View source for Gamma ray

{{Short description|Penetrating form of electromagnetic radiation}}
{{About|the term's use in physics}}
{{Pp-semi-indef}}
{{more citations needed|date=January 2024}}
[[File:Gamma Decay.svg|thumb|upright=1.35|An emission of a gamma ray (''γ'') from an atomic nucleus]]
{{Nuclear physics}}

A '''gamma ray''', also known as '''gamma radiation''' (symbol {{math|{{Subatomic particle|link=yes|Gamma}}}}), is a penetrating form of [[electromagnetic radiation]] arising from high energy interactions like the [[radioactive decay]] of [[atomic nuclei]] or astronomical events like solar flares. It consists of the shortest [[wavelength]] electromagnetic waves, typically shorter than those of [[X-ray]]s. With [[frequencies]] above 30&nbsp;exahertz ({{val|3|e=19|u=Hz}}) and wavelengths less than 10&nbsp;picometers ({{val|1|e=-11|u=meter}}),  gamma ray [[photon]]s have the highest [[photon energy]] of any form of electromagnetic radiation. [[Paul Villard]], a French [[chemist]] and [[physicist]], discovered gamma radiation in 1900 while studying [[radiation]] emitted by [[radium]]. In 1903, [[Ernest Rutherford]] named this radiation ''gamma rays'' based on their relatively strong penetration of [[matter]]; in 1900, he had already named two less penetrating types of decay radiation (discovered by [[Henri Becquerel]]) [[alpha rays]] and [[beta rays]] in ascending order of penetrating power.

Gamma rays from radioactive decay are in the energy range from a few kilo[[electronvolts]] (keV) to approximately 8 megaelectronvolts (MeV), corresponding to the typical energy levels in nuclei with reasonably long lifetimes. The energy spectrum of gamma rays can be used to identify the decaying [[radionuclides]] using [[gamma spectroscopy]]. [[Very-high-energy gamma ray]]s in the 100–1000 teraelectronvolt (TeV) range have been observed from astronomical sources such as the [[Cygnus X-3]] [[microquasar]].

Natural sources of gamma rays originating on Earth are mostly a result of radioactive decay and secondary radiation from atmospheric interactions with [[cosmic ray]] particles. However, there are other rare natural sources, such as [[terrestrial gamma-ray flash]]es, which produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays include [[nuclear fission|fission]], such as that which occurs in [[nuclear reactor]]s, and [[high energy physics]] experiments, such as [[pion|neutral pion decay]] and [[nuclear fusion]].

The energy ranges of gamma rays and X-rays overlap in the [[electromagnetic spectrum]], so the terminology for these electromagnetic waves varies between scientific disciplines. In some fields of physics, they are distinguished by their origin: gamma rays are created by nuclear decay while X-rays originate outside the nucleus. In [[astrophysics]], gamma rays are conventionally defined as having photon energies above 100&nbsp;keV and are the subject of [[gamma-ray astronomy]], while radiation below 100&nbsp;keV is classified as X-rays and is the subject of [[X-ray astronomy]].

Gamma rays are [[ionizing radiation]] and are thus hazardous to life. They can cause [[DNA]] [[mutation]]s, [[cancer]] and [[tumor]]s, and at high doses burns and [[radiation sickness]]. Due to their high penetration power, they can damage bone marrow and internal organs. Unlike alpha and beta rays, they easily pass through the body and thus pose a formidable [[radiation protection]] challenge, requiring shielding made from dense materials such as lead or concrete. On [[Earth]], the [[Earth's magnetic field|magnetosphere]] protects life from most types of lethal cosmic radiation other than gamma rays.

==History of discovery==
The first gamma ray source to be discovered was the [[radioactive decay]] process called ''gamma decay''. In this type of decay, an [[excited state|excited]] nucleus emits a gamma ray almost immediately upon formation.<ref group="note">It is now understood that a nuclear [[isomeric transition]], however, can produce inhibited gamma decay with a measurable and much longer half-life.</ref> [[Paul Villard]], a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from [[radium]]. Villard knew that his described radiation was more powerful than previously described types of rays from radium, which included beta rays, first noted as "radioactivity" by [[Henri Becquerel]] in 1896, and alpha rays, discovered as a less penetrating form of radiation by Rutherford, in 1899. However, Villard did not consider naming them as a different fundamental type.<ref>{{cite journal | last1 = Villard | first1 = P. | year = 1900 | title = Sur la réflexion et la réfraction des rayons cathodiques et des rayons déviables du radium | url = https://books.google.com/books?id=W1oDAAAAYAAJ&pg=PA1010 | journal = Comptes rendus | volume = 130 | pages = 1010–1012 }} See also: {{cite journal | last1 = Villard | first1 = P. | year = 1900 | title = Sur le rayonnement du radium | url = https://books.google.com/books?id=W1oDAAAAYAAJ&pg=PA1179 | journal = Comptes rendus | volume = 130 | pages = 1178–1179 }}</ref><ref>{{cite book|last=L'Annunziata|first=Michael F.|title=Radioactivity: introduction and history|url=https://archive.org/details/radioactivityint00lann|url-access=limited|publisher=Elsevier BV|location=Amsterdam, Netherlands|year=2007|pages=[https://archive.org/details/radioactivityint00lann/page/n77 55]–58|isbn=978-0-444-52715-8}}</ref> Later, in 1903, Villard's radiation was recognized as being of a type fundamentally different from previously named rays by [[Ernest Rutherford]], who named Villard's rays "gamma rays" by analogy with the beta and alpha rays that Rutherford had differentiated in 1899.<ref>Rutherford named γ rays on page 177 of {{cite journal |first=E. |last=Rutherford |year=1903 |url=https://books.google.com/books?id=otXPAAAAMAAJ&pg=PA177 |title=The magnetic and electric deviation of the easily absorbed rays from radium |journal=Philosophical Magazine |volume=5 |number=26 |series=6 |pages=177–187 |doi=10.1080/14786440309462912|url-access=subscription }}</ref> The "rays" emitted by radioactive elements were named in order of their power to penetrate various materials, using the first three letters of the Greek alphabet: alpha rays as the least penetrating, followed by beta rays, followed by gamma rays as the most penetrating. Rutherford also noted that gamma rays were not deflected (or at least, not {{em|easily}} deflected) by a magnetic field, another property making them unlike alpha and beta rays.

Gamma rays were first thought to be particles with mass, like alpha and beta rays. Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by a magnetic field indicated that they had no charge.<ref name=RaP>{{cite web|url=http://galileo.phys.virginia.edu/classes/252/rays_and_particles.html |title=Rays and Particles |publisher=Galileo.phys.virginia.edu |access-date=2013-08-27}}</ref> In 1914, gamma rays were observed to be reflected from crystal surfaces, proving that they were electromagnetic radiation.<ref name=RaP/> Rutherford and his co-worker [[Edward Andrade]] measured the wavelengths of gamma rays from radium, and found they were similar to [[X-ray]]s, but with shorter wavelengths and thus, higher frequency. This was eventually recognized as giving them more energy per [[photon]], as soon as the latter term became generally accepted. A gamma decay was then understood to usually emit a gamma photon.

