Gamma ray

This article is about the term's use in physics. For other uses, seeGamma ray (disambiguation).

Illustration of an emission of a gamma ray (γ) from an atomic nucleus

Gamma rays are emitted duringnuclear fissionin nuclear explosions.

NASA guide to electromagnetic spectrum showing overlap of frequency between X-rays and gamma rays

Agamma ray,also known asgamma radiation(symbol
γ
), is a penetrating form ofelectromagnetic radiationarising from theradioactive decayofatomic nuclei.It consists of the shortestwavelengthelectromagnetic waves, typically shorter than those ofX-rays.Withfrequenciesabove 30 exahertz (3×10¹⁹Hz) and wavelengths less than 10 picometers (1×10⁻¹¹m), gamma rayphotonshave the highestphoton energyof any form of electromagnetic radiation.Paul Villard,a Frenchchemistandphysicist,discovered gamma radiation in 1900 while studyingradiationemitted byradium.In 1903,Ernest Rutherfordnamed this radiationgamma raysbased on their relatively strong penetration ofmatter;in 1900, he had already named two less penetrating types of decay radiation (discovered byHenri Becquerel)Alpha raysandbeta raysin ascending order of penetrating power.

Gamma rays from radioactive decay are in the energy range from a few kiloelectronvolts(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 decayingradionuclidesusinggamma spectroscopy.Very-high-energy gamma raysin the 100–1000 teraelectronvolt (TeV) range have been observed from astronomical sources such as theCygnus X-3microquasar.

Natural sources of gamma rays originating on Earth are mostly a result of radioactive decay and secondary radiation from atmospheric interactions withcosmic rayparticles. However, there are other rare natural sources, such asterrestrial gamma-ray flashes,which produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays includefission,such as that which occurs innuclear reactors,andhigh energy physicsexperiments, such asneutral pion decayandnuclear fusion.

Gamma rays and X-rays are both electromagnetic radiation, and since they overlap in theelectromagnetic spectrum,the terminology 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. Inastrophysics,gamma rays are conventionally defined as having photon energies above 100 keV and are the subject ofgamma-ray astronomy,while radiation below 100 keV is classified as X-rays and is the subject ofX-ray astronomy.

Gamma rays areionizing radiationand are thus hazardous to life. They can cause DNAmutations,cancerandtumors,and at high doses burns andradiation 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 formidableradiation protectionchallenge, requiring shielding made from dense materials such as lead or concrete. OnEarth,themagnetosphereprotects life from most types of lethal cosmic radiation other than gamma rays.

History of discovery

The first gamma ray source to be discovered was theradioactive decayprocess calledgamma decay.In this type of decay, anexcitednucleus emits a gamma ray almost immediately upon formation.Paul Villard,a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted fromradium.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" byHenri Becquerelin 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.Later, in 1903, Villard's radiation was recognized as being of a type fundamentally different from previously named rays byErnest Rutherford,who named Villard's rays "gamma rays" by analogy with the beta and Alpha rays that Rutherford had differentiated in 1899.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 Alpha bet: 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, noteasilydeflected) 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.In 1914, gamma rays were observed to be reflected from crystal surfaces, proving that they were electromagnetic radiation.Rutherford and his co-workerEdward Andrademeasured the wavelengths of gamma rays from radium, and found they were similar toX-rays,but with shorter wavelengths and thus, higher frequency. This was eventually recognized as giving them more energy perphoton,as soon as the latter term became generally accepted. A gamma decay was then understood to usually emit a gamma photon.

Sources

This animation tracks several gamma rays through space and time, from their emission in the jet of a distantblazarto their arrival inFermi'sLarge Area Telescope (LAT).

Natural sources of gamma rays on Earth include gamma decay from naturally occurringradioisotopessuch aspotassium-40,and also as a secondary radiation from various atmospheric interactions withcosmic rayparticles. Natural terrestrial sources that produce gamma rays includelightning strikesandterrestrial gamma-ray flashes,which produce high energy emissions from natural high-energy voltages.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 ofbremsstrahlung,inverseCompton scatteringandsynchrotron radiation.A large fraction of such astronomical gamma rays are screened by Earth's atmosphere. Notable artificial sources of gamma rays includefission,such as occurs innuclear reactors,as well ashigh energy physicsexperiments, such asneutral pion decayandnuclear 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 aradioactive 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 preventradioactive contamination,and transported in heavy shielding.

