Editing Cosmic ray (section)

==History==
After the discovery of [[Radioactive decay|radioactivity]] by [[Henri Becquerel]] in 1896, it was generally believed that atmospheric electricity, [[ionization]] of the [[Earth's atmosphere|air]], was caused only by [[radiation]] from radioactive elements in the ground or the radioactive gases or isotopes of [[radon]] they produce.<ref>{{cite book|last=Malley|first=Marjorie C.|date=25 August 2011|title=Radioactivity: A History of a Mysterious Science|pages=78–79|publisher=Oxford University Press|isbn=9780199766413|url=https://books.google.com/books?id=yvtjtKSfX54C&pg=PA78}}</ref> Measurements of increasing ionization rates at increasing heights above the ground during the decade from 1900 to 1910 could be explained as due to absorption of the ionizing radiation by the intervening air.<ref>{{cite book|first=John|last=North|date=15 July 2008|title=Cosmos: An Illustrated History of Astronomy and Cosmology|page=686|publisher=University of Chicago Press|isbn=9780226594415|url=https://books.google.com/books?id=qq8Luhs7rTUC&pg=PA686}}</ref>

===Discovery===
[[File:Pacini measurement.jpg|thumb|left|Pacini makes a measurement in 1910.|upright=0.8]]
In 1909, [[Theodor Wulf]] developed an [[electrometer]], a device to measure the rate of ion production inside a hermetically sealed container, and used it to show higher levels of radiation at the top of the [[Eiffel Tower]] than at its base.<ref>{{cite journal |last1=Wulf |first1=Theodor |title=Beobachtungen über die Strahlung hoher Durchdringungsfähigkeit auf dem Eiffelturm |journal=Physikalische Zeitschrift |date=1910 |volume=11 |pages=811–813 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101054770845&view=1up&seq=349&skin=2021 |trans-title=Observations of radiation of high penetration power at the Eiffel tower |language=German}}</ref> However, his paper published in ''[[Physikalische Zeitschrift]]'' was not widely accepted. In 1911, [[Domenico Pacini]] observed simultaneous variations of the rate of ionization over a lake, over the sea, and at a depth of 3 metres from the surface. Pacini concluded from the decrease of radioactivity underwater that a certain part of the ionization must be due to sources other than the radioactivity of the Earth.<ref>{{cite journal|author=Pacini, D.|year=1912|title=La radiazione penetrante alla superficie ed in seno alle acque|journal=[[Il Nuovo Cimento]]|volume=3|issue=1|pages=93–100|doi=10.1007/BF02957440|arxiv=1002.1810|bibcode=1912NCim....3...93P|s2cid=118487938}}: Translated with commentary in
{{cite journal|author=de Angelis, A.|year=2010|title=Penetrating Radiation at the Surface of and in Water|journal=[[Il Nuovo Cimento]]|volume=3|issue=1|pages=93–100|arxiv=1002.1810|bibcode=1912NCim....3...93P|doi=10.1007/BF02957440|s2cid=118487938}}</ref>

