cosmic ray

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{{Redirect|Cosmic radiation|some other types of cosmic radiation|Cosmic background radiation|and|Cosmic background (disambiguation)|the film|Cosmic Ray (film)}}{{short description|High-energy particle, mainly originating outside the Solar system}}{{Use dmy dates|date=December 2012}}File:Cosmic ray flux versus particle energy.svg|360px|thumb|Cosmic fluxfluxCosmic rays are a form of high-energy radiation, mainly originating outside the Solar SystemBOOK
, Sharma
, Atomic And Nuclear Physics
, 2008
, Pearson Education India
, 978-81-317-1924-4
, 478
, and even from distant galaxies.WEB
, Detecting cosmic rays from a galaxy far, far away
, Science Daily
, 21 September 2017
, 26 December 2017
, Upon impact with the Earth's atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface. Composed primarily of high-energy protons and atomic nuclei, they are originated either from the sun or from outside of our solar system. Data from the Fermi Space Telescope (2013)JOURNAL
, 339
, 807–811
, Detection of the Characteristic Pion-Decay Signature in Supernova Remnants
, 2013-02-15
, 10.1126/science.1231160
, 6424
, Ackermann, M.
, Ajello, M.
, Allafort, A.
, Baldini, L.
, Ballet, J.
, Barbiellini, G.
, Baring, M. G.
, Bastieri, D.
, Bechtol, K.
, Science
, 1302.3307, 2013Sci...339..807A
, Bellazzini, R.
, Blandford, R. D.
, Bloom, E.D.
, Bonamente, E.
, Borgland, A. W.
, Bottacini, E.
, Brandt, T. J.
, Bregeon, J.
, Brigida, M.
, Bruel, P.
, Buehler, R.
, Busetto, G.
, Buson, S.
, Caliandro, G. A.
, Cameron, R. A.
, Caraveo, P. A.
, Patrizia A. Caraveo
, Casandjian, J. M.
, Cecchi, C.
, Celik, O.
, Charles, E.
, Chaty, S., 29
, 23413352
, have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova explosions of stars.WEB
, Evidence Shows that Cosmic Rays Come from Exploding Stars
, Ginger Pinholster
, 2013-02-13
, Active galactic nuclei also appear to produce cosmic rays, based on observations of neutrinos and gamma rays from blazar TXS 0506+056 in 2018.JOURNAL, Acceleration of petaelectronvolt protons in the Galactic Centre, HESS collaboration, Nature (journal), Nature, 531, 7595, 10.1038/nature17147, 2016, 1603.07730, 2016Natur.531..476H, 476–479, 26982725, JOURNAL, Collaboration, IceCube, 2018-07-12, Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert, Science, 361, 6398, en, 147–151, 10.1126/science.aat2890, 0036-8075, 30002248, 2018Sci...361..147I, 1807.08794,


The term ray is somewhat of a misnomer due to a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. In common scientific usage,WEB
, Are Cosmic Rays Electromagnetic radiation?
, Eric Christian
, 2012-12-11
, high-energy particles with intrinsic mass are known as "cosmic" rays, while 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 photon energy.


Of primary cosmic rays, which originate outside of Earth's atmosphere, about 99% are the nuclei of well-known atoms (stripped of their electron shells), 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, identical to helium nuclei; and 1% are the nuclei of heavier elements, called HZE ions.WEB,weblink What are cosmic rays?, NASA, Goddard Space Flight Center, 31 October 2012, yes,weblink" title="">weblink 28 October 2012, copy These fractions vary highly over the energy range of cosmic rays.JOURNAL, Data-driven model of the cosmic-ray flux and mass composition from 10 GeV to 10^11 GeV, H. Dembinski, etal, Proceedings of Science, ICRC2017, 10.22323/1.301.0533, 2018, 1711.11432, 533, 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.WEB, Cosmic Rays,weblink National Aeronautics and Space Administration, Nasa, 23 March 2019,


