Editing Cosmic ray (section)

===Energy distribution===
Measurements of the energy and arrival directions of the ultra-high-energy primary cosmic rays by the techniques of ''density sampling'' and ''fast timing'' of [[air shower (physics)|extensive air showers]] were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the [[Massachusetts Institute of Technology]].<ref>{{Cite journal|last1=Clark|first1=G.|last2=Earl|first2=J.|last3=Kraushaar|first3=W.|last4=Linsley|first4=J.|last5=Rossi|first5=B.|last6=Scherb|first6=F.|last7=Scott|first7=D.|doi=10.1103/PhysRev.122.637|title=Cosmic-Ray Air Showers at Sea Level|journal=Physical Review|volume=122|issue=2|pages=637–654|year=1961|bibcode=1961PhRv..122..637C}}</ref> The experiment employed eleven [[Scintillator|scintillation detectors]] arranged within a circle 460 metres in diameter on the grounds of the Agassiz Station of the [[Harvard College Observatory]]. From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 10<sup>20</sup>&nbsp;eV. A huge air shower experiment called the [[Pierre Auger Observatory|Auger Project]] is currently operated at a site on the [[Pampa]]s of Argentina by an international consortium of physicists. The project was first led by [[James Cronin]], winner of the 1980 Nobel Prize in Physics from the [[University of Chicago]], and [[Alan Andrew Watson|Alan Watson]] of the [[University of Leeds]], and later by scientists of the international Pierre Auger Collaboration. Their aim is to explore the properties and arrival directions of the very highest-energy primary cosmic rays.<ref>{{cite web|url=https://www.auger.org/index.php/observatory|title=The Pierre Auger Observatory|publisher=Auger Project|url-status=live|archive-url=https://web.archive.org/web/20180903033644/https://www.auger.org/index.php/observatory|archive-date=3 September 2018}}</ref> The results are expected to have important implications for particle physics and cosmology, due to a theoretical [[Greisen–Zatsepin–Kuzmin limit]] to the energies of cosmic rays from long distances (about 160 million light years) which occurs above 10<sup>20</sup>&nbsp;eV because of interactions with the remnant photons from the [[Big Bang]] origin of the universe. Currently the Pierre Auger Observatory is undergoing an upgrade to improve its accuracy and find evidence for the yet unconfirmed origin of the most energetic cosmic rays.

High-energy gamma rays (>50{{nbsp}}MeV photons) were finally discovered in the primary cosmic radiation by an MIT experiment carried on the OSO-3 satellite in 1967.<ref>{{cite journal|title=(none)|author=Kraushaar, W. L.| journal=The Astrophysical Journal|date=1972|volume=177|page=341|doi=10.1086/151713|bibcode=1972ApJ...177..341K|display-authors=etal|doi-access=free}}</ref> Components of both galactic and extra-galactic origins were separately identified at intensities much less than 1% of the primary charged particles. Since then, numerous satellite gamma-ray observatories have mapped the gamma-ray sky. The most recent is the Fermi Observatory, which has produced a map showing a narrow band of gamma ray intensity produced in discrete and diffuse sources in our galaxy, and numerous point-like extra-galactic sources distributed over the celestial sphere.