==Sources==
[[File:NASA's Fermi Explores the Early Universe.ogv|thumb|300px|This animation tracks several gamma rays through space and time, from their emission in the jet of a distant [[blazar]] to their arrival in [[Fermi Gamma-ray Space Telescope|Fermi's]] Large Area Telescope (LAT).]]
Natural sources of gamma rays on Earth include gamma decay from naturally occurring [[radioisotope]]s such as [[potassium-40]], and also as a secondary radiation from various atmospheric interactions with [[cosmic ray]] particles. Natural terrestrial sources that produce gamma rays include [[lightning strike]]s and [[terrestrial gamma-ray flash]]es, which produce high energy emissions from natural high-energy voltages.<ref name=Fishman-1994>{{Cite journal | last1 = Fishman | first1 = G. J. | last2 = Bhat | first2 = P. N. | last3 = Mallozzi | first3 = R. | last4 = Horack | first4 = J. M. | last5 = Koshut | first5 = T. | last6 = Kouveliotou | first6 = C. | last7 = Pendleton | first7 = G. N. | last8 = Meegan | first8 = C. A. | last9 = Wilson | first9 = R. B. | last10 = Paciesas | first10 = W. S. | last11 = Goodman | first11 = S. J. | last12 = Christian | first12 = H. J. | doi = 10.1126/science.264.5163.1313 | title = Discovery of Intense Gamma-Ray Flashes of Atmospheric Origin | url = http://nova.stanford.edu/~vlf/IHY_Test/Tutorials/TGFs/Papers/Fishman1994.pdf | journal = Science | volume = 264 | issue = 5163 | pages = 1313–1316 | date = May 27, 1994 | pmid = 17780850 | bibcode = 1994STIN...9611316F | hdl = 2060/19960001309 | s2cid = 20848006 | hdl-access = free | access-date = August 28, 2015 | archive-date = March 10, 2012 | archive-url = https://web.archive.org/web/20120310172124/http://nova.stanford.edu/~vlf/IHY_Test/Tutorials/TGFs/Papers/Fishman1994.pdf | url-status = dead }}</ref> Gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of ''[[bremsstrahlung]]'', inverse [[Compton scattering]] and [[synchrotron radiation]]. A large fraction of such astronomical gamma rays are screened by Earth's atmosphere. Notable artificial sources of gamma rays include [[nuclear fission|fission]], such as occurs in [[nuclear reactor]]s, as well as [[high energy physics]] experiments, such as [[pion|neutral pion decay]] and [[nuclear fusion]].

A sample of gamma ray-emitting material that is used for irradiating or imaging is known as a gamma source. It is also called a [[radioactive source]], isotope source, or radiation source, though these more general terms also apply to alpha and beta-emitting devices. Gamma sources are usually sealed to prevent [[radioactive contamination]], and transported in heavy shielding.

===Radioactive decay (gamma decay)===
{{Main|Nuclear isomer}}
Gamma rays are produced during gamma decay, which normally occurs after other forms of decay occur, such as [[alpha decay|alpha]] or [[beta decay|beta]] decay. A radioactive nucleus can decay by the emission of an [[alpha particle|{{Subatomic particle|alpha}}]] or [[beta particle|{{Subatomic particle|beta}}]] particle. The [[daughter nucleus]] that results is usually left in an excited state. It can then decay to a lower energy state by emitting a gamma ray photon, in a process called gamma decay.

The emission of a gamma ray from an excited nucleus typically requires only 10<sup>−12</sup> seconds. Gamma decay may also follow [[nuclear reaction]]s such as [[neutron capture]], [[nuclear fission]], or nuclear fusion. Gamma decay is also a mode of relaxation of many excited states of atomic nuclei following other types of radioactive decay, such as beta decay, so long as these states possess the necessary component of nuclear [[Spin (physics)|spin]]. When high-energy gamma rays, electrons, or protons bombard materials, the excited atoms emit characteristic "secondary" gamma rays, which are products of the creation of excited nuclear states in the bombarded atoms. Such transitions, a form of nuclear gamma [[fluorescence]], form a topic in [[nuclear physics]] called [[gamma spectroscopy]]. Formation of fluorescent gamma rays are a rapid subtype of radioactive gamma decay.

In certain cases, the excited nuclear state that follows the emission of a beta particle or other type of excitation, may be more stable than average, and is termed a [[metastability#Atomic and molecular physics|metastable]] excited state, if its decay takes (at least) 100 to 1000 times longer than the average 10<sup>−12</sup> seconds. Such relatively long-lived excited nuclei are termed [[nuclear isomer]]s, and their decays are termed [[isomeric transition]]s. Such nuclei have [[half-life]]s that are more easily measurable, and rare nuclear isomers are able to stay in their excited state for minutes, hours, days, or occasionally far longer, before emitting a gamma ray. The process of isomeric transition is therefore similar to any gamma emission, but differs in that it involves the intermediate metastable excited state(s) of the nuclei. Metastable states are often characterized by high [[nuclear spin]], requiring a change in spin of several units or more with gamma decay, instead of a single unit transition that occurs in only 10<sup>−12</sup> seconds. The rate of gamma decay is also slowed when the energy of excitation of the nucleus is small.<ref>{{Cite web|url=https://web1.eng.famu.fsu.edu/~dommelen/quantum/style_a/ntgd.html |title=14.20 Draft: Gamma Decay |access-date=2023-02-19|website=Quantum Mechanics for Engineers |first=Leon |last=van Dommelen |publisher=FAMU-FSU College of Engineering }}</ref>

An emitted gamma ray from any type of excited state may transfer its energy directly to any [[electron]]s, but most probably to one of the K shell electrons of the atom, causing it to be ejected from that atom, in a process generally termed the [[photoelectric effect]] (external gamma rays and ultraviolet rays may also cause this effect). The photoelectric effect should not be confused with the [[internal conversion]] process, in which a gamma ray photon is not produced as an intermediate particle (rather, a "virtual gamma ray" may be thought to mediate the process).

====Decay schemes====
[[File:Cobalt-60 Decay Scheme.svg|thumb|Radioactive decay scheme of {{SimpleNuclide|Cobalt|60}}]]
[[File:60Co gamma spectrum energy-de.svg|thumb|Gamma emission spectrum of cobalt-60]]
One example of gamma ray production due to radionuclide decay is the decay scheme for cobalt-60, as illustrated in the accompanying diagram. First, {{SimpleNuclide|link=yes|Cobalt|60}} decays to [[excited state|excited]] {{SimpleNuclide|link=yes|Nickel|60}} by [[beta rays|beta decay]] emission of an electron of {{val|0.31|ul=MeV}}. Then the excited {{SimpleNuclide|Nickel|60}} decays to the ground state (see [[nuclear shell model]]) by emitting gamma rays in succession of 1.17 MeV followed by {{val|1.33|u=MeV}}. This path is followed 99.88% of the time:
<!-- Autogenerated using Phykiformulae 0.12 [[User:SkyLined#Phykiformulae]]
Co-60 -> Ni-60* + e- + !ve + y + 1.17MeV
Ni-60* -> Ni-60 _ _ _ _ + y + 1.33MeV
-->:{| border="0"
|- style="height:2em;"
|{{nuclide|link=yes|cobalt|60}}&nbsp;||→&nbsp;||{{nuclide|link=yes|nickel|60|charge=*}}&nbsp;||+&nbsp;||{{Subatomic particle|link=yes|Electron}}&nbsp;||+&nbsp;||{{math|{{Subatomic particle|link=yes|Electron Antineutrino}}}}&nbsp;||+&nbsp;||{{math|{{Subatomic particle|link=yes|Gamma}}}}&nbsp;||+&nbsp;||{{val|1.17|ul=MeV}}
|- style="height:2em;"
|{{nuclide|link=yes|nickel|60|charge=*}}&nbsp;||→&nbsp;||{{nuclide|link=yes|nickel|60}}&nbsp;||&nbsp;||&nbsp;||&nbsp;||&nbsp;||+&nbsp;||{{math|{{Subatomic particle|link=yes|Gamma}}}}&nbsp;||+&nbsp;||{{val|1.33|u=MeV}}
|}

Another example is the alpha decay of {{SimpleNuclide|link=yes|americium|241}} to form {{SimpleNuclide|link=yes|neptunium|237}}; which is followed by gamma emission. In some cases, the gamma emission spectrum of the daughter nucleus is quite simple, (e.g. {{SimpleNuclide|Cobalt|60}}/{{SimpleNuclide|Nickel|60}}) while in other cases, such as with ({{SimpleNuclide|americium|241}}/{{SimpleNuclide|neptunium|237}} and {{SimpleNuclide|link=yes|iridium|192}}/{{SimpleNuclide|platinum|192}}), the gamma emission spectrum is complex, revealing that a series of nuclear energy levels exist.

