View source for Radioactive decay

{{short description|Emissions from unstable atomic nuclei}}
{{Redirect2|Radioactive|Radioactivity|other uses|Radioactive (disambiguation)|and|Radioactivity (disambiguation)}}
{{for-multi|particle decay in a more general context|Particle decay|more information on hazards of various kinds of radiation from decay|Ionizing radiation}}
{{Use British English|date=May 2014}}
{{Use dmy dates|date=May 2014}}
[[File:Alpha Decay.svg|thumb|[[Alpha decay]] by a nucleus emits an [[alpha particle]] of two neutrons and two protons; also known as a [[helium|helium-4]] nucleus.|300x300px]]{{Nuclear physics}}'''Radioactive decay''' (also known as '''nuclear decay''', '''radioactivity''', '''radioactive disintegration''', or '''nuclear disintegration''') is the process by which an unstable [[atomic nucleus]] loses energy by [[radiation]]. A material containing unstable nuclei is considered ''radioactive''. Three of the most common types of decay are [[Alpha decay|alpha]], [[Beta decay|beta]], and [[Gamma ray|gamma decay]]. The [[weak force]] is the [[Fundamental interactions|mechanism]] that is responsible for beta decay, while the other two are governed by the [[electromagnetic force|electromagnetic]] and [[nuclear force]]s.<ref name="edp">{{cite web |title=Radioactivity: Weak Forces |url=https://www.radioactivity.eu.com/site/pages/Weak_Forces.htm |website=Radioactivity |publisher=EDP Sciences |access-date=4 March 2020 |archive-date=12 August 2021 |archive-url=https://web.archive.org/web/20210812235150/https://www.radioactivity.eu.com/site/pages/Weak_Forces.htm |url-status=dead }}</ref>

Radioactive decay is a [[randomness|random]] process at the level of single atoms. According to [[quantum mechanics|quantum theory]], it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed.<ref name="IntroductionToHealthPhysics">{{cite book |title=Radiation Protection and Dosimetry: An Introduction to Health Physics |last1=Stabin |first1=Michael G. |editor1-first=Michael G |editor1-last=Stabin |isbn=978-0-387-49982-6 |year=2007 |publisher=[[Springer Publishing|Springer]] |chapter=3 |doi=10.1007/978-0-387-49983-3|chapter-url=https://cds.cern.ch/record/1105894 }}</ref><ref name="RadiationOncologyPrimer">{{cite book |title=Radiation Oncology Primer and Review |isbn=978-1-62070-004-4 |last1=Best |first1=Lara |last2=Rodrigues |first2=George |last3=Velker |first3=Vikram |publisher=[[Demos Medical Publishing]] |year=2013 |chapter=1.3}}</ref><ref>{{cite book |title=Modern Nuclear Chemistry |isbn=978-0-471-11532-8 |last1=Loveland |first1=W. |last2=Morrissey |first2=D. |author3-link=Glenn T. Seaborg |last3=Seaborg |first3=G.T. |publisher=Wiley-Interscience |year=2006 |page=57|bibcode=2005mnc..book.....L }}</ref> However, for a significant number of identical atoms, the overall decay rate can be expressed as a [[decay constant]] or as a [[half-life]]. The half-lives of radioactive atoms have a huge range: from nearly instantaneous to far longer than the [[age of the universe]].

The decaying nucleus is called the parent [[radionuclide]] (or parent [[Isotope#Isotope vs. nuclide|radioisotope]]), and the process produces at least one [[Decay product|daughter nuclide]]. Except for gamma decay or [[internal conversion]] from a nuclear [[excited state]], the decay is a [[nuclear transmutation]] resulting in a daughter containing a different number of [[proton]]s or [[neutron]]s (or both). When the number of protons changes, an atom of a different [[chemical element]] is created.

There are 28 naturally occurring chemical elements on Earth that are radioactive, consisting of 35 [[radionuclide]]s (seven elements have two different radionuclides each) that date before the time of formation of the [[Solar System]]. These 35 are known as [[primordial radionuclide]]s. Well-known examples are [[uranium]] and [[thorium]], but also included are naturally occurring long-lived radioisotopes, such as [[potassium-40]]. Each of the heavy [[Primordial nuclide|primordial radionuclides]] participates in one of the four [[decay chain]]s.

==History of discovery==
[[File:Pierre and Marie Curie.jpg|thumb|upright=0.9|Pierre and Marie Curie in their Paris laboratory, before 1907|left]]