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 {{nowrap|3 × 1020 eV}},NEWS,weblink Astronomy Without A Telescope – Oh-My-God Particles, Universe Today, 12 June 2011, Universe Today, 17 February 2013, Nerlich, Steve, about 40 million times the energy of particles accelerated by the Large Hadron Collider.WEB,weblink Facts and figures, European Organization for Nuclear Research, The LHC, 2008, 17 February 2013, One can show that such enormous energies might be achieved by means of the centrifugal mechanism of acceleration in active galactic nuclei. At 50{{nbsp}}J,JOURNAL,weblinkweblink" title="">weblink yes, 2013-04-07, Extreme speed, Gaensler, Brian, COSMOS, November 2011, 41, the highest-energy ultra-high-energy cosmic rays have energies comparable to the kinetic energy of a {{convert|90|kph|mph|adj=on}} baseball. As a result of these discoveries, there has been interest in investigating cosmic rays of even greater energies.JOURNAL
, L. Anchordoqui, T. Paul, S. Reucroft, J. Swain
, 2003
, Ultrahigh Energy Cosmic Rays: The state of the art before the Auger Observatory
, International Journal of Modern Physics A
, 18, 13, 2229–2366
, 10.1142/S0217751X03013879
, hep-ph/0206072, 2003IJMPA..18.2229A
, Most cosmic rays, however, do not have such extreme energies; the energy distribution of cosmic rays peaks on {{convert|0.3|GeV|J}}.WEB,weblink Cosmic rays, Georgia State University, HyperPhysics Concepts, 17 February 2013, Nave, Carl R.,


After the discovery of radioactivity by Henri Becquerel 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.{{citation
| first1=Marjorie C. | last1=Malley
| title=Radioactivity: A History of a Mysterious Science
| pages=78–79 | date=August 25, 2011 | publisher=Oxford University Press
| url=
| postscript=.| isbn=9780199766413
}} 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.{{citation
| first1=John | last1=North
| title=Cosmos: An Illustrated History of Astronomy and Cosmology
| page=686 | date=July 15, 2008 | publisher=University of Chicago Press
| url=
| postscript=.| isbn=9780226594415


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. 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.JOURNAL
, D. Pacini
, 1912
, La radiazione penetrante alla superficie ed in seno alle acque
, Il Nuovo Cimento
, 3, 1
, 93–100
, 10.1007/BF02957440
, 1002.1810, 1912NCim....3...93P,

Translated and commented in JOURNAL
, A. de Angelis, 2010
, Penetrating Radiation at the Surface of and in Water
, Il Nuovo Cimento
, 3
, 93–100
, 1002.1810
, 1912NCim....3...93P, 10.1007/BF02957440, (File:Pacini measurement.jpg|thumb|left|Pacini makes a measurement in 1910.|160px)
In 1912, Victor Hess carried three enhanced-accuracy Wulf electrometersWEB,weblink Nobel Prize in Physics 1936 – Presentation Speech,, 1936-12-10, 2013-02-27, to an altitude of 5,300 metres in a free balloon flight. He found the ionization rate increased approximately fourfold over the rate at ground level. 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. 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."JOURNAL
, V. F. Hess
, 1912
, Ãœber Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten (English translation)
, English
, Physikalische Zeitschrift
, 13, 1084–1091, 1808.02927
, In 1913–1914, Werner Kolhörster confirmed Victor Hess's earlier results by measuring the increased ionization enthalpy rate at an altitude of 9 km.(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.
, V.F. Hess
, 1936
, The Nobel Prize in Physics 1936
, The Nobel Foundation
, 2010-02-11
, V.F. Hess
, 1936
, Unsolved Problems in Physics: Tasks for the Immediate Future in Cosmic Ray Studies
, Nobel Lectures
, The Nobel Foundation
, 2010-02-11
, The Hess balloon flight took place on 7 August 1912. By sheer coincidence, exactly 100 years later on 7 August 2012, the Mars Science Laboratory rover used its Radiation Assessment Detector (RAD) instrument to begin measuring the radiation levels on another planet for the first time. On 31 May 2013, NASA scientists reported that a possible manned mission to Mars may involve a greater radiation risk than previously believed, based on the amount of energetic particle radiation detected by the RAD on the Mars Science Laboratory while traveling from the Earth to Mars in 2011–2012.(File:Hessballon.jpg|thumb|Hess lands after his balloon flight in 1912.)