===Particle physics===
Gamma rays are produced in many processes of [[particle physics]]. Typically, gamma rays are the products of [[electrically neutral|neutral]] systems which decay through [[electromagnetic interaction]]s (rather than a [[weak interaction|weak]] or [[strong interaction|strong]] interaction). For example, in an [[electron–positron annihilation]], the usual products are two gamma ray photons. If the annihilating electron and [[positron]] are at rest, each of the resulting gamma rays has an energy of ~ 511 [[kiloelectronvolt|keV]] and frequency of ~ {{val|1.24|e=20|u=Hz}}. Conversely, gamma rays above 1022 keV can interact with nuclei via [[pair production]] of an electron and positron.<ref name="r195">{{cite journal |last1=Pecko |first1=Stanislav |last2=Slugeň |first2=Vladimír |last3=Chung |first3=Won Sang |last4=Algin |first4=Abdullah |last5=Yamashita |first5=Osamu |last6=Sekimoto |first6=Takeyuki |last7=Terakawa |first7=Akira |last8=Torchigin |first8=V.P. |last9=Torchigin |first9=A.V. |last10=Oliveira |first10=Felipe M.F. de |last11=Limousin |first11=O. |date=2006-06-01 |title=Electron–positron pair production by photons: A historical overview |url=https://www.sciencedirect.com/science/article/abs/pii/S0969806X0500263X |journal=Radiation Physics and Chemistry |publisher=Pergamon |volume=75 |issue=6 |pages=614–623 |doi=10.1016/j.radphyschem.2005.10.008 |bibcode=2006RaPC...75..614H |issn=0969-806X |access-date=2025-04-30|url-access=subscription }}</ref> Similarly, a neutral [[pion]] most often decays into two photons. Many other [[hadron]]s and massive [[boson]]s also decay electromagnetically. High energy physics experiments, such as the [[Large Hadron Collider]], accordingly employ substantial radiation shielding.<ref>{{cite conference |title=Radiation protection considerations in the design of the LHC, CERN's Large Hadron Collider |last1= Höfert |first1=Manfred |last2=Huhtinen |first2=M | last3=Moritz |first3=L E |date=17 Oct 1996 |url=https://cds.cern.ch/record/313585?ln=en |id=CERN-TIS-96-014-RP-CF
|display-authors=2 |conference=American Health Physics Society Topical Meeting on the Health Physics of Radiation Generating Machines, San José, CA, USA, 5 - 8 Jan 1997 | pages=343–352}}</ref> Because [[subatomic particle]]s mostly have far shorter wavelengths than atomic nuclei, particle physics gamma rays are generally several orders of magnitude more energetic than nuclear decay gamma rays. Since gamma rays are at the top of the electromagnetic spectrum in terms of energy, all extremely high-energy photons are gamma rays; for example, a photon having the [[Planck energy]] would be a gamma ray.

===Other sources===
{{Main|Gamma-ray astronomy}}
A few gamma rays in astronomy are known to arise from gamma decay (see discussion of [[SN1987A]]), but most do not.

Photons from astrophysical sources that carry energy in the gamma radiation range are often explicitly called gamma-radiation. In addition to nuclear emissions, they are often produced by sub-atomic particle and particle-photon interactions. Those include [[electron-positron annihilation]], [[neutral pion decay]], [[bremsstrahlung]], inverse [[Compton scattering]], and [[synchrotron radiation]].
[[File:Antimatter Explosions.ogv|thumb|right|200px|The red dots show some of the ~500 terrestrial gamma-ray flashes daily detected by the Fermi Gamma-ray Space Telescope through 2010. Credit: NASA/Goddard Space Flight Center.]]

====Laboratory sources====
In October 2017, scientists from various European universities proposed a means for sources of GeV photons using lasers as exciters through a controlled interplay between the cascade and anomalous [[radiation trapping|radiative trapping]].<ref name="2017-10-07_PRX">{{Cite journal | doi=10.1103/PhysRevX.7.041003| arxiv=1610.06404| bibcode=2017PhRvX...7d1003G| title=Ultrabright GeV Photon Source via Controlled Electromagnetic Cascades in Laser-Dipole Waves| year=2017| last1=Gonoskov| first1=A.| last2=Bashinov| first2=A.| last3=Bastrakov| first3=S.| last4=Efimenko| first4=E.| last5=Ilderton| first5=A.| last6=Kim| first6=A.| last7=Marklund| first7=M.| last8=Meyerov| first8=I.| last9=Muraviev| first9=A.| last10=Sergeev| first10=A.| journal=[[Physical Review X]]| volume=7| issue=4| pages=041003| s2cid=55569348}}</ref>

====Terrestrial thunderstorms====
[[Thunderstorm]]s can produce a brief pulse of gamma radiation called a [[terrestrial gamma-ray flash]]. These gamma rays are thought to be produced by high intensity static electric fields accelerating electrons, which then produce gamma rays by [[bremsstrahlung]] as they collide with and are slowed by atoms in the atmosphere. Gamma rays up to 100 MeV can be emitted by terrestrial thunderstorms, and were discovered by space-borne observatories. This raises the possibility of health risks to passengers and crew on aircraft flying in or near thunderclouds.<ref>{{cite magazine|last=Smith|first=Joseph|author2=David M. Smith|title=Deadly Rays From Clouds|magazine=Scientific American|date=August 2012|volume=307|issue=2|pages=55–59|bibcode=2012SciAm.307b..54D|doi=10.1038/scientificamerican0812-54}}</ref>

====Solar flares====
The most effusive [[solar flare]]s emit across the entire EM spectrum, including γ-rays. The first confident observation occurred in [[Solar storm of August 1972|1972]].<ref>{{cite journal |last1 = Chupp |first1 = E. L. |first2= D. J. |last2= Forrest |first3= P. R. |last3= Higbie |first4= A. N. |last4= Suri |first5= C. |last5= Tsai |first6= P. P. |last6= Dunphy |title = Solar Gamma Ray Lines observed during the Solar Activity of August 2 to August 11, 1972 |journal = Nature |volume = 241 |issue = 5388 |pages = 333–335 |date = 1973 |doi = 10.1038/241333a0 |bibcode = 1973Natur.241..333C |s2cid = 4172523 }}</ref>

====Cosmic rays====
Extraterrestrial, high energy gamma rays include the gamma ray background produced when cosmic rays (either high speed electrons or protons) collide with ordinary matter, producing pair-production gamma rays at 511 keV. Alternatively, [[bremsstrahlung]] are produced at energies of tens of MeV or more when cosmic ray electrons interact with nuclei of sufficiently high atomic number (see gamma ray image of the Moon near the end of this article, for illustration).
[[File:Egret all sky gamma ray map from CGRO spacecraft.png|thumb|300px|right|Image of entire sky in 100 MeV or greater gamma rays as seen by the EGRET instrument aboard the [[CGRO]] spacecraft. Bright spots within the galactic plane are [[pulsar]]s while those above and below the plane are thought to be [[quasar]]s.]]