[[Henri Poincaré]] laid the seeds for the discovery of radioactivity through his interest in and studies of [[X-ray]]s, which significantly influenced physicist [[Henri Becquerel]].<ref>{{Cite journal |last1=Radvanyi |first1=Pierre |last2=Villain |first2=Jacques |date=2017-11-01 |title=The discovery of radioactivity |url=https://www.sciencedirect.com/science/article/pii/S1631070517300786 |journal=Comptes Rendus. Physique |language=en |volume=18 |issue=9–10 |pages=544–550 |doi=10.1016/j.crhy.2017.10.008 |bibcode=2017CRPhy..18..544R |issn=1878-1535|url-access=subscription }}</ref> Radioactivity was discovered in 1896 by Becquerel and independently by [[Marie Curie]], while working with [[Phosphorescence|phosphorescent]] materials.<ref>{{cite book |last=Mould |first=Richard F. |title=A century of X-rays and radioactivity in medicine : with emphasis on photographic records of the early years |date=1995 |publisher=Inst. of Physics Publ. |location=Bristol |isbn=978-0-7503-0224-1 |page=12 |edition=Reprint. with minor corr}}</ref><ref>{{cite journal|author=Henri Becquerel|title=Sur les radiations émises par phosphorescence|journal=Comptes Rendus|volume=122|pages=420–421|year=1896|url=http://gallica.bnf.fr/ark:/12148/bpt6k30780/f422.chemindefer}}</ref><ref>''Comptes Rendus'' '''122''': 420 (1896), [http://web.lemoyne.edu/~giunta/becquerel.html translated by Carmen Giunta]. Retrieved 12 April 2021.</ref><ref>{{cite journal|author=Henri Becquerel|title=Sur les radiations invisibles émises par les corps phosphorescents|journal=Comptes Rendus|volume=122|pages=501–503|year=1896|url=https://gallica.bnf.fr/ark:/12148/bpt6k30780/f503.item}}</ref><ref>''Comptes Rendus'' '''122''': 501–503 (1896), [http://web.lemoyne.edu/~giunta/becquerel.html translated by Carmen Giunta]. Retrieved 12 April 2021.</ref> These materials glow in the dark after exposure to light, and Becquerel suspected that the glow produced in [[cathode-ray tube]]s by X-rays might be associated with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent [[salt (chemistry)|salts]] on it. All results were negative until he used [[uranium]] salts. The uranium salts caused a blackening of the plate in spite of the plate being wrapped in black paper.<ref name=PaisInward>{{Cite book |last=Pais |first=Abraham |title=Inward bound: of matter and forces in the physical world |date=2002 |publisher=Clarendon Press [u.a.] |isbn=978-0-19-851997-3 |edition=Reprint |location=Oxford}}</ref>{{rp|48}} Curie named the radiation ''rayons de Becquerel'', "Becquerel Rays" and showed these rays where a property of atoms.<ref name=PaisInward/>{{rp|54}}

While X-rays were produced using electrical energy, the source of energy for radiation was a mystery.
In 1899, [[Julius Elster]] and [[Hans Geitel]] performed key experiments to find the energy source for radioactivity, excluding extraction of energy from air by measurements in a vacuum and extraction of energy from outer space by measurements 300m down a mine in the [[Harz mountains]].<ref name=PaisInward/>{{rp|100}} If the atoms themselves were the source of energy, this meant the seemingly immutable atoms must be altered when emitting the rays. In 1900 Curie summarized the puzzle of radioactivity as a choice between two equally unlikely possibilities: either energy was not conserved or chemical elements could be transmuted.<ref name=PaisInward/>{{rp|112}} 

Rutherford was the first to realize that all such elements decay in accordance with the same mathematical exponential formula. Rutherford and his student [[Frederick Soddy]] were the first to realize that many decay processes resulted in the [[nuclear transmutation|transmutation]] of one element to another. Subsequently, the [[radioactive displacement law of Fajans and Soddy]] was formulated to describe the products of alpha and [[beta decay]].<ref>Kasimir Fajans, "Radioactive transformations and the periodic system of the elements". [[Chemische Berichte|Berichte der Deutschen Chemischen Gesellschaft]], Nr. 46, 1913, pp. 422–439</ref><ref>Frederick Soddy, "The Radio Elements and the Periodic Law", Chem. News, Nr. 107, 1913, pp. 97–99</ref>

The early researchers also discovered that many other [[chemical element]]s, besides uranium, have radioactive isotopes. A systematic search for the total radioactivity in uranium ores also guided Pierre and Marie Curie to isolate two new elements: [[polonium]] and [[radium]]. Except for the radioactivity of radium, the chemical similarity of radium to [[barium]] made these two elements difficult to distinguish.

Marie and Pierre Curie's study of radioactivity is an important factor in science and medicine. After their research on Becquerel's rays led them to the discovery of both radium and polonium, they coined the term "radioactivity"<ref name="L'Annunziata-2007">{{Cite book|title=Radioactivity: Introduction and History|last=L'Annunziata|first=Michael F.|publisher=Elsevier Science|year=2007|isbn=9780080548883|location=Amsterdam, Netherlands|pages=2}}</ref> to define the emission of [[ionizing radiation]] by some heavy elements.<ref>{{cite book |last1=Petrucci |first1=Ralph H. |last2=Harwood |first2=William S. |last3=Herring |first3=F. Geoffrey |title=General chemistry |date=2002 |publisher=Prentice Hall |isbn=0-13-014329-4 |page=1025 |edition=8th}}</ref> (Later the term was generalized to all elements.) Their research on the penetrating rays in uranium and the discovery of radium launched an era of using radium for the treatment of cancer. Their exploration of radium could be seen as the first peaceful use of nuclear energy and the start of modern [[nuclear medicine]].<ref name="L'Annunziata-2007" />

==Early health dangers==
{{main|Ionizing radiation}}
[[File:Crookes tube xray experiment.jpg|thumb|upright=1.8|Taking an X-ray image with early [[Crookes tube]] apparatus in 1896. The Crookes tube is visible in the centre. The standing man is viewing his hand with a [[fluoroscope]] screen; this was a common way of setting up the tube. No precautions against radiation exposure are being taken; its hazards were not known at the time.]]
The dangers of [[ionizing radiation]] due to radioactivity and X-rays were not immediately recognized.

