatively charged particle, ordinarily occurring as part of an atom. The atom's electrons form a sort of cloud about the nucleus.

Electron volt (eV)

—The unit used to measure the energy of cosmic rays.


—A device for demonstrating the presence of an electric charge, which may be positive or negative.


—An atom or molecule which has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).


—Particle found in the nucleus of an atom, possessing no charge.


—The central mass of an atom. The nucleus is composed of neutrons and protons.

Primary cosmic ray

—Cosmic ray originating outside Earth's atmosphere.


—Positively charged particle composing part of the nucleus of an atom. Primary cosmic rays are mostly made up of single protons.

Secondary cosmic ray

—Cosmic ray originating within Earth's atmosphere as a result of a collision between another cosmic ray and some other particle or molecule.

Shower (also air shower or cascade shower)

—A chain reaction of collisions between cosmic rays and other particles, producing more cosmic rays.

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Science EncyclopediaScience & Philosophy: Cosine to Cyano groupCosmic Ray - Discovery Of Cosmic Rays, The Nature And Origin Of Cosmic Rays