====Pulsars and magnetars====
The gamma ray sky (see illustration at right) is dominated by the more common and longer-term production of gamma rays that emanate from [[pulsar]]s within the Milky Way. Sources from the rest of the sky are mostly [[quasar]]s. Pulsars are thought to be neutron stars with magnetic fields that produce focused beams of radiation, and are far less energetic, more common, and much nearer sources (typically seen only in our own galaxy) than are quasars or the rarer [[gamma-ray burst]] sources of gamma rays. Pulsars have relatively long-lived magnetic fields that produce focused beams of relativistic speed charged particles, which emit gamma rays (bremsstrahlung) when those strike gas or dust in their nearby medium, and are decelerated. This is a similar mechanism to the production of high-energy photons in [[megavoltage]] [[radiation therapy]] machines (see [[bremsstrahlung]]). [[Inverse Compton scattering]], in which charged particles (usually electrons) impart energy to low-energy photons boosting them to higher energy photons. Such impacts of photons on relativistic charged particle beams is another possible mechanism of gamma ray production. Neutron stars with a very high magnetic field ([[magnetar]]s), thought to produce astronomical [[soft gamma repeater]]s, are another relatively long-lived star-powered source of gamma radiation.

====Quasars and active galaxies====
More powerful gamma rays from very distant [[quasar]]s and closer active galaxies are thought to have a gamma ray production source similar to a [[particle accelerator]]. High energy electrons produced by the quasar, and subjected to inverse Compton scattering, [[synchrotron radiation]], or bremsstrahlung, are the likely source of the gamma rays from those objects. It is thought that a [[supermassive black hole]] at the center of such galaxies provides the power source that intermittently destroys stars and focuses the resulting charged particles into beams that emerge from their rotational poles. When those beams interact with gas, dust, and lower energy photons they produce X-rays and gamma rays. These sources are known to fluctuate with durations of a few weeks, suggesting their relatively small size (less than a few light-weeks across). Such sources of gamma and X-rays are the most commonly visible high intensity sources outside the Milky Way galaxy. They shine not in bursts (see illustration), but relatively continuously when viewed with gamma ray telescopes. The power of a typical quasar is about 10<sup>40</sup> watts, a small fraction of which is gamma radiation. Much of the rest is emitted as electromagnetic waves of all frequencies, including radio waves.
[[File:Gamma ray burst.jpg|thumb|300px|A [[hypernova]]. Artist's illustration showing the life of a [[massive star]] as [[nuclear fusion]] converts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses to form a [[black hole]]. Theoretically, energy may be released during the collapse along the axis of rotation to form a long duration [[gamma-ray burst]].]]

====Gamma-ray bursts====
{{See also|Gamma-ray burst}}
The most intense sources of gamma rays are also the most intense sources of any type of electromagnetic radiation presently known. They are the "long duration burst" sources of gamma rays in astronomy ("long" in this context, meaning a few tens of seconds), and they are rare compared with the sources discussed above. By contrast, "short" [[gamma-ray burst]]s of two seconds or less, which are not associated with supernovae, are thought to produce gamma rays during the collision of pairs of neutron stars, or a neutron star and a [[black hole]].<ref>{{Cite web|title=NASA - In a Flash NASA Helps Solve 35-year-old Cosmic Mystery|url=https://www.nasa.gov/mission_pages/swift/bursts/short_burst_oct5.html|access-date=2023-02-19|website=www.nasa.gov|language=en}}</ref>

The so-called ''long-duration'' gamma-ray bursts produce a total energy output of about 10<sup>44</sup> joules (as much energy as the [[Sun]] will produce in its entire life-time) but in a period of only 20 to 40 seconds. Gamma rays are approximately 50% of the total energy output. The leading hypotheses for the mechanism of production of these highest-known intensity beams of radiation, are inverse [[Compton scattering]] and [[synchrotron radiation]] from high-energy charged particles. These processes occur as relativistic charged particles leave the region of the event horizon of a newly formed [[black hole]] created during supernova explosion. The beam of particles moving at relativistic speeds are focused for a few tens of seconds by the magnetic field of the exploding [[hypernova]]. The fusion explosion of the hypernova drives the energetics of the process. If the narrowly directed beam happens to be pointed toward the Earth, it shines at gamma ray frequencies with such intensity, that it can be detected even at distances of up to 10 billion light years, which is close to the edge of the [[visible universe]].

==Properties==

===Penetration of matter===
{{See also|Radiation protection#Electromagnetic radiation}}
[[File:Alfa beta gamma radiation.svg|upright=1.2|thumb|[[Alpha particle|Alpha radiation]] consists of [[helium]] nuclei and is readily stopped by a sheet of paper. [[Beta radiation]], consisting of [[electron]]s or [[positron]]s, is stopped by an aluminium plate, but gamma radiation requires shielding by dense material such as lead or concrete.]]
{{unreferenced section|date=November 2022}}
Due to their penetrating nature, gamma rays require large amounts of shielding mass to reduce them to levels which are not harmful to living cells, in contrast to [[alpha particle]]s, which can be stopped by paper or skin, and [[beta particle]]s, which can be shielded by thin aluminium. Gamma rays are best absorbed by materials with high [[atomic number]]s (''Z'') and high density, which contribute to the total stopping power. Because of this, a lead (high ''Z'') shield is 20–30% better as a gamma shield than an equal mass of a low-''Z'' shielding material, such as aluminium, concrete, water, or soil; lead's major advantage is not in lower weight, but rather its compactness due to its higher density. Protective clothing, goggles and respirators can protect from internal contact with or ingestion of alpha or beta emitting particles, but provide no protection from gamma radiation from external sources.

The higher the energy of the gamma rays, the thicker the shielding made from the same shielding material is required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half (the [[half-value layer]] or HVL). For example, gamma rays that require 1&nbsp;cm (0.4&nbsp;inch) of [[lead]] to reduce their intensity by 50% will also have their intensity reduced in half by {{nowrap|4.1 cm}} of [[granite]] rock, 6&nbsp;cm (2.5&nbsp;inches) of [[concrete]], or 9&nbsp;cm (3.5&nbsp;inches) of packed [[soil]]. However, the mass of this much concrete or soil is only 20–30% greater than that of lead with the same absorption capability.

[[Depleted uranium]] is sometimes used for shielding in [[industrial radiography#Sources|portable gamma ray sources]], due to the smaller half-value layer when compared to lead (around 0.6 times the thickness for common gamma ray sources, i.e. Iridium-192 and Cobalt-60)<ref>{{cite web |url=https://www.nde-ed.org/Physics/X-Ray/HalfValueLayer.xhtml |title=Half-Value Layer |website=Iowa State University Center for Nondestructive Evaluation |access-date=2024-05-10 |language=en}}</ref> and cheaper cost compared to [[tungsten]].<ref>{{cite web |url=http://hps.org/publicinformation/ate/q8929.html |title=Answer to Question #8929 Submitted to "Ask the Experts" |website=Health Physics Society |access-date=2024-05-10 |language=en}}</ref>

In a nuclear power plant, shielding can be provided by steel and concrete in the pressure and particle containment vessel, while water provides a radiation shielding of fuel rods during storage or transport into the reactor core. The loss of water or removal of a "hot" fuel assembly into the air would result in much higher radiation levels than when kept under water.