===X-rays===
The discovery of X‑rays by [[Wilhelm Röntgen]] in 1895 led to widespread experimentation by scientists, physicians, and inventors. Many people began recounting stories of burns, hair loss and worse in technical journals as early as 1896. In February of that year, Professor Daniel and Dr. Dudley of [[Vanderbilt University]] performed an experiment involving X-raying Dudley's head that resulted in his hair loss. A report by Dr. H.D. Hawks, of his suffering severe hand and chest burns in an X-ray demonstration, was the first of many other reports in ''Electrical Review''.<ref name="SansareKhanna2011">{{cite journal |last1=Sansare |first1=K. |last2=Khanna |first2=V. |last3=Karjodkar |first3=F. |title=Early victims of X-rays: a tribute and current perception |journal=Dentomaxillofacial Radiology |volume=40 |issue=2 |year=2011 |pages=123–125 |issn=0250-832X |doi=10.1259/dmfr/73488299 |pmc=3520298 |pmid=21239576}}</ref>

Other experimenters, including [[Elihu Thomson]] and [[Nikola Tesla]], also reported burns. Thomson deliberately exposed a finger to an X-ray tube over a period of time and suffered pain, swelling, and blistering.<ref name="physics.isu.edu">{{Cite web |url=http://www.physics.isu.edu/radinf/50yrs.htm |title=Ronald L. Kathern and Paul L. Ziemer, he First Fifty Years of Radiation Protection, physics.isu.edu |access-date=25 November 2013 |archive-date=12 September 2017 |archive-url=https://web.archive.org/web/20170912164652/http://www.physics.isu.edu/radinf/50yrs.htm |url-status=dead }}</ref> Other effects, including ultraviolet rays and ozone, were sometimes blamed for the damage,<ref>{{Cite journal |title=Nikola Tesla and the Discovery of X-rays |journal=RadioGraphics |date=July 2008 |volume=28 |issue=4 |pmid=18635636 |pages=1189–92 |doi=10.1148/rg.284075206 |last1=Hrabak |first1=M. |last2=Padovan |first2=R.S. |last3=Kralik |first3=M. |last4=Ozretic |first4=D. |last5=Potocki |first5=K.|doi-access=free }}</ref> and many physicians still claimed that there were no effects from X-ray exposure at all.<ref name="physics.isu.edu" />

Despite this, there were some early systematic hazard investigations, and as early as 1902 [[William Herbert Rollins]] wrote almost despairingly that his warnings about the dangers involved in the careless use of X-rays were not being heeded, either by industry or by his colleagues. By this time, Rollins had proved that X-rays could kill experimental animals, could cause a pregnant guinea pig to abort, and that they could kill a foetus. He also stressed that "animals vary in susceptibility to the external action of X-light" and warned that these differences be considered when patients were treated by means of X-rays.{{citation needed|date=January 2023}}

===Radioactive substances===
[[File:Periodic Table Stability & Radioactivity.svg|upright=1.8|right|thumb
|Radioactivity is characteristic of elements with large atomic numbers. Elements with at least one stable isotope are shown in light blue. Green shows elements of which the most stable isotope has a half-life measured in millions of years. Yellow and orange are progressively less stable, with half-lives in thousands or hundreds of years, down toward one day. Red and purple show highly and extremely radioactive elements where the most stable isotopes exhibit half-lives measured on the order of one day and much less.]]
However, the biological effects of radiation due to radioactive substances were less easy to gauge. This gave the opportunity for many physicians and corporations to market radioactive substances as [[patent medicine]]s. Examples were radium [[enema]] treatments, and radium-containing waters to be drunk as tonics. Marie Curie protested against this sort of treatment, warning that "radium is dangerous in untrained hands".<ref>{{cite journal |last1=Rentetzi |first1=Maria |title=Marie Curie and the perils in radium |journal=Physics Today |date=7 November 2017 |issue=11 |page=30676 |doi=10.1063/PT.6.4.20171107a |bibcode=2017PhT..2017k0676R |url=https://physicstoday.scitation.org/do/10.1063/PT.6.4.20171107a/full/ |access-date=3 May 2022|url-access=subscription }}</ref> Curie later died from [[aplastic anaemia]], likely caused by exposure to ionizing radiation. By the 1930s, after a number of cases of bone necrosis and death of radium treatment enthusiasts, radium-containing medicinal products had been largely removed from the market ([[radioactive quackery]]).

===Radiation protection===
{{main|Radiation protection}}
{{see also|Sievert|Ionizing radiation}}
Only a year after [[Wilhelm Röntgen|Röntgen]]'s discovery of X-rays, the American engineer [[Wolfram Conrad Fuchs|Wolfram Fuchs]] (1896) gave what is probably the first protection advice, but it was not until 1925 that the first [[International Congress of Radiology]] (ICR) was held and considered establishing international protection standards. The effects of radiation on genes, including the effect of cancer risk, were recognized much later. In 1927, [[Hermann Joseph Muller]] published research showing genetic effects and, in 1946, was awarded the [[Nobel Prize in Physiology or Medicine]] for his findings.