In 1912, [[Victor Francis Hess|Victor Hess]] carried three enhanced-accuracy Wulf electrometers<ref name=HessNobelPresSp>{{cite web|url=http://nobelprize.org/nobel_prizes/physics/laureates/1936/press.html|title=Nobel Prize in Physics 1936 – Presentation Speech|publisher=Nobelprize.org|date=10 December 1936|access-date=27 February 2013}}</ref> to an altitude of 5,300&nbsp;metres in a [[hot air balloon|free balloon]] flight. He found the ionization rate increased to twice the rate at ground level.<ref name=HessNobelPresSp/> Hess ruled out the Sun as the radiation's source by making a balloon ascent during a near-total eclipse. With the moon blocking much of the Sun's visible radiation, Hess still measured rising radiation at rising altitudes.<ref name=HessNobelPresSp/> He concluded that "The results of the observations seem most likely to be explained by the assumption that radiation of very high penetrating power enters from above into our atmosphere."<ref>{{cite journal|author=Hess, V.F. |author-link=Victor Francis Hess |year=1912 |title=Über Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten |trans-title=On observations of penetrating radiation during seven free balloon flights |language=de |journal=Physikalische Zeitschrift |volume=13 |pages=1084–1091 |arxiv=1808.02927}}</ref> In 1913–1914, [[Werner Kolhörster]] confirmed Victor Hess's earlier results by measuring the increased ionization enthalpy rate at an altitude of 9&nbsp;km.<ref>{{cite journal |last1=Kolhörster |first1=Werner |title=Messungen der durchdringenden Strahlung im Freiballon in größeren Höhen |journal=Physikalische Zeitschrift |date=1913 |volume=14 |pages=1153–1156 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015021268936&view=1up&seq=1285&skin=2021 |trans-title=Measurements of the penetrating radiation in a free balloon at high altitudes |language=German}}</ref><ref>{{cite journal |last1=Kolhörster |first1=W. |title=Messungen der durchdringenden Strahlungen bis in Höhen von 9300 m. |journal=Verhandlungen der Deutschen Physikalischen Gesellschaft |date=1914 |volume=16 |pages=719–721 |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924056112083&view=1up&seq=741&skin=2021 |trans-title=Measurements of the penetrating radiation up to heights of 9300 m. |language=German}}</ref>[[File:HessKol.jpg|thumb|Increase of ionization with altitude as measured by Hess in 1912 (left) and by Kolhörster (right)]] Hess received the [[Nobel Prize in Physics]] in 1936 for his discovery.<ref>{{cite web|author=Hess, V.F.|author-link=Victor Francis Hess|year=1936|title=The Nobel Prize in Physics 1936|publisher=[[The Nobel Foundation]]|url=http://nobelprize.org/nobel_prizes/physics/laureates/1936/index.html|access-date=11 February 2010}}</ref><ref>{{cite web|author=Hess, V.F.|author-link=Victor Francis Hess|year=1936|title=Unsolved Problems in Physics: Tasks for the Immediate Future in Cosmic Ray Studies|series=Nobel Lectures|publisher=The Nobel Foundation|url=http://nobelprize.org/nobel_prizes/physics/laureates/1936/index.html|access-date=11 February 2010}}</ref>
[[File:Hessballon.jpg|thumb|Hess lands after his balloon flight in 1912.]]

===Identification===
[[Bruno Rossi]] wrote in 1964:
<blockquote>In the late 1920s and early 1930s the technique of self-recording electroscopes carried by balloons into the highest layers of the atmosphere or sunk to great depths under water was brought to an unprecedented degree of perfection by the German physicist [[Erich Regener]] and his group. To these scientists we owe some of the most accurate measurements ever made of cosmic-ray ionization as a function of altitude and depth.<ref>{{cite book|last=Rossi|first=Bruno Benedetto|title=Cosmic Rays|location=New York|publisher=McGraw-Hill|year=1964|isbn=978-0-07-053890-0}}</ref></blockquote>

[[Ernest Rutherford]] stated in 1931 that "thanks to the fine experiments of Professor Millikan and the even more far-reaching experiments of Professor Regener, we have now got for the first time, a curve of absorption of these radiations in water which we may safely rely upon".<ref>{{cite journal|last1=Geiger|first1=H.|last2=Rutherford|first2=Lord|last3=Regener|first3=E.|last4=Lindemann|first4=F.A.|last5=Wilson|first5=C.T.R.|last6=Chadwick|first6=J.|last7=Gray|first7=L.H.|last8=Tarrant|first8=G.T.P.|last9=Dobson|first9=G.M.B.|display-authors=6|year=1931|title=Discussion on Ultra-Penetrating Rays|journal=Proceedings of the Royal Society of London A|volume=132|issue=819|page=331|bibcode=1931RSPSA.132..331G|doi=10.1098/rspa.1931.0104|doi-access=free}}</ref>