===Matter interaction===
{{See also|Gamma ray cross section}}
[[File:Al-gamma-xs.svg|upright=1.35|thumb|The total absorption coefficient of aluminium (atomic number 13) for gamma rays, plotted versus gamma energy, and the contributions by the three effects. As is usual, the photoelectric effect is largest at low energies, Compton scattering dominates at intermediate energies, and pair production dominates at high energies.]]
[[File:Pb-gamma-xs.svg|upright=1.35|thumb|The total absorption coefficient of lead (atomic number 82) for gamma rays, plotted versus gamma energy, and the contributions by the three effects. Here, the photoelectric effect dominates at low energy. Above 5 MeV, pair production starts to dominate.]]
{{unreferenced section|date=November 2022}}
When a gamma ray passes through matter, the probability for absorption is proportional to the thickness of the layer, the density of the material, and the absorption cross section of the material. The total absorption shows an [[exponential decay|exponential decrease]] of intensity with distance from the incident surface:
:<math>I(x)= I_0 \cdot e ^{-\mu x}</math>
where x is the thickness of the material from the incident surface, μ= ''n''σ is the absorption coefficient, measured in cm<sup>−1</sup>, ''n'' the number of atoms per cm<sup>3</sup> of the material (atomic density) and σ the absorption [[cross section (physics)|cross section]] in cm<sup>2</sup>.

As it passes through matter, gamma radiation ionizes via three processes:
* '''The [[photoelectric effect]]''': This describes the case in which a gamma photon interacts with and transfers its energy to an atomic electron, causing the ejection of that electron from the atom. The kinetic energy of the resulting [[photoelectron]] is equal to the energy of the incident gamma photon minus the energy that originally bound the electron to the atom (binding energy). The photoelectric effect is the dominant energy transfer mechanism for X-ray and gamma ray photons with energies below 50 keV (thousand electronvolts), but it is much less important at higher energies.
* '''[[Compton scattering]]''': This is an interaction in which an incident gamma photon loses enough energy to an atomic electron to cause its ejection, with the remainder of the original photon's energy emitted as a new, lower energy gamma photon whose emission direction is different from that of the incident gamma photon, hence the term "scattering". The probability of Compton scattering decreases with increasing photon energy. It is thought to be the principal absorption mechanism for gamma rays in the intermediate energy range 100 keV to 10 MeV. It is relatively independent of the [[atomic number]] of the absorbing material, which is why very dense materials like lead are only modestly better shields, on a ''per weight'' basis, than are less dense materials.
* '''[[Pair production]]''': This becomes possible with gamma energies exceeding 1.02 MeV, and becomes important as an absorption mechanism at energies over 5 MeV (see illustration at right, for lead). By interaction with the [[electric field]] of a nucleus, the energy of the incident photon is converted into the mass of an electron-positron pair. Any gamma energy in excess of the equivalent rest mass of the two particles (totaling at least 1.02 MeV) appears as the kinetic energy of the pair and in the recoil of the emitting nucleus. At the end of the positron's [[range (particle radiation)|range]], it combines with a free electron, and the two annihilate, and the entire mass of these two is then converted into two gamma photons of at least 0.51 MeV energy each (or higher according to the kinetic energy of the annihilated particles).

The secondary electrons (and/or positrons) produced in any of these three processes frequently have enough energy to produce much [[ionization]] themselves.

Additionally, gamma rays, particularly high energy ones, can interact with atomic nuclei resulting in ejection of particles in [[photodisintegration]], or in some cases, even nuclear fission ([[photofission]]).

===Light interaction===
High-energy (from 80 GeV to ~10 [[TeV]]) gamma rays arriving from far-distant quasars are used to estimate the [[extragalactic background light]] in the universe: The highest-energy rays interact more readily with the background light photons and thus the density of the background light may be estimated by analyzing the incoming gamma ray spectra.<ref>{{cite journal|last=Bock|first=R. K. |date=2008-06-27|title=Very-High-Energy Gamma Rays from a Distant Quasar: How Transparent Is the Universe?|journal=[[Science (journal)|Science]]|volume=320|issue=5884|pages=1752–1754|issn=0036-8075|doi=10.1126/science.1157087|pmid=18583607|bibcode= 2008Sci...320.1752M |arxiv= 0807.2822 |s2cid=16886668 |display-authors=etal}}</ref><ref>{{cite magazine|last=Domínguez|first=Alberto|date=2015-06-01|title=All the Light There Ever Was|magazine=Scientific American|volume=312|issue=6|pages=38–43|issn=0036-8075|display-authors=etal}}</ref>

===Gamma spectroscopy===
{{Main|Gamma spectroscopy}}
Gamma spectroscopy is the study of the energetic transitions in atomic nuclei, which are generally associated with the absorption or emission of gamma rays. As in optical [[spectroscopy]] (see [[Franck–Condon]] effect) the absorption of gamma rays by a nucleus is especially likely (i.e., peaks in a "resonance") when the energy of the gamma ray is the same as that of an energy transition in the nucleus. In the case of gamma rays, such a resonance is seen in the technique of [[Mössbauer spectroscopy]]. In the [[Mössbauer effect]] the narrow resonance absorption for nuclear gamma absorption can be successfully attained by physically immobilizing atomic nuclei in a crystal. The immobilization of nuclei at both ends of a gamma resonance interaction is required so that no gamma energy is lost to the kinetic energy of recoiling nuclei at either the emitting or absorbing end of a gamma transition. Such loss of energy causes gamma ray resonance absorption to fail. However, when emitted gamma rays carry essentially all of the energy of the atomic nuclear de-excitation that produces them, this energy is also sufficient to excite the same energy state in a second immobilized nucleus of the same type.

==Applications==
[[File:VACIS Gamma-ray Image with stowaways.GIF|upright=1.3|thumb|Gamma-ray image of a truck with two stowaways taken with a [[cargo scanning|VACIS]] (vehicle and container imaging system)]]
Gamma rays provide information about some of the most energetic phenomena in the universe; however, they are largely absorbed by the Earth's atmosphere. Instruments aboard high-altitude balloons and satellites missions, such as the [[Fermi Gamma-ray Space Telescope]], provide our only view of the universe in gamma rays.

Gamma-induced molecular changes can also be used to alter the properties of [[semi-precious stone]]s, and is often used to change white [[topaz]] into [[blue topaz]].

Non-contact industrial sensors commonly use sources of gamma radiation in refining, mining, chemicals, food, soaps and detergents, and pulp and paper industries, for the measurement of levels, density, and thicknesses.<ref>{{cite journal |last1=Beigzadeh |first1=A.M. |title=Design and improvement of a simple and easy-to-use gamma-ray densitometer for application in wood industry |journal=Measurement |date=2019 |volume=138 |pages=157–161 |doi=10.1016/j.measurement.2019.02.017 |bibcode=2019Meas..138..157B |s2cid=115945689 }}</ref> Gamma-ray sensors are also used for measuring the fluid levels in water and oil industries.<ref>{{cite journal |last1=Falahati |first1=M. |title=Design, modelling and construction of a continuous nuclear gauge for measuring the fluid levels |journal=Journal of Instrumentation |date=2018 |volume=13 |issue=2 |page=02028 |doi=10.1088/1748-0221/13/02/P02028 |bibcode=2018JInst..13P2028F |s2cid=125779702 }}</ref> Typically, these use Co-60 or Cs-137 isotopes as the radiation source.