The second ICR was held in Stockholm in 1928 and proposed the adoption of the [[Roentgen (unit)|röntgen]] unit, and the [[International X-ray and Radium Protection Committee]] (IXRPC) was formed. [[Rolf Maximilian Sievert|Rolf Sievert]] was named chairman, but a driving force was [[G. W. C. Kaye|George Kaye]] of the British [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]]. The committee met in 1931, 1934, and 1937.

After [[World War II]], the increased range and quantity of radioactive substances being handled as a result of military and civil nuclear programs led to large groups of occupational workers and the public being potentially exposed to harmful levels of ionising radiation. This was considered at the first post-war ICR convened in London in 1950, when the present [[International Commission on Radiological Protection]] (ICRP) was born.<ref>{{cite journal|last=Clarke|first=R.H.|author2=J. Valentin|title=The History of ICRP and the Evolution of its Policies|journal=Annals of the ICRP|year=2009|volume=39|series=ICRP Publication 109|issue=1|pages=75–110|doi=10.1016/j.icrp.2009.07.009|s2cid=71278114|url=http://www.icrp.org/docs/The%20History%20of%20ICRP%20and%20the%20Evolution%20of%20its%20Policies.pdf|access-date=12 May 2012}}</ref>
Since then the ICRP has developed the present international system of radiation protection, covering all aspects of radiation hazards.

In 2020, Hauptmann and another 15 international researchers from eight nations (among them: Institutes of Biostatistics, Registry Research, Centers of Cancer Epidemiology, Radiation Epidemiology, and also the [[National Cancer Institute|U.S. National Cancer Institute]] (NCI), [[International Agency for Research on Cancer]] (IARC) and the [[Radiation Effects Research Foundation|Radiation Effects Research Foundation of Hiroshima]]) studied definitively through [[meta-analysis]] the damage resulting from the "low doses" that have afflicted survivors of the [[atomic bombings of Hiroshima and Nagasaki]] and also in numerous [[Nuclear and radiation accidents and incidents|accidents at nuclear plants]] that have occurred. These scientists reported, in ''JNCI Monographs: Epidemiological Studies of Low Dose Ionizing Radiation and Cancer Risk'', that the new epidemiological studies directly support excess cancer risks from low-dose ionizing radiation.<ref>{{cite journal |last1=Daniels |first1=M. |display-authors=et al. |title=Epidemiological Studies of Low-Dose Ionizing Radiation and Cancer: Summary Bias Assessment and Meta-Analysis.|journal=J Natl Cancer Inst Monogr|volume=56 |issue=July 1 |year=2020 |pages=188–200|issn=1434-6001 |doi=10.1093/jncimonographs/lgaa010|pmid=32657347 |pmc=8454205 }}</ref> In 2021, Italian researcher Sebastiano Venturi reported the first correlations between radio-caesium and [[pancreatic cancer]] with the role of [[caesium]] in biology, in pancreatitis and in diabetes of pancreatic origin.<ref>{{cite journal |last1=Venturi |first1=Sebastiano |title=Cesium in Biology, Pancreatic Cancer, and Controversy in High and Low Radiation Exposure Damage – Scientific, Environmental, Geopolitical, and Economic Aspects |journal=International Journal of Environmental Research and Public Health |date=January 2021 |volume=18 |issue=17 |pages=8934 | pmid=34501532 |doi=10.3390/ijerph18178934 |pmc=8431133 |language=en|doi-access=free }}  [[File:CC-BY icon.svg|50px]]  Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/  Creative Commons Attribution 4.0 International License].</ref>

==Units==
[[File:Radioactivity and radiation.png|thumb|upright=1.75|Graphic showing relationships between radioactivity and detected ionizing radiation]]
The [[International System of Units]] (SI) unit of radioactive activity is the [[becquerel]] (Bq), named in honor of the scientist [[Henri Becquerel]]. One Bq is defined as one transformation (or decay or disintegration) per second.

An older unit of radioactivity is the [[Curie (unit)|curie]], Ci, which was originally defined as "the quantity or mass of [[radium emanation]] in [[secular equilibrium|equilibrium]] with one gram of [[radium]] (element)".<ref>{{cite journal |last1=Rutherford |first1=Ernest |title=Radium Standards and Nomenclature |journal=Nature |date=6 October 1910 |volume=84 |issue=2136 |pages=430–431 |url=https://archive.org/details/nature841910lock |doi=10.1038/084430a0|bibcode = 1910Natur..84..430R |doi-access=free }}</ref> Today, the curie is defined as {{val|3.7|e=10}} disintegrations per second, so that 1&nbsp;[[Curie (unit)|curie]] (Ci) = {{val|3.7|e=10|u=Bq}}.
For radiological protection purposes, although the United States Nuclear Regulatory Commission permits the use of the unit curie alongside SI units,<ref>{{cite book
 |title=10 CFR 20.1005
 |year=2009
 |publisher=US Nuclear Regulatory Commission
 |url=https://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1005.html}}
</ref> the [[European Union]] [[European units of measurement directives]] required that its use for "public health ... purposes" be phased out by 31 December 1985.<ref>{{cite web
|url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31980L0181:EN:NOT
| author = The Council of the European Communities
| title = Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC
| date=1979-12-21
| access-date=19 May 2012}}</ref>

The effects of ionizing radiation are often measured in units of [[Gray (unit)|gray]] for mechanical or [[sievert]] for damage to tissue.