Bruno Rossi wrote that: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.BOOK, Rossi, Bruno Benedetto, Cosmic Rays, New York, McGraw-Hill, 1964, 978-0-07-053890-0, 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".JOURNAL, 1931, Discussion on Ultra-Penetrating Rays, Proceedings of the Royal Society of London A, 132, 331, 10.1098/rspa.1931.0104, Geiger, H., Rutherford, Lord, Regener, E., Lindemann, F. A., Wilson, C. T. R., Chadwick, J., Gray, L. H., Tarrant, G. T. P., Dobson, G. M. B., 8, 1931RSPSA.132..331G, 819, In the 1920s, the term cosmic rays 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 electrons were produced in the atmosphere by Compton scattering of gamma rays. But then, sailing from Java to the Netherlands in 1927, Jacob Clay found evidence,JOURNAL, Penetrating Radiation, Clay, J., Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen Te Amsterdam, 1927, 30, 9–10, 1115–1127,weblink 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, Bothe and Kolhörster discovered charged cosmic-ray particles that could penetrate 4.1 cm of gold.JOURNAL, Das Wesen der Höhenstrahlung, Bothe, Walther, Werner Kolhörster, Zeitschrift für Physik, November 1929, 56, 11–12, 751–777, 10.1007/BF01340137, 1929ZPhy...56..751B, 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."JOURNAL, On the Magnetic Deflection of Cosmic Rays, Rossi, Bruno, Physical Review, August 1930, 36, 3, 606, 10.1103/PhysRev.36.606, 1930PhRv...36..606R, Three independent experimentsJOURNAL, The Azimuthal Asymmetry of the Cosmic Radiation, Johnson, Thomas H., Physical Review, May 1933, 43, 10, 834–835, 10.1103/PhysRev.43.834, 1933PhRv...43..834J, JOURNAL, A Positively Charged Component of Cosmic Rays, Alvarez, Luis, Compton, Arthur Holly, Physical Review, May 1933, 43, 10, 835–836, 10.1103/PhysRev.43.835, 1933PhRv...43..835A, JOURNAL, Directional Measurements on the Cosmic Rays Near the Geomagnetic Equator, Rossi, Bruno, Physical Review, May 1934, 45, 3, 212–214, 10.1103/PhysRev.45.212, 1934PhRv...45..212R, 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 muons. In 1948, observations with nuclear emulsions 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 heavier nuclei of the elements such as carbon, iron, and lead.JOURNAL, Evidence for Heavy Nuclei in the Primary Cosmic radiation, Freier, Phyllis, Physical Review, July 1948, 74, 2, 213–217, 10.1103/PhysRev.74.213, Lofgren, E., Ney, E., Oppenheimer, F., Bradt, H., Peters, B., 1948PhRv...74..213F, etal, JOURNAL, Investigation of the Primary Cosmic Radiation with Nuclear Photographic Emulsions, Freier, Phyllis, Physical Review, December 1948, 74, 12, 1828–1837, 10.1103/PhysRev.74.1828, Peters, B., 1948PhRv...74.1828B, etal, 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 counters was larger than the expected accidental rate. In his report on the experiment, Rossi wrote "... 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."JOURNAL, Misure sulla distribuzione angolare di intensita della radiazione penetrante all'Asmara, Rossi, Bruno, Ricerca Scientifica, 1934, 5, 1, 579–589, In 1937 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.{{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=.
}}Soviet physicist Sergey Vernov was the first to use radiosondes 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 counters in an anti-coincidence circuit to avoid counting secondary ray showers.BOOK
, J.L. DuBois, R.P. Multhauf, C.A. Ziegler, 2002
, The Invention and Development of the Radiosonde
, Smithsonian Studies in History and Technology
, 53,
, Smithsonian Institution Press
, S. Vernoff
, 1935
, Radio-Transmission of Cosmic Ray Data from the Stratosphere
, Nature (journal), Nature
, 135, 3426, 1072–1073
, 10.1038/1351072c0
, 1935Natur.135.1072V
, 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.JOURNAL, Bhabha, H. J., Heitler, W., The Passage of Fast Electrons and the Theory of Cosmic Showers, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 159, 898, 1937, 432–458, 1364-5021, 10.1098/rspa.1937.0082,weblink 1937RSPSA.159..432B, {{citation needed|date=April 2015}}JOURNAL, Braunschweig, W., etal, A study of Bhabha scattering at PETRA energies, Zeitschrift für Physik C Particles and Fields, 37, 2, 1988, 171–177, 10.1007/BF01579904,

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 extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the Massachusetts Institute of Technology.JOURNAL, Clark, G., Earl, J., Kraushaar, W., Linsley, J., Rossi, B., Scherb, F., Scott, D., 10.1103/PhysRev.122.637, Cosmic-Ray Air Showers at Sea Level, Physical Review, 122, 2, 637–654, 1961, 1961PhRv..122..637C, The experiment employed eleven 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 1020 eV. A huge air shower experiment called the Auger Project is currently operated at a site on the pampas 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 Watson of the University of Leeds, and later by other 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.WEB,weblink The Pierre Auger Observatory, Auger Project, no,weblink 13 September 2018, dmy-all, 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 1020 eV because of interactions with the remnant photons from the Big Bang origin of the universe. Currently the Pierre Auger Observatory undergoes 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.JOURNAL, Title unknown, Kraushaar, W. L., The Astrophysical Journal, 1972, 177, 341, 10.1086/151713, 1972ApJ...177..341K, etal, 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.


