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Cosmic rays are immensely high-energy radiation, mainly originating outside the Solar System. They may produce showers of secondary particles that penetrate and impact the Earth's atmosphere and sometimes even reach the surface. Composed primarily of high-energy protons and atomic nuclei, they are of mysterious origin. Data from the ''Fermi'' space telescope (2013) has been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernovae of massive stars.〔(【引用サイトリンク】 title=Evidence Shows that Cosmic Rays Come from Exploding Stars )〕 However, this is not thought to be their only source. Active galactic nuclei probably also produce cosmic rays. The term ''ray'' is a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. In common scientific usage high-energy particles with intrinsic mass are known as "cosmic" rays, and photons, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as "gamma rays" or "X-rays", depending on their origin. Cosmic rays attract great interest practically, due to the damage they inflict on microelectronics and life outside the protection of an atmosphere and magnetic field, and scientifically, because the energies of the most energetic ultra-high-energy cosmic rays (UHECRs) have been observed to approach 3 × 1020 eV, about 40 million times the energy of particles accelerated by the Large Hadron Collider. One can show that such enormous energies might be achieved by means of the Centrifugal mechanism of acceleration in Active galactic nuclei. At 50 J, the highest-energy ultra-high-energy cosmic rays have energies comparable to the kinetic energy of a baseball. As a result of these discoveries, there has been interest in investigating cosmic rays of even greater energies.〔 〕 Most cosmic rays, however, do not have such extreme energies; the energy distribution of cosmic rays peaks at . Of primary cosmic rays, which originate outside of Earth's atmosphere, 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 alpha particles, and 1% are the nuclei of heavier elements, called HZE ions.〔(【引用サイトリンク】 title=What are cosmic rays? ) (copy )〕 A very small fraction are stable particles of antimatter, such as positrons or antiprotons. The precise nature of this remaining fraction is an area of active research. An active search from Earth orbit for anti-alpha particles has failed to detect them. == History == After the discovery of radioactivity by Henri Becquerel and Marie Curie in 1896, it was generally believed that atmospheric electricity, ionization of the air, was caused only by radiation from radioactive elements in the ground or the radioactive gases or isotopes of radon they produce. Measurements of ionization rates at increasing heights above the ground during the decade from 1900 to 1910 showed a decrease that could be explained as due to absorption of the ionizing radiation by the intervening air. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cosmic ray」の詳細全文を読む スポンサード リンク
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