==Types==
{{More citations needed|section|date=May 2023}}
Radioactive decay results in a reduction of summed rest [[mass]], once the released energy (the ''disintegration energy'') has escaped in some way. Although [[decay energy]] is sometimes defined as associated with the difference between the mass of the parent nuclide products and the mass of the decay products, this is true only of rest mass measurements, where some energy has been removed from the product system. This is true because the decay energy must always carry mass with it, wherever it appears (see [[mass in special relativity]]) according to the formula [[E=mc2|''E''&nbsp;=&nbsp;''mc''<sup>2</sup>]]. The decay energy is initially released as the energy of emitted photons plus the kinetic energy of massive emitted particles (that is, particles that have rest mass). If these particles come to [[thermal equilibrium]] with their surroundings and photons are absorbed, then the decay energy is transformed to thermal energy, which retains its mass.

Decay energy, therefore, remains associated with a certain measure of the mass of the decay system, called [[invariant mass]], which does not change during the decay, even though the energy of decay is distributed among decay particles. The energy of photons, the kinetic energy of emitted particles, and, later, the thermal energy of the surrounding matter, all contribute to the [[invariant mass]] of the system. Thus, while the sum of the rest masses of the particles is not conserved in radioactive decay, the ''system'' mass and system invariant mass (and also the system total energy) is conserved throughout any decay process. This is a restatement of the equivalent laws of [[conservation of energy]] and [[conservation of mass]].

=== Alpha, beta and gamma decay ===
{{Main|Alpha decay|Beta decay|Gamma decay}}
[[File:Alfa beta gamma radiation.svg|upright=0.8|thumb|[[Alpha particle]]s may be completely stopped by a sheet of paper, [[beta particle]]s by aluminium shielding. [[Gamma ray]]s can only be reduced by much more substantial mass, such as a very thick layer of [[lead]].]]Early researchers found that an [[Electric field|electric]] or [[magnetic field]] could split radioactive emissions into three types of beams. The rays were given the names [[Alpha particle|alpha]], [[Beta particle|beta]], and gamma, in increasing order of their ability to penetrate matter. Alpha decay is observed only in heavier elements of atomic number 52 ([[tellurium]]) and greater, with the exception of [[beryllium-8]] (which decays to two alpha particles). The other two types of decay are observed in all the elements. Lead, [[atomic number]] 82, is the heaviest element to have any isotopes stable (to the limit of measurement) to radioactive decay. Radioactive decay is seen in all isotopes of all elements of atomic number 83 ([[bismuth]]) or greater. [[Bismuth-209]], however, is only very slightly radioactive, with a half-life greater than the age of the universe; radioisotopes with extremely long half-lives are considered effectively stable for practical purposes.
[[File:Radioactive decay modes.svg|upright=0.9|thumb|Transition diagram for decay modes of a radionuclide, with neutron number ''N'' and [[atomic number]] ''Z'' (shown are [[Alpha particle|α]], [[electron|β<sup>±</sup>]], [[proton|p<sup>+</sup>]], and [[neutron|n<sup>0</sup>]] emissions, EC denotes [[electron capture]]).]]In analyzing the nature of the decay products, it was obvious from the direction of the [[electromagnetic force]]s applied to the radiations by external magnetic and electric fields that alpha particles carried a positive charge, beta particles carried a negative charge, and gamma rays were neutral. From the magnitude of deflection, it was clear that [[alpha particles]] were much more massive than [[beta particles]]. Passing alpha particles through a very thin glass window and trapping them in a [[neon lamp|discharge tube]] allowed researchers to study the [[emission spectrum]] of the captured particles, and ultimately proved that alpha particles are [[helium]] nuclei. Other experiments showed beta radiation, resulting from decay and [[cathode ray]]s, were high-speed [[electrons]]. Likewise, gamma radiation and X-rays were found to be high-energy [[electromagnetic radiation]].

The relationship between the types of decays also began to be examined: For example, gamma decay was almost always found to be associated with other types of decay, and occurred at about the same time, or afterwards. Gamma decay as a separate phenomenon, with its own half-life (now termed [[isomeric transition]]), was found in natural radioactivity to be a result of the gamma decay of excited metastable [[nuclear isomer]]s, which were in turn created from other types of decay. Although alpha, beta, and gamma radiations were most commonly found, other types of emission were eventually discovered. Shortly after the discovery of the [[positron]] in cosmic ray products, it was realized that the same process that operates in classical beta decay can also produce positrons ([[positron emission]]), along with [[neutrino]]s (classical beta decay produces antineutrinos).

