cosmic radiation (cosmic rays) Charged particles from space that constantly bombard the Earth at velocities approaching the speed of light. The extraterrestrial nature of cosmic radiation was discovered (c.1912) by Victor Hess and has contributed greatly to the development of particle physics. Primary cosmic radiation consists mainly of protons (hydrogen nuclei) and some alpha particles (helium nuclei). These are the most energetic particles known; as high as 10²⁰ electron volts (eV), or nearly a billion times more energetic than the highest energy yet produced in a particle accelerator. There are two main types of primary radiation, galactic and solar. It is believed that galactic rays originate chiefly from supernovae. The energy for solar rays appears to be obtained from solar flares. Some primary nuclei penetrate Earth's magnetic field and enter the upper atmosphere, where they collide with other nuclei to produce secondary cosmic radiation of nucleons (protons and neutrons), mesons, leptons (such as electrons), and high-energy gamma radiation). Nuclear reactions, or cascades, continue until all the energy of the primary particle is dissipated. Cosmic radiation contributes to background radiation. See also supernova

radiation, cosmic (cosmic rays) Streams of subatomic particles from space that constantly bombard the Earth at velocities approaching the speed of light. Primary cosmic rays are high-energy radiation that comes from the Sun and other sources in outer space. This includes ultraviolet rays, X-rays and radio waves. They consist mainly of atomic nuclei and protons. When primary cosmic rays strike gas molecules in the upper atmosphere they yield showers of secondary cosmic rays, which consist of energetic protons, neutrons, and pions. Further collisions yield muons, alpha particles, positrons, electrons, gamma radiation, and photons.

cosmic radiation Ionizing radiation from space, comprised principally of protons, alpha particles, and 1–2% heavier atomic nucleii, as well as some high-energy photons and electrons. On encountering the Earth's atmosphere, secondary radiation is produced, mainly gamma rays, electrons, pions, and muons. Three sources are identified: (a) galactic cosmic rays, from outside the solar system, with energies in the range 1–10 GeV per nucleon; (b) solar cosmic rays, mainly associated with solar flares, with energies in the range 1–100 MeV per nucleon; and (c) solar wind, with energies of about 1000 eV per nucleon.

cosmic radiation Ionizing radiation from space, consisting principally of protons, alpha particles, and 1–2 per cent heavier atomic nuclei, as well as some high-energy photons and electrons. On encountering the Earth's atmosphere, secondary radiation is produced, mainly gamma rays, electrons, pions, and muons. Three sources are identified: (a)galactic cosmic rays, from outside the solar system, with energies in the range 1–10 GeV per nucleon;(b)solar cosmic rays, mainly associated with solar flares, with energies in the range 1–100 MeV per nucleon; and(c)solar wind, with energies of about 1000 eV per nucleon.