In the US, gamma ray detectors are beginning to be used as part of the [[Container Security Initiative]] (CSI). These machines are advertised to be able to scan 30 containers per hour.

Gamma radiation is often used to kill living organisms, in a process called [[irradiation]]. Applications of this include the sterilization of medical equipment (as an alternative to [[autoclave]]s or chemical means), the removal of decay-causing [[bacteria]] from many foods and the prevention of the sprouting of fruit and vegetables to maintain freshness and flavor.

Despite their cancer-causing properties, gamma rays are also used to treat some types of [[cancer]], since the rays also kill cancer cells. In the procedure called [[gamma knife|gamma-knife]] surgery, multiple concentrated beams of gamma rays are directed to the growth in order to kill the cancerous cells. The beams are aimed from different angles to concentrate the radiation on the growth while minimizing damage to surrounding tissues.

Gamma rays are also used for diagnostic purposes in [[nuclear medicine]] in imaging techniques. A number of different gamma-emitting radioisotopes are used. For example, in a [[PET scan]] a radiolabeled sugar called [[fluorodeoxyglucose]] emits [[positron]]s that are annihilated by electrons, producing pairs of gamma rays that highlight cancer as the cancer often has a higher metabolic rate than the surrounding tissues. The most common gamma emitter used in medical applications is the [[nuclear isomer]] [[technetium-99m]] which emits gamma rays in the same energy range as diagnostic X-rays. When this radionuclide tracer is administered to a patient, a [[gamma camera]] can be used to form an image of the radioisotope's distribution by detecting the gamma radiation emitted (see also [[SPECT]]). Depending on which molecule has been labeled with the tracer, such techniques can be employed to diagnose a wide range of conditions (for example, the spread of cancer to the bones via [[bone scan]]).

==Health effects==
{{See also|Ionizing_radiation#Health_effects}}
Gamma rays cause damage at a cellular level and are penetrating, causing diffuse damage throughout the body. However, they are less ionising than alpha or beta particles, which are less penetrating.{{cn|date=June 2025}}

Low levels of gamma rays cause a [[stochastic]] health risk, which for radiation dose assessment is defined as the ''probability'' of cancer induction and genetic damage. The [[International Commission on Radiological Protection]] says "In the low dose range, below about 100 mSv, it is scientifically plausible to assume that the incidence of cancer or heritable effects will rise in direct proportion to an increase in the equivalent dose in the relevant organs and tissues"<ref name=ICRP_103/>{{rp|51}} (The unit mSv is milli-[[Seivert]]). High doses produce [[deterministic]] effects, which is the ''severity'' of acute tissue damage that is certain to happen. These effects are compared to the physical quantity [[absorbed dose]] measured by the unit [[Gray (unit)|gray]] (Gy).<ref name=ICRP_103>{{Cite book |title=The 2007 recommendations of the International Commission on Radiological Protection
| url=https://www.icrp.org/publication.asp?id=ICRP%20Publication%20103%20(Users%20Edition) |date=2007 |publisher=Elsevier |isbn=978-0-7020-3048-2 |editor-last=Valentin |editor-first=J. |series=ICRP publication |location=Oxford |editor-last2=International Commission on Radiological Protection}}</ref>{{rp|61}}

===Effects and body response===
When gamma radiation breaks DNA molecules, a cell may be able to [[DNA repair#DNA repair mechanisms|repair the damaged]] genetic material, within limits. However, a study of Rothkamm and Lobrich concerning X-ray radiation has shown that this repair process works well after high-dose exposure but is much slower in the case of a low-dose exposure.<ref name= "mcuaqz">{{cite journal|last1=Rothkamm|first1=K|last2=Löbrich|first2=M|title=Evidence for a lack of DNA double-strand break repair in human cells exposed to very low x-ray doses|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=100|issue=9|pages=5057–62|year=2003|pmid=12679524|pmc=154297|doi=10.1073/pnas.0830918100|bibcode= 2003PNAS..100.5057R|doi-access=free}}</ref>

Studies have shown low-dose gamma radiation may be enough to cause cancer.<ref name="Graupner-2016">{{Cite journal |last1=Graupner |first1=Anne |last2=Eide |first2=Dag M. |last3=Instanes |first3=Christine |last4=Andersen |first4=Jill M. |last5=Brede |first5=Dag A. |last6=Dertinger |first6=Stephen D. |last7=Lind |first7=Ole C. |last8=Brandt-Kjelsen |first8=Anicke |last9=Bjerke |first9=Hans |last10=Salbu |first10=Brit |last11=Oughton |first11=Deborah |last12=Brunborg |first12=Gunnar |last13=Olsen |first13=Ann K. |date=2016-09-06 |title=Gamma radiation at a human relevant low dose rate is genotoxic in mice |journal=Scientific Reports |volume=6 |issue=1 |pages=32977 |doi=10.1038/srep32977 |issn=2045-2322 |pmc=5011728 |pmid=27596356|bibcode=2016NatSR...632977G }}</ref> In a study of mice, they were given human-relevant low-dose gamma radiation, with genotoxic effects 45 days after continuous low-dose gamma radiation, with significant increases of chromosomal damage, DNA lesions and phenotypic mutations in blood cells of irradiated animals, covering the three types of genotoxic activity.<ref name="Graupner-2016" /> Another study studied the effects of acute ionizing gamma radiation in rats, up to 10 [[Gray (unit)|Gy]], and who ended up showing acute oxidative protein damage, DNA damage, cardiac troponin T carbonylation, and long-term [[cardiomyopathy]].<ref>{{Cite journal |last1=Rosen |first1=Elliot |last2=Kryndushkin |first2=Dmitry |last3=Aryal |first3=Baikuntha |last4=Gonzalez |first4=Yanira |last5=Chehab |first5=Leena |last6=Dickey |first6=Jennifer |last7=Rao |first7=V. Ashutosh |date=2020-06-04 |title=Acute total body ionizing gamma radiation induces long-term adverse effects and immediate changes in cardiac protein oxidative carbonylation in the rat |journal=PLOS ONE |volume=15 |issue=6 |pages=e0233967 |doi=10.1371/journal.pone.0233967 |doi-access=free |issn=1932-6203 |pmc=7272027 |pmid=32497067|bibcode=2020PLoSO..1533967R }}</ref>

===Risk assessment===
Natural gamma radiation in the United Kingdom of accounts for about 13% of the average radiation dose.<ref>{{Cite web|title=Radioactivity in food and the environment (RIFE) reports|url=https://www.gov.uk/government/publications/radioactivity-in-food-and-the-environment-rife-reports|access-date=2023-02-19|website=GOV.UK|language=en}}</ref> Natural exposure to gamma rays is about 1 to 2 mSv per year, and the average total amount of radiation received in one year per inhabitant in the USA is 3.6 mSv.<ref>United Nations Scientific Committee on the Effects of Atomic Radiation Annex E: Medical radiation exposures – Sources and Effects of Ionizing – 1993, p. 249, New York, UN</ref> There might be a small increase in the dose around small particles of [[depleted uranium]] should these enter the human body from spent munitions, caused by increased effects of natural gamma radiation.<ref>{{cite journal|last1=Pattison|first1=J. E.|last2=Hugtenburg|first2=R. P.|last3=Green|first3=S.|title=Enhancement of natural background gamma-radiation dose around uranium microparticles in the human body|journal=Journal of the Royal Society Interface|volume=7|pages=603–611|year=2009|doi=10.1098/rsif.2009.0300|pmid=19776147|issue=45|pmc=2842777}}</ref>

By comparison, the radiation dose from chest [[radiography]] (about 0.06 mSv) is a fraction of the annual naturally occurring background radiation dose.<ref>US National Council on Radiation Protection and Measurements – NCRP Report No. 93 – pp 53–55, 1987. Bethesda, Maryland, USA, NCRP</ref> A chest CT delivers 5 to 8 mSv. A whole-body [[positron emission tomography|PET]]/CT scan can deliver 14 to 32 mSv depending on the protocol.<ref>{{cite web |url=http://radiology.rsna.org/content/251/1/166.full.pdf |title=PET/CT total radiation dose calculations. |access-date=2011-11-08 |archive-url=https://web.archive.org/web/20130123140458/http://radiology.rsna.org/content/251/1/166.full.pdf |archive-date=2013-01-23 |url-status=dead }}</ref> The dose from [[fluoroscopy]] of the stomach is much higher, approximately 50 mSv (14 times the annual background).