In the 1920s, the term ''cosmic ray'' was coined by [[Robert Millikan]] who made measurements of ionization due to cosmic rays from deep under water to high altitudes and around the globe. Millikan believed that his measurements proved that the primary cosmic rays were gamma rays; i.e., energetic photons. And he proposed a theory that they were produced in interstellar space as by-products of the fusion of hydrogen atoms into the heavier elements, and that secondary [[electron]]s were produced in the atmosphere by [[Compton scattering]] of gamma rays. In 1927, while sailing from [[Java]] to the Netherlands, [[Jacob Clay]] found evidence,<ref>{{cite journal|author=Clay, J.|year=1927|title=Penetrating Radiation|journal=Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam| volume=30|issue=9–10|pages=1115–1127|url=http://www.dwc.knaw.nl/DL/publications/PU00011919.pdf |archive-url=https://web.archive.org/web/20160206055736/http://www.dwc.knaw.nl/DL/publications/PU00011919.pdf |archive-date=2016-02-06 |url-status=live}}</ref> later confirmed in many experiments, that cosmic ray intensity increases from the tropics to mid-latitudes, which indicated that the primary cosmic rays are deflected by the geomagnetic field and must therefore be charged particles, not photons. In 1929, [[Walther Bothe|Bothe]] and [[Werner Kolhörster|Kolhörster]] discovered charged cosmic-ray particles that could penetrate 4.1&nbsp;cm of gold.<ref>{{cite journal|author1=Bothe, Walther| author2=Werner Kolhörster|date=November 1929|title=Das Wesen der Höhenstrahlung|journal=Zeitschrift für Physik|volume=56|issue=11–12|pages=751–777|doi=10.1007/BF01340137|bibcode=1929ZPhy...56..751B|s2cid=123901197}}</ref> Charged particles of such high energy could not possibly be produced by photons from Millikan's proposed interstellar fusion process.{{citation needed|date=April 2015}}

In 1930, [[Bruno Rossi]] predicted a difference between the intensities of cosmic rays arriving from the east and the west that depends upon the charge of the primary particles—the so-called "east–west effect".<ref>{{cite journal|title=On the Magnetic Deflection of Cosmic Rays|author=Rossi, Bruno|journal=Physical Review|date=August 1930|volume=36|issue=3|page=606|doi=10.1103/PhysRev.36.606|bibcode=1930PhRv...36..606R}}</ref> Three independent experiments<ref>{{cite journal|title=The Azimuthal Asymmetry of the Cosmic Radiation|author=Johnson, Thomas H.|journal=Physical Review|date=May 1933|volume=43|issue=10|pages=834–835|doi=10.1103/PhysRev.43.834|bibcode=1933PhRv...43..834J}}</ref><ref>{{cite journal|title=A Positively Charged Component of Cosmic Rays|author=Alvarez, Luis|author2=Compton, Arthur Holly|journal=Physical Review|date=May 1933|volume=43|issue=10|pages=835–836|doi=10.1103/PhysRev.43.835|bibcode=1933PhRv...43..835A}}</ref><ref>{{cite journal|title=Directional Measurements on the Cosmic Rays Near the Geomagnetic Equator|author=Rossi, Bruno| journal=Physical Review|date=May 1934|volume=45|issue=3|pages=212–214|doi=10.1103/PhysRev.45.212|bibcode=1934PhRv...45..212R}}</ref> found that the intensity is, in fact, greater from the west, proving that most primaries are positive. During the years from 1930 to 1945, a wide variety of investigations confirmed that the primary cosmic rays are mostly protons, and the secondary radiation produced in the atmosphere is primarily electrons, photons and [[muon]]s. In 1948, observations with [[nuclear emulsion]]s carried by balloons to near the top of the atmosphere showed that approximately 10% of the primaries are helium nuclei (alpha particles) and 1% are nuclei of heavier elements such as carbon, iron, and lead.<ref>{{cite journal|title=Evidence for Heavy Nuclei in the Primary Cosmic radiation|author=Freier, Phyllis|journal=Physical Review|date=July 1948|volume=74|issue=2|pages=213–217|doi=10.1103/PhysRev.74.213|last2=Lofgren|first2=E.|last3=Ney|first3=E.|last4=Oppenheimer|first4=F.|last5=Bradt|first5=H.|last6=Peters|first6=B.|bibcode=1948PhRv...74..213F|display-authors=etal}}</ref><ref>{{cite journal|title=Investigation of the Primary Cosmic Radiation with Nuclear Photographic Emulsions|author=Freier, Phyllis|journal=Physical Review|date=December 1948|volume=74|issue=12|pages=1828–1837|doi=10.1103/PhysRev.74.1828|last2=Peters|first2=B.|bibcode=1948PhRv...74.1828B|display-authors=etal}}</ref>