Radioactive decay (gamma decay)

Main article:Nuclear isomer

Gamma rays are produced during gamma decay, which normally occurs after other forms of decay occur, such asAlphaorbetadecay. A radioactive nucleus can decay by the emission of an
α
or
β
particle. Thedaughter nucleusthat 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 10⁻¹²seconds. Gamma decay may also follownuclear reactionssuch asneutron 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 nuclearspin.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 gammafluorescence,form a topic innuclear physicscalledgamma 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 ametastableexcited state, if its decay takes (at least) 100 to 1000 times longer than the average 10⁻¹²seconds. Such relatively long-lived excited nuclei are termednuclear isomers,and their decays are termedisomeric transitions.Such nuclei havehalf-lifesthat 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 highnuclear spin,requiring a change in spin of several units or more with gamma decay, instead of a single unit transition that occurs in only 10⁻¹²seconds. The rate of gamma decay is also slowed when the energy of excitation of the nucleus is small.

An emitted gamma ray from any type of excited state may transfer its energy directly to anyelectrons,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 thephotoelectric effect(external gamma rays and ultraviolet rays may also cause this effect). The photoelectric effect should not be confused with theinternal conversionprocess, 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

Radioactive decay scheme of⁶⁰
Co

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,⁶⁰
Co
decays toexcited⁶⁰
Ni
bybeta decayemission of an electron of0.31MeV.Then the excited⁶⁰
Ni
decays to the ground state (seenuclear shell model) by emitting gamma rays in succession of 1.17 MeV followed by1.33 MeV.This path is followed 99.88% of the time:

60 27Co	→	60 28Ni*	+	e−	+	ν e	+	γ	+	1.17MeV
60 28Ni*	→	60 28Ni					+	γ	+	1.33 MeV

Another example is the Alpha decay of²⁴¹
Am
to form²³⁷
Np
;which is followed by gamma emission. In some cases, the gamma emission spectrum of the daughter nucleus is quite simple, (e.g.⁶⁰
Co
/⁶⁰
Ni
) while in other cases, such as with (²⁴¹
Am
/²³⁷
Np
and¹⁹²
Ir
/¹⁹²
Pt
), the gamma emission spectrum is complex, revealing that a series of nuclear energy levels exist.

Particle physics

Gamma rays are produced in many processes ofparticle physics.Typically, gamma rays are the products ofneutralsystems which decay throughelectromagnetic interactions(rather than aweakorstronginteraction). For example, in anelectron–positron annihilation,the usual products are two gamma ray photons. If the annihilating electron andpositronare at rest, each of the resulting gamma rays has an energy of ~ 511keVand frequency of ~1.24×10²⁰Hz.Similarly, a neutralpionmost often decays into two photons. Many otherhadronsand massivebosonsalso decay electromagnetically. High energy physics experiments, such as theLarge Hadron Collider,accordingly employ substantial radiation shielding.Becausesubatomic particlesmostly 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 thePlanck energywould be a gamma ray.

Other sources

Main article:Gamma-ray astronomy

A few gamma rays in astronomy are known to arise from gamma decay (see discussion ofSN1987A), 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 includeelectron-positron annihilation,neutral pion decay,bremsstrahlung,inverseCompton scattering,andsynchrotron radiation.

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 anomalousradiative trapping.

Terrestrial thunderstorms

Thunderstormscan produce a brief pulse of gamma radiation called aterrestrial gamma-ray flash.These gamma rays are thought to be produced by high intensity static electric fields accelerating electrons, which then produce gamma rays bybremsstrahlungas 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.

Solar flares

The most effusivesolar flaresemit across the entire EM spectrum, including γ-rays. The first confident observation occurred in1972.

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,bremsstrahlungare 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).

Image of entire sky in 100 MeV or greater gamma rays as seen by the EGRET instrument aboard theCGROspacecraft. Bright spots within the galactic plane arepulsarswhile those above and below the plane are thought to bequasars.

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 frompulsarswithin the Milky Way. Sources from the rest of the sky are mostlyquasars.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 rarergamma-ray burstsources 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 inmegavoltageradiation therapymachines (seebremsstrahlung).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 (magnetars), thought to produce astronomicalsoft gamma repeaters,are another relatively long-lived star-powered source of gamma radiation.

Quasars and active galaxies

More powerful gamma rays from very distantquasarsand closer active galaxies are thought to have a gamma ray production source similar to aparticle 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 asupermassive black holeat 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 10⁴⁰watts, a small fraction of which is gamma radiation. Much of the rest is emitted as electromagnetic waves of all frequencies, including radio waves.

Ahypernova.Artist's illustration showing the life of amassive starasnuclear fusionconverts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses to form ablack hole.Theoretically, energy may be released during the collapse along the axis of rotation to form a long durationgamma-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 burstsof 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 ablack hole.

