What is Cosmic ray?
'''Cosmic rays''' are very high-energy [[radiation]], mostly from outside the [[Solar System]].<ref>{{cite book| last=Sharma| title=Atomic and nuclear physics| year=2008| publisher=Pearson Education India| isbn=978-81-317-1924-4| page=478}}</ref> The term ''ray'' is a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly [[electromagnetic radiation]]. They are [[particles]]. Most are [[protons]] and [[alpha particle]]s, which are the nuclei of [[helium]] atoms. Some are [[electrons]] (beta particles), [[gamma rays]] or [[photons]] and a tiny fraction are even heavier particles. == Origin == Their origin is not exactly known. There is evidence that many primary cosmic rays come from the [[supernova]]e of massive stars.<ref name=pinholster-2013>{{cite web| url=http://www.aaas.org/news/releases/2013/0214_supernova_cosmicrays.shtml| title=Evidence shows that cosmic rays come from exploding stars| author=Ginger Pinholster| date=2013}}</ref> However, this is not thought to be their only source. [[Active galactic nucleus|Active galactic nuclei]] probably also produce cosmic rays. Galactic cosmic rays are Particles trapped in the magnetic field of Earth, Particles sent flying into space during a solar flare, and High-energy protons and heavy ions that originate outside our solar system == Secondary cosmic rays == Cosmic rays can have as much energy as 10<sup>20</sup> eV, far higher than the 10<sup>12</sup> to 10<sup>13</sup> eV that man-made [[particle accelerator]]s can produce. When cosmic ray particles enter the [[Earth]]'s atmosphere they hit other particles, like molecules of [[oxygen]] and [[nitrogen]]. This causes a shower of lighter particles, a so-called 'air shower'. The number of particles made in an air shower event can reach billions. All of the particles in the shower stay within about one degree of the first particle's path. In a shower are [[x-rays]], [[muon]]s, protons, [[alpha particle]]s, [[pion]]s, [[electron]]s, and [[neutron]]s.<ref>{{cite book|last=Morison|first