An acute full-body equivalent single exposure dose of 1 Sv (1000 mSv), or 1 Gy, will cause mild symptoms of [[acute radiation sickness]], such as nausea and vomiting; and a dose of 2.0–3.5 Sv (2.0–3.5 Gy) causes more severe symptoms (i.e. nausea, diarrhea, hair loss, [[hemorrhaging]], and inability to fight infections), and will cause death in a sizable number of cases—about 10% to 35% without medical treatment. A dose of 3–5 Sv (3–5 Gy) is considered approximately the [[median lethal dose|LD<sub>50</sub>]] (or the lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment.<ref>{{cite journal |vauthors=Ryan JL |date=March 2012 |title=Ionizing radiation: the good, the bad, and the ugly |journal=The Journal of Investigative Dermatology |volume=132 |issue=3 Pt 2 |pages=985–993 |doi=10.1038/jid.2011.411 |pmc=3779131 |pmid=22217743}}</ref><ref>{{Cite web |date=2022-12-13 |title=Radiation Exposure - Dose and Dose Rate (the Gray & Sievert) |url=https://ionactive.co.uk/resource-hub/guidance/radiation-exposure-dose-and-dose-rate-the-gray-sievert |access-date=2024-07-27 |website=Ionactive}}</ref> A dose higher than 5 Sv (5 Gy) brings an increasing chance of death above 50%. Above 7.5–10 Sv (7.5–10 Gy) to the entire body, even extraordinary treatment, such as bone-marrow transplants, will not prevent the death of the individual exposed (see [[radiation poisoning]]).<ref name="Rodgerson-2012">{{cite journal |last1=Rodgerson |first1=D.O. |last2=Reidenberg |first2=B.E. |last3=Harris |first3=A.g. |last4=Pecora |first4=A.L. |date=2012 |title=Potential for a pluripotent adult stem cell treatment for acute radiation sickness |journal=World Journal of Experimental Medicine |volume=2 |issue=3 |pages=37–44 |doi=10.5493/wjem.v2.i3.37 |pmid=24520532 |pmc=3905584 |doi-access=free }}</ref> (Doses much larger than this may, however, be delivered to selected parts of the body in the course of [[radiation therapy]].)

For low-dose exposure, for example among nuclear workers, who receive an average yearly radiation dose of 19 mSv,{{Clarify|date=February 2009}} <!--annual dose? lifetime dose?--> the risk of dying from cancer (excluding [[leukemia]]) increases by 2 percent. For a dose of 100 mSv, the risk increase is 10 percent. By comparison, risk of dying from cancer was increased by 32 percent for the survivors of the [[atomic bombing of Hiroshima and Nagasaki]].<ref>{{cite journal|last1=Cardis|first1=E|title=Risk of cancer after low doses of ionising radiation: retrospective cohort study in 15 countries|journal=BMJ|date=9 July 2005|volume=331|issue=7508|pages=77–0|doi=10.1136/bmj.38499.599861.E0|pmid=15987704|pmc=558612}}</ref>

==Units of measurement and exposure==
The following table shows radiation quantities in SI and non-SI units:
{{Radiation related quantities}}

The measure of the [[ionizing]] effect of gamma and X-rays in dry air is called the exposure, for which a legacy unit, the [[Roentgen (unit)|röntgen]], was used from 1928. This has been replaced by [[kerma (physics)|kerma]], now mainly used for instrument calibration purposes but not for received dose effect. The effect of gamma and other ionizing radiation on living tissue is more closely related to the amount of [[energy]] deposited in tissue rather than the ionisation of air, and replacement radiometric units and quantities for [[radiation protection]] have been defined and developed from 1953 onwards. These are:
* The [[gray (unit)|gray]] (Gy), is the SI unit of [[absorbed dose]], which is the amount of radiation energy deposited in the irradiated material. For gamma radiation this is numerically equivalent to [[equivalent dose]] measured by the [[sievert]], which indicates the stochastic biological effect of low levels of radiation on human tissue. The radiation weighting conversion factor from absorbed dose to equivalent dose is 1 for gamma, whereas alpha particles have a factor of 20, reflecting their greater ionising effect on tissue.
* The [[rad (unit)|rad]] is the deprecated [[CGS]] unit for absorbed dose and the [[Röntgen equivalent man|rem]] is the deprecated [[CGS]] unit of equivalent dose, used mainly in the USA.

==Distinction from X-rays==
[[File:Moon egret.jpg|thumb|upright|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 usually be seen at all at these energies, which are too high to emerge from primary nuclear reactions, such as solar nuclear fusion (though occasionally the Sun produces gamma rays by [[cyclotron]]-type mechanisms, during [[solar flare]]s). Gamma rays typically have higher energy than X-rays.<ref>{{cite web|url=http://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/gammoon.html |title=CGRO SSC >> EGRET Detection of Gamma Rays from the Moon |publisher=Heasarc.gsfc.nasa.gov |date=2005-08-01 |access-date=2011-11-08}}</ref>]]

The conventional distinction between X-rays and gamma rays has changed over time. Originally, the electromagnetic radiation emitted by [[X-ray tube]]s almost invariably had a longer [[wavelength]] than the radiation (gamma rays) emitted by [[radioactive]] [[atomic nucleus|nuclei]].<ref name="Dendy">{{cite book
|last= Dendy
|first= P. P.
|author2=B. Heaton
|title= Physics for Diagnostic Radiology
|publisher= CRC Press
|year= 1999
|location= US
|page= 12
|url= https://books.google.com/books?id=1BTQvsQIs4wC&pg=PA12
|isbn= 0-7503-0591-6}}</ref> Older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10<sup>−11</sup>&nbsp;m, defined as gamma rays.<ref>{{cite book
|last= Charles Hodgman, Ed.
|title= CRC Handbook of Chemistry and Physics, 44th Ed.
|publisher= Chemical Rubber Co.
|year= 1961
|location= US
|page= 2850
}}</ref> Since the [[photon energy|energy of photons]] is proportional to their frequency and inversely proportional to wavelength, this past distinction between X-rays and gamma rays can also be thought of in terms of its energy, with gamma rays considered to be higher energy electromagnetic radiation than are X-rays.