During a test of his equipment for measuring the east–west effect, Rossi observed that the rate of near-simultaneous discharges of two widely separated [[Geiger counter]]s was larger than the expected accidental rate. In his report on the experiment, Rossi wrote "...&nbsp;it seems that once in a while the recording equipment is struck by very extensive showers of particles, which causes coincidences between the counters, even placed at large distances from one another."<ref>{{cite journal|title=Misure sulla distribuzione angolare di intensita della radiazione penetrante all'Asmara|author=Rossi, Bruno|journal=Ricerca Scientifica|date=1934|volume=5|issue=1|pages=579–589}}</ref> In 1937, [[Pierre Victor Auger|Pierre Auger]], unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that high-energy primary cosmic-ray particles interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, and photons that reach ground level.<ref>{{citation|display-authors=1|last1=Auger|first1=P.|last2=Ehrenfest|first2=P.|last3=Maze|first3=R.|last4=Daudin|first4=J.|last5=Fréon|first5=R. A.|title=Extensive Cosmic-Ray Showers| journal=Reviews of Modern Physics|volume=11|issue=3–4|pages=288–291|date=July 1939|doi=10.1103/RevModPhys.11.288|bibcode=1939RvMP...11..288A| postscript=.}}</ref>

Soviet physicist [[Sergei Vernov]] was the first to use [[radiosonde]]s to perform cosmic ray readings with an instrument carried to high altitude by a balloon. On 1 April 1935, he took measurements at heights up to 13.6 kilometres using a pair of [[Geiger counter]]s in an anti-coincidence circuit to avoid counting secondary ray showers.<ref>{{cite book|author1=J.L. DuBois|author2=R.P. Multhauf|author3=C.A. Ziegler|date=2002|title=The Invention and Development of the Radiosonde|url=http://www.sil.si.edu/smithsoniancontributions/HistoryTechnology/pdf_lo/SSHT-0053.pdf |archive-url=https://web.archive.org/web/20110605132224/http://www.sil.si.edu/smithsoniancontributions/HistoryTechnology/pdf_lo/SSHT-0053.pdf |archive-date=2011-06-05 |url-status=live|series=Smithsonian Studies in History and Technology|volume=53|publisher=[[Smithsonian Institution Press]]}}</ref><ref>{{cite journal|author=S. Vernoff|date=1935|title=Radio-Transmission of Cosmic Ray Data from the Stratosphere|journal=Nature|volume=135|issue=3426|pages=1072–1073|doi=10.1038/1351072c0|bibcode=1935Natur.135.1072V|s2cid=4132258}}</ref>

[[Homi J. Bhabha]] derived an expression for the probability of scattering positrons by electrons, a process now known as [[Bhabha scattering]]. His classic paper, jointly with [[Walter Heitler]], published in 1937 described how primary cosmic rays from space interact with the upper atmosphere to produce particles observed at the ground level. Bhabha and Heitler explained the cosmic ray shower formation by the cascade production of gamma rays and positive and negative electron pairs.<ref>{{cite journal|last1=Bhabha|first1=H. J.|last2=Heitler|first2=W.|title=The Passage of Fast Electrons and the Theory of Cosmic Showers|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=159|issue=898|year=1937|pages=432–458|issn=1364-5021|doi=10.1098/rspa.1937.0082|url=http://repository.ias.ac.in/2364/1/2364.pdf |archive-url=https://web.archive.org/web/20160102023556/http://repository.ias.ac.in/2364/1/2364.pdf |archive-date=2016-01-02 |url-status=live|bibcode=1937RSPSA.159..432B|doi-access=free}}</ref><ref>{{cite journal|last=Braunschweig|first=W.|display-authors=etal|title=A study of Bhabha scattering at PETRA energies|journal=Zeitschrift für Physik C|volume=37|issue=2|date=1988|pages=171–177|doi=10.1007/BF01579904|s2cid=121904361}}</ref>

===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.