The so-calledlong-durationgamma-ray bursts produce a total energy output of about 10⁴⁴joules (as much energy as theSunwill 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 inverseCompton scatteringandsynchrotron radiationfrom high-energy charged particles. These processes occur as relativistic charged particles leave the region of the event horizon of a newly formedblack holecreated 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 explodinghypernova.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 thevisible universe.

Properties

Penetration of matter

See also:Radiation protection § Electromagnetic radiation

Alpha radiationconsists ofheliumnuclei and is readily stopped by a sheet of paper.Beta radiation,consisting ofelectronsorpositrons,is stopped by an aluminium plate, but gamma radiation requires shielding by dense material such as lead or concrete.

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 toAlpha particles,which can be stopped by paper or skin, andbeta particles,which can be shielded by thin aluminium. Gamma rays are best absorbed by materials with highatomic numbers(Z) and high density, which contribute to the total stopping power. Because of this, a lead (highZ) shield is 20–30% better as a gamma shield than an equal mass of another low-Zshielding 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 (thehalf-value layeror HVL). For example, gamma rays that require 1 cm (0.4 inch) ofleadto reduce their intensity by 50% will also have their intensity reduced in half by4.1 cmofgraniterock, 6 cm (2.5 inches) ofconcrete,or 9 cm (3.5 inches) of packedsoil.However, the mass of this much concrete or soil is only 20–30% greater than that of lead with the same absorption capability.

Depleted uraniumis sometimes used for shielding inportable 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)and cheaper cost compared totungsten.

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

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.

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.

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 anexponential decreaseof intensity with distance from the incident surface:

$I(x)=I_{0}\cdot e^{-\mu x}$

where x is the thickness of the material from the incident surface, μ=nσ is the absorption coefficient, measured in cm⁻¹,nthe number of atoms per cm³of the material (atomic density) and σ the absorptioncross sectionin cm².

As it passes through matter, gamma radiation ionizes via three processes:

• Thephotoelectric 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 resultingphotoelectronis 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 theatomic numberof the absorbing material, which is why very dense materials like lead are only modestly better shields, on aper weightbasis, 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 theelectric fieldof 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'srange,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 muchionizationthemselves.

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

Light interaction

High-energy (from 80 GeV to ~10TeV) gamma rays arriving from far-distant quasars are used to estimate theextragalactic background lightin 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.

Gamma spectroscopy

Main article: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 opticalspectroscopy(seeFranck–Condoneffect) 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 ofMössbauer spectroscopy.In theMössbauer effectthe 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

Gamma-ray image of a truck with two stowaways taken with aVACIS(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 theFermi 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 ofsemi-precious stones,and is often used to change whitetopazintoblue 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.Gamma-ray sensors are also used for measuring the fluid levels in water and oil industries.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 theContainer 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 calledirradiation.Applications of this include the sterilization of medical equipment (as an alternative toautoclavesor chemical means), the removal of decay-causingbacteriafrom 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 ofcancer,since the rays also kill cancer cells. In the procedure calledgamma-knifesurgery, 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 innuclear medicinein imaging techniques. A number of different gamma-emitting radioisotopes are used. For example, in aPET scana radiolabeled sugar calledfluorodeoxyglucoseemitspositronsthat 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 thenuclear isomertechnetium-99mwhich emits gamma rays in the same energy range as diagnostic X-rays. When this radionuclide tracer is administered to a patient, agamma cameracan be used to form an image of the radioisotope's distribution by detecting the gamma radiation emitted (see alsoSPECT). 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 viabone scan).

Health effects

See also:Sievert

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.

Low levels of gamma rays cause astochastichealth risk, which for radiation dose assessment is defined as theprobabilityof cancer induction and genetic damage. TheInternational Commission on Radiological Protectionsays "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"High doses producedeterministiceffects, which is theseverityof acute tissue damage that is certain to happen. These effects are compared to the physical quantityabsorbed dosemeasured by the unitgray(Gy).

Body response

When gamma radiation breaks DNA molecules, a cell may be able torepair the damagedgenetic material, within limits. However, a study of Rothkamm and Lobrich has shown that this repair process works well after high-dose exposure but is much slower in the case of a low-dose exposure.

Risk assessment

The natural outdoor exposure in the United Kingdom ranges from 0.1 to 0.5 μSv/h with significant increase around known nuclear and contaminated sites.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.There is a small increase in the dose, due to naturally occurring gamma radiation, around small particles of high atomic number materials in the human body caused by the photoelectric effect.