=== Electron capture ===
{{Main|Electron capture}}
In electron capture, some proton-rich nuclides were found to capture their own atomic electrons instead of emitting positrons, and subsequently, these nuclides emit only a neutrino and a gamma ray from the excited nucleus (and often also [[Auger electron]]s and [[characteristic X-ray]]s, as a result of the re-ordering of electrons to fill the place of the missing captured electron). These types of decay involve the nuclear capture of electrons or emission of electrons or positrons, and thus acts to move a nucleus toward the ratio of neutrons to protons that has the least energy for a given total number of [[nucleon]]s. This consequently produces a more stable (lower energy) nucleus.

A hypothetical process of positron capture, analogous to electron capture, is theoretically possible in antimatter atoms, but has not been observed, as complex antimatter atoms beyond [[antihelium]] are not experimentally available.<ref>{{Cite web |title=Radioactive Decay |url=http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch23/modes.php#fission |access-date=2022-05-05 |website=chemed.chem.purdue.edu}}</ref> Such a decay would require antimatter atoms at least as complex as [[beryllium-7]], which is the lightest known isotope of normal matter to undergo decay by electron capture.<ref>{{Cite web |title=CH103 – Chapter 3: Radioactivity and Nuclear Chemistry – Chemistry |url=https://wou.edu/chemistry/courses/online-chemistry-textbooks/ch103-allied-health-chemistry/ch103-chapter-3-radioactivity/ |access-date=2022-07-05 |language=en-US}}</ref>

=== Nucleon emission ===
{{Main|Neutron emission|Cluster emission|Nuclear fission}}
Shortly after the discovery of the neutron in 1932, [[Enrico Fermi]] realized that certain rare beta-decay reactions immediately yield neutrons as an additional decay particle, so called beta-delayed [[neutron emission]]. Neutron emission usually happens from nuclei that are in an excited state, such as the excited <sup>17</sup>O* produced from the beta decay of <sup>17</sup>N. The neutron emission process itself is controlled by the [[nuclear force]] and therefore is extremely fast, sometimes referred to as "nearly instantaneous". Isolated [[proton emission]] was eventually observed in some elements. It was also found that some heavy elements may undergo [[spontaneous fission]] into products that vary in composition. In a phenomenon called [[cluster decay]], specific combinations of neutrons and protons other than alpha particles (helium nuclei) were found to be spontaneously emitted from atoms.

=== More exotic types of decay ===
Other types of radioactive decay were found to emit previously seen particles but via different mechanisms. An example is [[internal conversion]], which results in an initial electron emission, and then often further [[characteristic X-ray]]s and [[Auger electron]]s emissions, although the internal conversion process involves neither beta nor gamma decay. A neutrino is not emitted, and none of the electron(s) and photon(s) emitted originate in the nucleus, even though the energy to emit all of them does originate there. Internal conversion decay, like [[isomeric transition]] gamma decay and neutron emission, involves the release of energy by an excited nuclide, without the transmutation of one element into another.

Rare events that involve a combination of two beta-decay-type events happening simultaneously are known (see below). Any decay process that does not violate the conservation of energy or momentum laws (and perhaps other particle conservation laws) is permitted to happen, although not all have been detected. An interesting example discussed in a final section, is [[Beta decay#Bound-state β− decay|bound state beta decay]] of [[rhenium-187]]. In this process, the beta electron-decay of the parent nuclide is not accompanied by beta electron emission, because the beta particle has been captured into the K-shell of the emitting atom. An antineutrino is emitted, as in all negative beta decays.

If energy circumstances are favorable, a given radionuclide may undergo many competing types of decay, with some atoms decaying by one route, and others decaying by another. An example is [[copper-64]], which has 29 protons, and 35 neutrons, which decays with a half-life of {{val|12.7004|(13)}} hours.<ref name=NUBASE2020 /> This isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay to the other particle, which has opposite [[isospin]]. This particular nuclide (though not all nuclides in this situation) is more likely to decay through [[beta plus decay]] ({{val|61.52|(26)}}%<ref name=NUBASE2020/>) than through [[electron capture]] ({{val|38.48|(26)}}%<ref name=NUBASE2020/>). The excited energy states resulting from these decays which fail to end in a ground energy state, also produce later internal conversion and [[gamma decay]] in almost 0.5% of the time.