Early speculation on the sources of cosmic rays included a 1934 proposal by Baade and Zwicky suggesting cosmic rays originated from supernovae.JOURNAL, Baade, W., Zwicky, F., 1934, Cosmic Rays from Super-novae, Proceedings of the National Academy of Sciences of the United States of America, 20, 5, 259–263, 86841, 10.1073/pnas.20.5.259, 1934PNAS...20..259B, 1076396, A 1948 proposal by Horace W. Babcock suggested that magnetic variable stars could be a source of cosmic rays.JOURNAL, Babcock, H., Magnetic Variable Stars as Sources of Cosmic Rays, 10.1103/PhysRev.74.489, Physical Review, 74, 4, 489, 1948, 1948PhRv...74..489B, Subsequently, in 1951, Y. Sekido et al. identified the Crab Nebula as a source of cosmic rays.JOURNAL, Sekido, Y., Masuda, T., Yoshida, S., Wada, M., The Crab Nebula as an Observed Point Source of Cosmic Rays, 10.1103/PhysRev.83.658.2, Physical Review, 83, 3, 658–659, 1951, 1951PhRv...83..658S, Since then, a wide variety of potential sources for cosmic rays began to surface, including supernovae, active galactic nuclei, quasars, and gamma-ray bursts.WEB, Gibb, Meredith, Cosmic Rays,weblink Imagine the Universe, NASA Goddard Space Flight Center, 17 March 2013, 3 February 2010, (File:PIA16938-RadiationSources-InterplanetarySpace.jpg|thumb|left|Sources of ionizing radiation in interplanetary space.)(File:Shockfrontacceleration.svg|thumb|Shock front acceleration (theoretical model for supernovae and active galactic nuclei): Incident proton gets accelerated between two shock fronts up to energies of the high-energy component of cosmic rays.)Later experiments have helped to identify the sources of cosmic rays with greater certainty. In 2009, a paper presented at the International Cosmic Ray Conference (ICRC) by scientists at the Pierre Auger Observatory showed ultra-high energy cosmic rays (UHECRs) originating from a location in the sky very close to the radio galaxy Centaurus A, although the authors specifically stated that further investigation would be required to confirm Cen A as a source of cosmic rays.CONFERENCE,weblink Correlation of the Highest Energy Cosmic Rays with Nearby Extragalactic Objects in Pierre Auger Observatory Data, 17 March 2013, Hague, J. D., Proceedings of the 31st ICRC, Łódź 2009, July 2009, International Cosmic Ray Conference, Łódź, Poland, 6–9, yes,weblink" title="">weblink 28 May 2013, dmy-all, However, no correlation was found between the incidence of gamma-ray bursts and cosmic rays, causing the authors to set upper limits as low as 3.4 Ã— 10−6 erg·cm−2 on the flux of {{nowrap|1 GeV – 1 TeV}} cosmic rays from gamma-ray bursts.JOURNAL, Hague, J. D., Correlation of the Highest Energy Cosmic Rays with Nearby Extragalactic Objects in Pierre Auger Observatory Data,weblink Proceedings of the 31st ICRC, Łódź, Poland 2009 - International Cosmic Ray Conference, July 2009, 36–39, 17 March 2013, yes,weblink" title="">weblink 28 May 2013, dmy-all, In 2009, supernovae were said to have been "pinned down" as a source of cosmic rays, a discovery made by a group using data from the Very Large Telescope.WEB,weblink Source of Cosmic Rays Pinned Down, TechMediaNetwork,, 25 June 2009, 20 March 2013, Moskowitz, Clara, This analysis, however, was disputed in 2011 with data from PAMELA, which revealed that "spectral shapes of [hydrogen and helium nuclei] are different and cannot be described well by a single power law", suggesting a more complex process of cosmic ray formation.JOURNAL, Adriani, O., Barbarino, G. C., Bazilevskaya, G. A., Bellotti, R., Boezio, M., Bogomolov, E. A., Bonechi, L., Bongi, M., Bonvicini, V., Borisov, 10.1126/science.1199172, S., Bottai, S., Bruno, A., Cafagna, F., Campana, D., Carbone, R., Carlson, P., Casolino, M., Castellini, G., Consiglio, L., De Pascale, M. P., De Santis, C., De Simone, N., Di Felice, V., Galper, A. M., Gillard, W., Grishantseva, L., Jerse, G., Karelin, A. V., Koldashov, S. V., Krutkov, S. Y., PAMELA Measurements of Cosmic-Ray Proton and Helium Spectra, Science, 332, 6025, 69–72, 2011, 21385721, 1103.4055, 2011Sci...332...69A, 29, 2108/55474, In February 2013, though, research analyzing data from Fermi revealed through an observation of neutral pion decay that supernovae were indeed a source of cosmic rays, with each explosion producing roughly 3 Ã— 1042 – 3 Ã— 1043{{nbsp}}J of cosmic rays. However, supernovae do not produce all cosmic rays, and the proportion of cosmic rays that they do produce is a question which cannot be answered without further study.NEWS,weblink Cosmic ray mystery solved, Guardian News and Media Limited, The Guardian, 14 February 2013, 21 March 2013, Jha, Alok, As an explanation of the acceleration in supernovae and active galactic nuclei the model of shock front acceleration is used.In 2017 the Pierre Auger Collaboration published the observation of a weak anisotropy in the arrival directions of the highest energy cosmic rays.JOURNAL, The Pierre Auger Collaboration, Observation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above 8×10^18 eV, Science, 2017, 1266–1270, 357, 6357, 1709.07321, 10.1126/science.aan4338, 28935800, Since the Galactic Center is in the deficit region, this anisotropy can be interpreted as evidence for the extragalactic origin of cosmic rays at the highest energies. This implies that there must be a transition energy from galactic to extragalactic sources, and there may be different types of cosmic-ray sources contributing to different energy ranges.


