Cosmic Rays
| Professors: Finley, Lyutikov Discovered in 1912 by Victor Hess, cosmic Rays are energetic particles from outer space. They produce secondary particles that penetrate Earth's atmosphere and surface. Most cosmic rays are composed of familiar particles, such as protons and electrons. However, a small fraction are composed of antimatter, such as positrons and antiprotons. The nature of this fraction is an area of active research. The nuclei that make up cosmic rays are able to travel from distant sources to the Earth due to the low density of matter in space. When cosmic rays approach Earth they collide with the nuclei of atmospheric gases, which results in a shower of pions and kaons: unstable mesons which quickly decay into muons. When cosmic rays impact elements heavier than hydrogen and helium they can be detected indirectly by observing high energy gamma ray emissions from these elements using a gamma-ray telescope, such as the VERITAS array. |
 The Moon as seen by the Compton Gamma Ray Observatory, in gamma rays of greater than 20 MeV. These are produced by cosmic ray bombardment of its surface. The Sun, which has no similar surface of high atomic number to act as target for cosmic rays, cannot be seen at all at these energies, which are too high to emerge from primary nuclear reactions, such as solar nuclear fusion. |