charged particles moving at nearly the speed of light reaching the earth from outer space. Primary cosmic rays consist mostly of protons (nuclei of hydrogen atoms), some alpha particles (helium nuclei), and lesser amounts of nuclei of carbon, nitrogen, oxygen, and heavier atoms. These nuclei collide with nuclei in the upper atmosphere, producing secondary cosmic rays of protons, neutrons, mesons, electrons, and gamma rays of high energy, which in turn hit nuclei lower in the atmosphere to produce more particles (see elementary particles). These cascade processes continue until all the energy of the primary particle is dissipated. The secondary particles shower down through the atmosphere in diminishing intensity to the earth's surface and even penetrate it. The size of the shower indicates the energy of the primary ray, which may be as high as 10²⁰ electron volts (eV) or more, almost a billion times higher than the highest energy yet produced in a man-made particle accelerator; however, cosmic rays of lower energy predominate. Cosmic rays were long used as a source of high-energy particles in the study of nuclear reactions. The positron, the muon, the pion (or pi meson), and some of the so-called strange particles were initially discovered in studies of this radiation. Cosmic rays were first found to be of extraterrestrial origin by Victor F. Hess (c.1912) when he recorded them with electrometers carried to high altitudes in balloons, an achievement for which he won the Nobel Prize in 1936. They were so named in 1925 by R. A. Millikan, who did extensive research on them. Since then much pertinent information has been collected that have been of use in studying the chemical composition of the universe, but the origin of cosmic rays remains a mystery. However, when they react with interstellar gases, the result is a gamma ray that can be traced back. Spacecraft results indicate that many of the gamma rays appear to come from the direction of supernova remnants. The nature of the acceleration processes by which the primary particles achieve great velocities (very nearly the speed of light) is also still highly speculative. Modern electronic detectors called charge coupled devices (CCDs) are effective cosmic ray detectors; a ray can strike a single pixel, making it much brighter than the surrounding ones. Cosmic rays play a significant role in the natural mutation and evolution of life on earth.

See B. B. Rossi, Cosmic Rays (1964); L. I. Dorman, Cosmic Rays (1974); M. W. Friedlander, Cosmic Rays (1989).

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