Cosmic rays can be divided into three types, galactic cosmic rays (GCR) and extragalactic cosmic rays, i.e., high-energy particles originating outside the solar system, and solar energetic particles, high-energy particles (predominantly protons) emitted by the sun, primarily in solar particle events. However, the term "cosmic ray" is often used to refer to only the extrasolar flux.(File:Atmospheric Collision.svg|right|400 px|thumb|Primary cosmic particle collides with a molecule of atmosphere.)Cosmic rays originate as primary cosmic rays, which are those originally produced in various astrophysical processes. Primary cosmic rays are composed primarily of protons and alpha particles (99%), with a small amount of heavier nuclei (≈1%) and an extremely minute proportion of positrons and antiprotons. Secondary cosmic rays, caused by a decay of primary cosmic rays as they impact an atmosphere, include photons, leptons, and hadrons, such as electrons, positrons, muons, and pions. The latter three of these were first detected in cosmic rays.

Primary cosmic rays

Primary cosmic rays primarily originate from outside the Solar System and sometimes even the Milky Way. When they interact with Earth's atmosphere, they are converted to secondary particles. The mass ratio of helium to hydrogen nuclei, 28%, is similar to the primordial elemental abundance ratio of these elements, 24%.WEB
, Mewaldt, Richard A., 1996
, Cosmic Rays
, California Institute of Technology.
, The remaining fraction is made up of the other heavier nuclei that are typical nucleosynthesis end products, primarily lithium, beryllium, and boron. These nuclei appear in cosmic rays in much greater abundance (≈1%) than in the solar atmosphere, where they are only about 10−11 as abundant as helium. Cosmic rays made up of charged nuclei heavier than helium are called HZE ions. Due to the high charge and heavy nature of HZE ions, their contribution to an astronaut's radiation dose in space is significant even though they are relatively scarce.This abundance difference is a result of the way secondary cosmic rays are formed. Carbon and oxygen nuclei collide with interstellar matter to form lithium, beryllium and boron in a process termed cosmic ray spallation. Spallation is also responsible for the abundances of scandium, titanium, vanadium, and manganese ions in cosmic rays produced by collisions of iron and nickel nuclei with interstellar matter.JOURNAL, The relative abundances of the elements scandium to manganese in relativistic cosmic rays and the possible radioactive decay of manganese 54, Koch, L., Engelmann, J. J., Goret, P., Juliusson, E., Petrou, N., Rio, Y., Soutoul, A., Byrnak, B., Lund, N., Peters, B., Astronomy and Astrophysics, October 1981, 102, 11, 1981A&A...102L...9K, L9, At high energies the composition changes and heavier nuclei have larger abundances in some energy ranges. Current experiments aim at more accurate measurements of the composition at high energies.