===List of decay modes {{anchor|1=Modes}}<!-- 2022-03-03 -->===
{{anchor|Decay modes in table form|Decay modes}}
{| class="wikitable collapsible uncollapsed sortable" style="font-size: 95%;"
|+Decay modes in NUBASE2020<ref name=NUBASE2020>{{NUBASE2020}}</ref>
|- style="background:#eee0e0; white-space:nowrap;"
! Mode
! Name
! style="max-width:30%;" | Action
! Nucleus changes<!-- A, Z daughter nucleus effect -->
|-
| data-sort-value="10" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=α|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=α|type=dm}}|type=dm|property=dm-symbol-alt}}<!-- for 2b-, 2b+ -->
   }}
| [[Alpha decay|alpha emission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=α|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=α|type=dm}}|type=dm|property=AZ-changes}}
|-
| data-sort-value="20" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=p|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=p|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[proton emission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=p|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=p|type=dm}}|type=dm|property=AZ-changes}}
|-
|-
| data-sort-value="30" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2p|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2p|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[Proton emission|2-proton emission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2p|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2p|type=dm}}|type=dm|property=AZ-changes}}
|-
|-
| data-sort-value="40" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=n|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=n|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[neutron emission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=n|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=n|type=dm}}|type=dm|property=AZ-changes}}
|-
|-
| data-sort-value="50" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2n|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2n|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[Neutron emission|2-neutron emission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2n|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2n|type=dm}}|type=dm|property=AZ-changes}}
|-
|-
| data-sort-value="60" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=epsi|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=epsi|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[electron capture]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=epsi|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=epsi|type=dm}}|type=dm|property=AZ-changes}}
|-
|-
| data-sort-value="70" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=e+|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=e+|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[positron emission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=e+|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=e+|type=dm}}|type=dm|property=AZ-changes}}
|-
|-
| data-sort-value="80" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[positron emission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="90" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[Beta decay|β<sup>−</sup> decay]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="100" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2β<sup>−</sup>|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2β<sup>−</sup>|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[Double beta decay|double β<sup>−</sup> decay]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2β<sup>−</sup>|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2β<sup>−</sup>|type=dm}}|type=dm|property=AZ-changes}}
|-
|-
| data-sort-value="110" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2β<sup>+</sup>|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2β<sup>+</sup>|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[Double beta decay|double β<sup>+</sup> decay]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2β<sup>+</sup>|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=2β<sup>+</sup>|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="120" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>n|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>n|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[Delayed neutron|β<sup>−</sup>-delayed neutron emission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>n|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>n|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="130" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>2n|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>2n|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>−</sup>-delayed 2-neutron emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>2n|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>2n|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="140" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>3n|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>3n|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>−</sup>-delayed 3-neutron emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>3n|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>3n|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="150" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>p|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>p|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>+</sup>-delayed proton emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>p|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>p|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="160" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>2p|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>2p|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>+</sup>-delayed 2-proton emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>2p|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>2p|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="170" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>3p|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>3p|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>+</sup>-delayed 3-proton emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>3p|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>3p|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="180" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>α|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>α|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>−</sup>-delayed alpha emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>α|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>α|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="190" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>α|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>α|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>+</sup>-delayed alpha emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>α|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>α|type=dm}}|type=dm|property=AZ-changes}}
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| data-sort-value="200" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>d|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>d|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>−</sup>-delayed deuteron emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>d|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>d|type=dm}}|type=dm|property=AZ-changes}}
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| data-sort-value="210" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>t|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>t|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>−</sup>-delayed triton emission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>t|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>t|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="262" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=CD|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=CD|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[cluster decay]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=CD|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=CD|type=dm}}|type=dm|property=AZ-changes}}
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| data-sort-value="305" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=IT|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=IT|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[Nuclear isomer#Decay processes|internal (isomeric) transition]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=IT|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=IT|type=dm}}|type=dm|property=AZ-changes}}
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| data-sort-value="310" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=SF|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=SF|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| [[spontaneous fission]]
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=SF|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=SF|type=dm}}|type=dm|property=AZ-changes}}
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| data-sort-value="320" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>SF|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>SF|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>+</sup>-delayed fission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>SF|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>+</sup>SF|type=dm}}|type=dm|property=AZ-changes}}
|-
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| data-sort-value="330" | {{ubl|style=text-align: center;font-size:120%
   |1={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>SF|type=dm}}|type=dm|property=dm-symbol}}
   |2={{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>SF|type=dm}}|type=dm|property=dm-symbol-alt}}
   }}
| β<sup>−</sup>-delayed fission
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>SF|type=dm}}|type=dm|property=action-text}}
| {{Isotopes/decay-mode/property|normal-code={{Isotopes/decay-mode/property|property=normal-code|raw-code=β<sup>−</sup>SF|type=dm}}|type=dm|property=AZ-changes}}
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=== Decay chains and multiple modes ===
{{See also|Valley of stability}}
[[File:Decay_Chain(4n+1,_Neptunium_Series).svg|right|thumb|600x600px|The decay chain of [[neptunium]]-237]]
The daughter nuclide of a decay event may also be unstable (radioactive). In this case, it too will decay, producing radiation. The resulting second daughter nuclide may also be radioactive. This can lead to a sequence of several decay events called a ''[[decay chain]]'' (see this article for specific details of important natural decay chains). Eventually, a stable nuclide is produced. Any decay daughters that are the result of an alpha decay will also result in helium atoms being created.

Some radionuclides may have several different paths of decay. For example, {{val|35.94|(6)}}%<ref name="NUBASE2020" /> of [[bismuth-212]] decays, through alpha-emission, to [[thallium-208]] while {{val|64.06|(6)}}%<ref name="NUBASE2020" /> of [[bismuth-212]] decays, through beta-emission, to [[polonium-212]]. Both [[thallium-208]] and [[polonium-212]] are radioactive daughter products of bismuth-212, and both decay directly to stable [[lead-208]].