However, since current artificial sources are now able to duplicate any electromagnetic radiation that originates in the nucleus, as well as far higher energies, the wavelengths characteristic of radioactive gamma ray sources vs. other types now completely overlap. Thus, gamma rays are now usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus.<ref name="Dendy"/><ref>{{cite book
|last= Feynman
|first= Richard
|author2=Robert Leighton |author3=Matthew Sands
|title= The Feynman Lectures on Physics, Vol.1
|url= https://archive.org/details/feynmanlectureso01feyn
|url-access= limited
|publisher= Addison-Wesley
|year= 1963
|location= US
|pages= [https://archive.org/details/feynmanlectureso01feyn/page/2 2]–5
|isbn= 0-201-02116-1}}</ref><ref>{{cite book
|last= L'Annunziata
|first= Michael
|author2=Mohammad Baradei
|title= Handbook of Radioactivity Analysis
|publisher= Academic Press
|year= 2003
|page= 58
|url= https://books.google.com/books?id=b519e10OPT0C&pg=PA58 |isbn= 0-12-436603-1}}</ref><ref>{{cite book
|last= Grupen
|first= Claus
|author2=G. Cowan |author3=S. D. Eidelman |author4=T. Stroh
|title= Astroparticle Physics
|url= https://archive.org/details/astroparticlephy00grup
|url-access= limited
|publisher= Springer
|year= 2005
|page= [https://archive.org/details/astroparticlephy00grup/page/n123 109]
|isbn= 3-540-25312-2}}</ref> Exceptions to this convention occur in astronomy, where gamma decay is seen in the afterglow of certain supernovas, but radiation from high energy processes known to involve other radiation sources than radioactive decay is still classed as gamma radiation.

For example, modern high-energy X-rays produced by [[linear accelerators]] for [[megavoltage]] treatment in cancer often have higher energy (4 to 25 MeV) than do most classical gamma rays produced by nuclear [[gamma decay]]. One of the most common gamma ray emitting isotopes used in diagnostic [[nuclear medicine]], [[technetium-99m]], produces gamma radiation of the same energy (140 keV) as that produced by diagnostic X-ray machines, but of significantly lower energy than therapeutic photons from linear particle accelerators. In the medical community today, the convention that radiation produced by nuclear decay is the only type referred to as "gamma" radiation is still respected.

Due to this broad overlap in energy ranges, in physics the two types of electromagnetic radiation are now often defined by their origin: X-rays are emitted by electrons (either in orbitals outside of the nucleus, or while being accelerated to produce [[bremsstrahlung]]-type radiation),<ref>"Bremsstrahlung radiation" is "braking radiation", but "acceleration" is being used here in the specific sense of the ''deflection'' of an electron from its course: {{cite book|last1=Serway|first1=Raymond A|title=College Physics|url=https://archive.org/details/collegephysics00serw_139|url-access=limited|year=2009|publisher=Brooks Cole|location=Belmont, CA|isbn=978-0-03-023798-0|page=[https://archive.org/details/collegephysics00serw_139/page/n907 876]|display-authors=etal}}</ref> while gamma rays are emitted by the nucleus or by means of other [[particle decay]]s or annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thus [[ultraviolet]] or lower energy photons produced by these processes would also be defined as "gamma rays" (indeed, this happens for the isomeric transition of the extremely low-energy isomer [[thorium-229m|<sup>229m</sup>Th]]).<ref name=Shaw>{{cite journal|journal=Physical Review Letters|volume=82|issue=6|pages=1109–1111|year=1999 |author1=Shaw, R. W. |author2=Young, J. P. |author3=Cooper, S. P. |author4=Webb, O. F. |title=Spontaneous Ultraviolet Emission from <sup>233</sup>Uranium/<sup>229</sup>Thorium Samples|doi= 10.1103/PhysRevLett.82.1109|bibcode=1999PhRvL..82.1109S|url=https://zenodo.org/record/1233913}}</ref> The only naming-convention that is still universally respected is the rule that electromagnetic radiation that is known to be of atomic nuclear origin is ''always'' referred to as "gamma rays", and never as X-rays. However, in physics and astronomy, the converse convention (that all gamma rays are considered to be of nuclear origin) is frequently violated.

In astronomy, higher energy gamma and X-rays are defined by energy, since the processes that produce them may be uncertain and photon energy, not origin, determines the required astronomical detectors needed.<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/how_l2/gamma_detectors.html |title=Gamma-Ray Telescopes & Detectors |publisher=NASA GSFC |access-date=2011-11-22}}</ref> High-energy photons occur in nature that are known to be produced by processes other than nuclear decay but are still referred to as gamma radiation. An example is "gamma rays" from lightning discharges at 10 to 20 MeV, and known to be produced by the bremsstrahlung mechanism.

Another example is gamma-ray bursts, now known to be produced from processes too powerful to involve simple collections of atoms undergoing radioactive decay. This is part and parcel of the general realization that many gamma rays produced in astronomical processes result not from radioactive decay or particle annihilation, but rather in non-radioactive processes similar to X-rays.{{clarify|reason=This is confusing. There is not just one manner of production. Electron jumps in atoms, bremstrahlung, or all other mechanisms?|date=July 2016}} Although the gamma rays of astronomy often come from non-radioactive events, a few gamma rays in astronomy are specifically known to originate from gamma decay of nuclei (as demonstrated by their spectra and emission half life). A classic example is that of supernova [[SN 1987A]], which emits an "afterglow" of gamma-ray photons from the decay of newly made radioactive [[nickel-56]] and [[cobalt-56]]. Most gamma rays in astronomy, however, arise by other mechanisms.
[[File:EM Spectrum Properties edit.svg|thumb|center|upright=1.7|In practice, gamma ray energies overlap with the range of X-rays, especially in the higher-frequency region referred to as "hard" X-rays. This depiction follows the older convention of distinguishing by wavelength.]]

==See also==
* [[Annihilation]]
* [[Galactic Center GeV excess]]
* [[Gaseous ionization detectors]]
* [[Very-high-energy gamma ray]]
* [[Ultra-high-energy gamma ray]]

== Explanatory notes ==
{{Reflist|group="note"}}

==References==
{{reflist}}

==External links==
{{Spoken Wikipedia|Gamma_ray.ogg|date=2019-8-16}}
* [http://www.rerf.or.jp/general/whatis_e/index.html Basic reference on several types of radiation] {{Webarchive|url=https://web.archive.org/web/20180425024814/http://www.rerf.or.jp/general/whatis_e/index.html |date=2018-04-25 }}
* [http://www.cancer.gov/cancertopics/factsheet/Therapy/radiation Radiation Q & A]
* [http://www.gcsechemistry.com/pwav46.htm GCSE information]
* [http://www.physics.isu.edu/radinf Radiation information] {{Webarchive|url=https://web.archive.org/web/20100611175635/http://www.physics.isu.edu/radinf/ |date=2010-06-11 }}
* [https://web.archive.org/web/20050414173513/http://www.astro.caltech.edu/~ejb/faq.html Gamma-ray bursts]
* [https://web.archive.org/web/20000815222730/http://nucleardata.nuclear.lu.se/nucleardata/toi/ The Lund/LBNL Nuclear Data Search] – Contains information on gamma-ray energies from isotopes.
* [http://grapevine.com.au/~pbeirwirth/gamma.html Mapping soils with airborne detectors] {{Webarchive|url=https://web.archive.org/web/20101111083656/http://grapevine.com.au/~pbeirwirth/gamma.html |date=2010-11-11 }}
* [http://www-nds.iaea.org/livechart The LIVEChart of Nuclides – IAEA] with filter on gamma-ray energy
* [http://www.radiationanswers.org/ Health Physics Society Public Education Website]

{{Electromagnetic spectrum}}
{{Nuclear processes}}
{{Radiation}}
{{Authority control}}

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