Primary cosmic ray antimatter

{{see also|Alpha Magnetic Spectrometer}}Satellite experiments have found evidence of positrons and a few antiprotons in primary cosmic rays, amounting to less than 1% of the particles in primary cosmic rays. These do not appear to be the products of large amounts of antimatter from the Big Bang, or indeed complex antimatter in the universe. Rather, they appear to consist of only these two elementary particles, newly made in energetic processes.Preliminary results from the presently operating Alpha Magnetic Spectrometer (AMS-02) on board the International Space Station show that positrons in the cosmic rays arrive with no directionality. In September, 2014, new results with almost twice as much data were presented in a talk at CERN and published in Physical Review Letters.JOURNAL, L. Accardo (AMS Collaboration), High Statistics Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–500 GeV with the Alpha Magnetic Spectrometer on the International Space Station, Physical Review Letters, 18 September 2014, 113, 12, 121101, 10.1103/PhysRevLett.113.121101, 25279616,weblink 2014PhRvL.113l1101A, etal, JOURNAL, Schirber, Michael, Synopsis: More Dark Matter Hints from Cosmic Rays?, Physical Review Letters, 113, 12, 121102, 10.1103/PhysRevLett.113.121102, 25279617, 2014, 1701.07305, 2014PhRvL.113l1102A,weblink A new measurement of positron fraction up to 500 GeV was reported, showing that positron fraction peaks at a maximum of about 16% of total electron+positron events, around an energy of {{nowrap|275±32 GeV}}. At higher energies, up to 500{{nbsp}}GeV, the ratio of positrons to electrons begins to fall again. The absolute flux of positrons also begins to fall before 500{{nbsp}}GeV, but peaks at energies far higher than electron energies, which peak about 10{{nbsp}}GeV.WEB, New results from the Alpha Magnetic$Spectrometer on the International Space Station,weblink AMS-02 at NASA, 21 September 2014, These results on interpretation have been suggested to be due to positron production in annihilation events of massive dark matter particles.JOURNAL, Aguilar, M., Alberti, G., Alpat, B., Alvino, A., Ambrosi, G., Andeen, K., Anderhub, H., Arruda, L., Azzarello, P., Bachlechner, A., Barao, F., Baret, B., Barrau, A., Barrin, L., Bartoloni, A., Basara, L., Basili, A., Batalha, L., Bates, J., Battiston, R., Bazo, J., Becker, R., Becker, U., Behlmann, M., Beischer, B., Berdugo, J., Berges, P., Bertucci, B., Bigongiari, G., Biland, A., First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV, 10.1103/PhysRevLett.110.141102, Physical Review Letters, 110, 14, 141102, 2013, 25166975, 2013PhRvL.110n1102A, 29,weblink Cosmic ray antiprotons also have a much higher average energy than their normal-matter counterparts (protons). They arrive at Earth with a characteristic energy maximum of 2 GeV, indicating their production in a fundamentally different process from cosmic ray protons, which on average have only one-sixth of the energy.JOURNAL,weblink Secondary antiprotons and propagation of cosmic rays in the Galaxy and heliosphere, Moskalenko, I. V., Strong, A. W., Ormes, J. F., Potgieter, M. S., The Astrophysical Journal, January 2002, 565, 1, 280–296, 10.1086/324402, astro-ph/0106567, 2002ApJ...565..280M, There is no evidence of complex antimatter atomic nuclei, such as antihelium nuclei (i.e., anti-alpha particles), in cosmic rays. These are actively being searched for. A prototype of the AMS-02 designated AMS-01, was flown into space aboard the {{OV|103}} on STS-91 in June 1998. By not detecting any antihelium at all, the AMS-01 established an upper limit of {{nowrap|1.1 × 10−6}} for the antihelium to helium flux ratio.JOURNAL, Physics Reports, 366, 6, 331–405, August 2002, AMS Collaboration, Aguilar, M., Alcaraz, J., Allaby, J., Alpat, B., Ambrosi, G., Anderhub, H., Ao, L., Arefiev, A., 8, The Alpha Magnetic Spectrometer (AMS) on the International Space Station: Part I – results from the test flight on the space shuttle, 2002PhR...366..331A, 10.1016/S0370-1573(02)00013-3, 2078.1/72661, {{multiple image|| align = left| direction = vertical| header = The moon in cosmic rays| height = 200| image1 = Moon's shadow in muons.gif| alt1 = The moon's muon shadow! Particle energy (eV)! Particle rate (m{{sup|−2}}s{{sup|−1}})
Moon's cosmic ray shadow, as seen in secondary muons detected 700 m below ground, at the Soudan 2>Soudan 2 detector| image2 = Moon egret.jpg| alt2= The moon as seen in gamma raysCompton Gamma Ray Observatory, in gamma rays with energies greater than 20 MeV. These are produced by cosmic ray bombardment on its surface.1 AUGUST 2005 URL=HTTP://HEASARC.GSFC.NASA.GOV/DOCS/CGRO/EPO/NEWS/GAMMOON.HTMLNASA/GSFC>ACCESSDATE=2010-02-11, }}