By comparison, the radiation dose from chestradiography(about 0.06 mSv) is a fraction of the annual naturally occurring background radiation dose.A chest CT delivers 5 to 8 mSv. A whole-bodyPET/CT scan can deliver 14 to 32 mSv depending on the protocol.The dose fromfluoroscopyof 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 ofacute 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 5 Sv(5 Gy) is considered approximately theLD₅₀(lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment. 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 (seeradiation poisoning).(Doses much larger than this may, however, be delivered to selected parts of the body in the course ofradiation therapy.)

For low-dose exposure, for example among nuclear workers, who receive an average yearly radiation dose of 19 mSv,the risk of dying from cancer (excludingleukemia) 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 theatomic bombing of Hiroshima and Nagasaki.

Units of measurement and exposure

The following table shows radiation quantities in SI and non-SI units:

Ionizing radiation related quantities

Quantity	Unit	Symbol	Derivation	Year	SIequivalent
Activity(A)	becquerel	Bq	s−1	1974	SI unit
	curie	Ci	3.7×1010s−1	1953	3.7×1010Bq
	rutherford	Rd	106s−1	1946	1000000Bq
Exposure(X)	coulombperkilogram	C/kg	C⋅kg−1of air	1974	SI unit
	röntgen	R	esu/0.001293gof air	1928	2.58×10−4C/kg
Absorbed dose(D)	gray	Gy	J⋅kg−1	1974	SI unit
	ergper gram	erg/g	erg⋅g−1	1950	1.0×10−4Gy
	rad	rad	100 erg⋅g−1	1953	0.010 Gy
Equivalent dose(H)	sievert	Sv	J⋅kg−1×WR	1977	SI unit
	röntgen equivalent man	rem	100 erg⋅g−1×WR	1971	0.010 Sv
Effective dose(E)	sievert	Sv	J⋅kg−1×WR×WT	1977	SI unit
	röntgen equivalent man	rem	100 erg⋅g−1×WR×WT	1971	0.010 Sv

The measure of theionizingeffect of gamma and X-rays in dry air is called the exposure, for which a legacy unit, theröntgen,was used from 1928. This has been replaced bykerma,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 ofenergydeposited in tissue rather than the ionisation of air, and replacement radiometric units and quantities forradiation protectionhave been defined and developed from 1953 onwards. These are:

• Thegray(Gy), is the SI unit ofabsorbed dose,which is the amount of radiation energy deposited in the irradiated material. For gamma radiation this is numerically equivalent toequivalent dosemeasured by thesievert,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.
• Theradis the deprecatedCGSunit for absorbed dose and theremis the deprecatedCGSunit of equivalent dose, used mainly in the USA.

Distinction from X-rays

TheMoonas seen by theCompton Gamma Ray Observatory,in gamma rays of greater than 20 MeV. These are produced bycosmic raybombardment of its surface. The Sun, which has no similar surface of highatomic numberto 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 bycyclotron-type mechanisms, duringsolar flares). Gamma rays typically have higher energy than X-rays.

The conventional distinction between X-rays and gamma rays has changed over time. Originally, the electromagnetic radiation emitted byX-ray tubesalmost invariably had a longerwavelengththan the radiation (gamma rays) emitted byradioactivenuclei.Older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10⁻¹¹m, defined as gamma rays.Since theenergy of photonsis 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.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 bylinear acceleratorsformegavoltagetreatment in cancer often have higher energy (4 to 25 MeV) than do most classical gamma rays produced by nucleargamma decay.One of the most common gamma ray emitting isotopes used in diagnosticnuclear 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 producebremsstrahlung-type radiation),while gamma rays are emitted by the nucleus or by means of otherparticle decaysor annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thusultravioletor 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^229mTh).The only naming-convention that is still universally respected is the rule that electromagnetic radiation that is known to be of atomic nuclear origin isalwaysreferred 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.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.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 supernovaSN 1987A,which emits an "afterglow" of gamma-ray photons from the decay of newly made radioactivenickel-56andcobalt-56.Most gamma rays in astronomy, however, arise by other mechanisms.

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

External links

• Basic reference on several types of radiationArchived2018-04-25 at theWayback Machine
• Radiation Q & A
• GCSE information
• Radiation informationArchived2010-06-11 at theWayback Machine
• Gamma-ray bursts
• The Lund/LBNL Nuclear Data Search– Contains information on gamma-ray energies from isotopes.
• Mapping soils with airborne detectorsArchived2010-11-11 at theWayback Machine
• The LIVEChart of Nuclides – IAEAwith filter on gamma-ray energy
• Health Physics Society Public Education Website