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Cosmic Rays

        Science | Astronomy Terms

Cosmic Rays, high-energy radiation consisting chiefly of subatomic particles moving at nearly the speed of light. Primary cosmic rays bombard the earth's atmosphere continuously from all directions. Few reach the surface of the earth, however, because most are stopped or destroyed in violent collisions with atoms of air. The collisions create showers of new subatomic particles called secondary cosmic rays, which move at very high speeds in the same general direction as the primary rays that produced them. Some secondary cosmic rays, in turn, collide with other air atoms lower in the atmosphere, creating even more secondary cosmic rays. By the time it reaches the earth's surface, a shower may consist of hundreds of thousands of secondary rays, all created within a fraction of a second.

Astronomers believe that most primary cosmic rays originate within our galaxy and that they reach their high speeds because of the action of magnetic forces in space. The sun and the magnetic fields of planets in the solar system have been shown to be sources of weak cosmic rays. Specific sources of other cosmic rays cannot be determined easily, because large magnetic fields in the galaxy scatter the rays in all directions. Some astronomical observations indicate that a principal source of cosmic rays is Cygnus X-3, a distant binary star system near the edge of our galaxy. Other sources may be supernovae and pulsars. The most powerful cosmic rays probably originate in other galaxies.

Most primary cosmic rays are the nuclei of atoms, in particular the nuclei of hydrogen atoms (protons) and helium atoms. Typically, the energy of primary cosmic rays that reach the vicinity of the earth ranges from 1 to 10 GeV (1 to 10 giga [billion] electron volts). Some cosmic rays, however, have energies more than a billion times greater (up to 1020 eV). These energies are much higher than the energies acquired by particles in particle accelerators.

Secondary cosmic rays consist of gamma rays and a large variety of elementary particles, including mesons, protons, neutrons, electrons, and positrons. They are constantly striking every object on earth. Though each secondary cosmic ray possesses only a small fraction of the energy of the primary ray, some secondary cosmic rays have enough energy to penetrate deep into solid rock.

As forms of high-energy radiation, both primary and secondary cosmic rays may affect living things by causing changes in their genes, which control heredity. Cosmic rays are involved in the formation of carbon 14, which is used in the radioactive dating of organic material. The carbon 14 is formed when cosmic rays strike nitrogen atoms in the air.

Cosmic rays provide scientists with information about the structure, operation, and history of the galaxy and of the universe. Studies of cosmic rays have led to the discovery of many elementary particles. Also, cosmic rays have provided a test of certain effects predicted by the Special Theory of Relativity, because the particles that constitute cosmic rays move at speeds close to that of light.

The Study of Cosmic Rays

Scientists in the early 1900's were baffled by an unknown form of radiation that was able to penetrate the most heavily insulated container. In 1911 and 1912, instruments carried on balloon flights high into the stratosphere showed that this mysterious radiation increases with altitude. In 1925 it was shown conclusively that the rays originate beyond the earth's atmosphere. Such Nobel Prize winners as Robert Millikan, Arthur Compton, and Victor Hess made important contributions to the early study of cosmic rays. Research on cosmic rays has been greatly aided not only by balloons but also by rockets and artificial satellites.

Studies of cosmic rays have been made with a variety of detection devices. The earliest studies were made with electroscopes; later such instruments as cloud chambers and Geiger-Mller counters were used. Today, instruments called scintillation counters are commonly used. A scintillation counter contains a fluorescent material that produces a flash of light when struck by radiation such as cosmic rays. An electronic device called a photomultiplier tube converts the flash of light into an electrical signal that is recorded by the counter. With an array of scintillation counters, the direction and energy of the radiation can be determined.