Secondary cosmic rays

When cosmic rays enter the Earth's atmosphere they collide with atoms and molecules, mainly oxygen and nitrogen. The interaction produces a cascade of lighter particles, a so-called air shower secondary radiation that rains down, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons.BOOK, Morison, Ian, Introduction to Astronomy and Cosmology, 2008, John Wiley & Sons, 978-0-470-03333-3, 198,, All of the produced particles stay within about one degree of the primary particle's path.Typical particles produced in such collisions are neutrons and charged mesons such as positive or negative pions and kaons. Some of these subsequently decay into muons and neutrinos, which are able to reach the surface of the Earth. Some high-energy muons even penetrate for some distance into shallow mines, and most neutrinos traverse the Earth without further interaction. Others decay into photon, subsequently producing electromagnetic cascades. Hence, next to photons electrons and positrons usually dominate in air showers. These particles as well as muons can be easily detected by many types of particle detectors, such as cloud chambers, bubble chambers, water-Cherenkov or scintillation detectors. The observation of a secondary shower of particles in multiple detectors at the same time is an indication that all of the particles came from that event.Cosmic rays impacting other planetary bodies in the Solar System are detected indirectly by observing high-energy gamma ray emissions by gamma-ray telescope. These are distinguished from radioactive decay processes by their higher energies above about 10 MeV.

Cosmic-ray flux

File:SpaceEnvironmentOverview From 19830101.jpg|thumb|right|400px|An overview of the space environment shows the relationship between the solar activity and galactic cosmic rays.WEB, Extreme Space Weather Events, National Geophysical Data CenterNational Geophysical Data CenterThe flux of incoming cosmic rays at the upper atmosphere is dependent on the solar wind, the Earth's magnetic field, and the energy of the cosmic rays. At distances of ≈94 AU from the Sun, the solar wind undergoes a transition, called the termination shock, from supersonic to subsonic speeds. The region between the termination shock and the heliopause acts as a barrier to cosmic rays, decreasing the flux at lower energies (≤{{nbsp}}1 GeV) by about 90%. However, the strength of the solar wind is not constant, and hence it has been observed that cosmic ray flux is correlated with solar activity.In addition, the Earth's magnetic field acts to deflect cosmic rays from its surface, giving rise to the observation that the flux is apparently dependent on latitude, longitude, and azimuth angle.The combined effects of all of the factors mentioned contribute to the flux of cosmic rays at Earth's surface. The following table of participial frequencies reach the planetWEB,weblink Pierre Auger Observatory,, 2012-08-17, yes,weblink" title="">weblink 12 October 2012, dmy-all, and are inferred from lower energy radiation reaching the ground.WEB,weblink Pierre Auger Observatory, The Mystery of High-Energy Cosmic Rays,, {| class="wikitable"
1|e=9}} (GeV)1|e=4}}
1|e=12}} (TeV)| 1
1|e=16}} (10{{nbsp}}PeV)1|e=-7}} (a few times a year)
1|e=20}} (100{{nbsp}}EeV)1|e=-15}} (once a century)
In the past, it was believed that the cosmic ray flux remained fairly constant over time. However, recent research suggests one-and-a-half- to two-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.JOURNAL
, D. Lal, A.J.T. Jull, D. Pollard, L. Vacher
, 2005
, Evidence for large century time-scale changes in solar activity in the past 32 Kyr, based on in-situ cosmogenic 14C in ice at Summit, Greenland
, Earth and Planetary Science Letters
, 234, 3–4, 335–349
, 10.1016/j.epsl.2005.02.011
, 2005E&PSL.234..335L
, The magnitude of the energy of cosmic ray flux in interstellar space is very comparable to that of other deep space energies: cosmic ray energy density averages about one electron-volt per cubic centimetre of interstellar space, or ≈1{{nbsp}}eV/cm3, which is comparable to the energy density of visible starlight at 0.3{{nbs}}eV/cm3, the galactic magnetic field energy density (assumed 3 microgauss) which is ≈0.25{{nbsp}}eV/cm3, or the cosmic microwave background (CMB) radiation energy density at ≈0.25{{nbsp}}eV/cm3.BOOK, Planets, Stars, and Stellar Systems, 2012, Springer, 978-90-481-8817-8, Castellina, Antonella, Donato, Fiorenza, 1, Astrophysics of Galactic charged cosmic rays, Oswalt, T.D., McLean, I.S., Bond, H.E., French, L., Kalas, P., Barstow, M., Gilmore, G.F., Keel, W.,

Detection methods

File:VERITAS array.jpg|thumb|600px|The VERITASVERITASThere are two main classes of detection methods. First, the direct detection of the primary cosmic rays in space or at high altitude by balloon-borne instruments. Second, the indirect detection of secondary particle, i.e., extensive air showers at higher energies. While there have been proposals and prototypes for space and ballon-borne detection of air showers, currently operating experiments for high-energy cosmic rays are ground based. Generally direct detection is more accurate than indirect detection. However the flux of cosmic rays decreases with energy, which hampers direct detection for the energy range above 1 PeV. Both, direct and indirect detection, is realized by several techniques.

Direct detection

Direct detection is possible by all kind of particle detectors at the ISS, on satellites, or high-altitude balloons. However, there are constraints in weight and size limiting the choices of detectors.An example for the direct detection technique is a method developed by Robert Fleischer, P. Buford Price, and Robert M. Walker for use in high-altitude balloons.BOOK
, R.L. Fleischer, P.B. Price, R.M. Walker, 1975
, Nuclear tracks in solids: Principles and applications
, University of California Press
, In this method, sheets of clear plastic, like 0.25 mm Lexan polycarbonate, are stacked together and exposed directly to cosmic rays in space or high altitude. The nuclear charge causes chemical bond breaking or ionization in the plastic. At the top of the plastic stack the ionization is less, due to the high cosmic ray speed. As the cosmic ray speed decreases due to deceleration in the stack, the ionization increases along the path. The resulting plastic sheets are "etched" or slowly dissolved in warm caustic sodium hydroxide solution, that removes the surface material at a slow, known rate. The caustic sodium hydroxide dissolves the plastic at a faster rate along the path of the ionized plastic. The net result is a conical etch pit in the plastic. The etch pits are measured under a high-power microscope (typically 1600× oil-immersion), and the etch rate is plotted as a function of the depth in the stacked plastic.This technique yields a unique curve for each atomic nucleus from 1 to 92, allowing identification of both the charge and energy of the cosmic ray that traverses the plastic stack. The more extensive the ionization along the path, the higher the charge. In addition to its uses for cosmic-ray detection, the technique is also used to detect nuclei created as products of nuclear fission.

Indirect detection

There are several ground-based methods of detecting cosmic rays currently in use, which can be divided in two main categories: the detection of secondary particles forming extensive air showers (EAS) by various types of particle detectors, and the detection of electromagnetic radiation emitted by EAS in the atmosphere.Extensive air shower arrays made of particle detectors measure the charged particles which pass through them. EAS arrays can observe a broad area of the sky and can be active more than 90% of the time. However, they are less able to segregate background effects from cosmic rays than can air Cherenkov telescopes. Most state-of-the-art EAS arrays employ plastic scintillators. Also water (liquid or frozen) is used as a detection medium through which particles pass and produce Cherenkov radiation to make them detectable.WEB,weblink What are cosmic rays?, Michigan State University National Superconducting Cyclotron Laboratory, 23 February 2013, yes,weblink" title="">weblink 12 July 2012, dmy-all, Therefore, several arrays use water/ice-Cherenkov detectors as alternative or in addition to scintillators.By the combination of several detectors, some EAS arrays have the capability to distinguish muons from lighter secondary particles (photons, electrons, positrons). The fraction of muons among the secondary particles in one traditional way to estimate the mass composition of the primary cosmic rays.A historic method of secondary particle detection still used for demonstration purposes involves the use of cloud chambersWEB,weblink Cloud Chambers and Cosmic Rays: A Lesson Plan and Laboratory Activity for the High School Science Classroom, Cornell University Laboratory for Elementary-Particle Physics, 2006, 23 February 2013, to detect the secondary muons created when a pion decays. Cloud chambers in particular can be built from widely available materials and can be constructed even in a high-school laboratory. A fifth method, involving bubble chambers, can be used to detect cosmic ray particles.JOURNAL, Chu, W., Kim, Y., Beam, W., Kwak, N., Evidence of a Quark in a High-Energy Cosmic-Ray Bubble-Chamber Picture, 10.1103/PhysRevLett.24.917, Physical Review Letters, 24, 16, 917–923, 1970, 1970PhRvL..24..917C, More recently, the CMOS devices in pervasive smartphone cameras have been proposed as a practical distributed network to detect air showers from ultra-high-energy cosmic rays (UHECRs).WEB,weblink Cosmic ray particle shower? There's an app for that., Ars Technica, Timmer, John, 13 October 2014, The first app, to exploit this proposition was the CRAYFIS (Cosmic RAYs Found In Smartphones) experiment.Collaboration website {{webarchive|url= |date=14 October 2014 }}CRAYFIS detector array paper. {{webarchive|url= |date=14 October 2014 }} Then, in 2017, the CREDO (Cosmic Ray Extremely Distributed Observatory) Collaboration Collaboration website released the first version of its completely open source app for Android devices. Since then the collaboration has attracted the interest and support of many scientific institutions, educational institutions and members of the public around the world.CREDO 'first light' press release. Future research has to show in what aspects this new technique can compete with dedicated EAS arrays.The first detection method in the second category is called the air Cherenkov telescope, designed to detect low-energy (

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