==Occurrence and applications==
{{See also|Nuclear drip line}}
According to the [[Big Bang theory]], stable isotopes of the lightest three elements ([[hydrogen|H]], He, and traces of [[lithium|Li]]) were produced very shortly after the emergence of the universe, in a process called [[Big Bang nucleosynthesis]]. These lightest stable nuclides (including [[deuterium]]) survive to today, but any radioactive isotopes of the light elements produced in the Big Bang (such as [[tritium]]) have long since decayed. Isotopes of elements heavier than boron were not produced at all in the Big Bang, and these first five elements do not have any long-lived radioisotopes. Thus, all radioactive nuclei are, therefore, relatively young with respect to the birth of the universe, having formed later in various other types of [[nucleosynthesis]] in [[star]]s (in particular, [[supernova]]e), and also during ongoing interactions between stable isotopes and energetic particles. For example, [[carbon-14]], a radioactive nuclide with a half-life of only {{val|5700|(30)}} years,<ref name="NUBASE2020" /> is constantly produced in Earth's upper atmosphere due to interactions between cosmic rays and nitrogen.

Nuclides that are produced by radioactive decay are called [[radiogenic nuclide]]s, whether they themselves are [[Stable isotope|stable]] or not. There exist stable radiogenic nuclides that were formed from short-lived [[extinct radionuclide]]s in the early Solar System.<ref>{{cite book |last=Clayton |first=Donald D. |url=https://archive.org/details/principlesofstel0000clay |title=Principles of Stellar Evolution and Nucleosynthesis |publisher=University of Chicago Press |year=1983 |isbn=978-0-226-10953-4 |edition=2nd |page=[https://archive.org/details/principlesofstel0000clay/page/75 75] |url-access=registration}}</ref><ref>{{cite web |author1=Bolt, B.A. |author2=Packard, R.E. |author3=Price, P.B. |year=2007 |title=John H. Reynolds, Physics: Berkeley |url=http://content.cdlib.org/xtf/view?docId=hb1r29n709&doc.view=content&chunk.id=div00061&toc.depth=1&brand=oac&anchor.id=0 |access-date=2007-10-01 |publisher=The University of California, Berkeley}}</ref> The extra presence of these stable radiogenic nuclides (such as xenon-129 from extinct [[iodine-129]]) against the background of primordial [[stable nuclide]]s can be inferred by various means.

Radioactive decay has been put to use in the technique of [[radioisotopic labeling]], which is used to track the passage of a chemical substance through a complex system (such as a living [[organism]]). A sample of the substance is synthesized with a high concentration of unstable atoms. The presence of the substance in one or another part of the system is determined by detecting the locations of decay events.

On the premise that radioactive decay is truly [[random]] (rather than merely [[chaos theory|chaotic]]), it has been used in [[hardware random-number generator]]s. Because the process is not thought to vary significantly in mechanism over time, it is also a valuable tool in estimating the absolute ages of certain materials. For geological materials, the radioisotopes and some of their decay products become trapped when a rock solidifies, and can then later be used (subject to many well-known qualifications) to estimate the date of the solidification. These include checking the results of several simultaneous processes and their products against each other, within the same sample. In a similar fashion, and also subject to qualification, the rate of formation of carbon-14 in various eras, the date of formation of organic matter within a certain period related to the isotope's half-life may be estimated, because the carbon-14 becomes trapped when the organic matter grows and incorporates the new carbon-14 from the air. Thereafter, the amount of carbon-14 in organic matter decreases according to decay processes that may also be independently cross-checked by other means (such as checking the carbon-14 in individual tree rings, for example).

===Szilard–Chalmers effect===
The Szilard–Chalmers effect is the breaking of a chemical bond as a result of a kinetic energy imparted from radioactive decay.  It operates by the absorption of neutrons by an atom and subsequent emission of gamma rays, often with significant amounts of kinetic energy. This kinetic energy, by [[Newton's laws of motion|Newton's third law]], pushes back on the decaying atom, which causes it to move with enough speed to break a chemical bond.<ref>{{Cite web |title=Szilard-Chalmers effect - Oxford Reference |url=https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100548450 |access-date=2019-12-27 |website=oxfordreference.com |language=en}}</ref> This effect can be used to separate isotopes by chemical means.

The Szilard–Chalmers effect was discovered in 1934 by [[Leó Szilárd]] and Thomas A. Chalmers.<ref>{{cite journal |last1=Szilard |first1=Leó |last2=Chalmers |first2=Thomas A. |year=1934 |title=Chemical separation of the radioactive element from its bombarded isotope in the Fermi effect |journal=Nature |volume=134 |issue=3386 |pages=462 |bibcode=1934Natur.134..462S |doi=10.1038/134462b0 |s2cid=4129460|doi-access=free }}</ref> They observed that after bombardment by neutrons,  the breaking of a bond in liquid ethyl iodide allowed radioactive iodine to be removed.<ref>{{Citation |last1=Harbottle |first1=Garman |title=The Szilard-Chalmers Reaction in Solids |date=1959-01-01 |url=http://www.sciencedirect.com/science/article/pii/S0065279208602563 |volume=1 |pages=267–314 |editor-last=Emeléus |editor-first=H. J. |access-date=2020-03-19 |series=Advances in Inorganic Chemistry and Radiochemistry |publisher=Academic Press |language=en |doi=10.1016/S0065-2792(08)60256-3 |isbn=9780120236015 |last2=Sutin |first2=Norman |editor2-last=Sharpe |editor2-first=A. G.|url-access=subscription }}</ref>

=== Origins of radioactive nuclides ===
{{Main|Nucleosynthesis}}
Radioactive [[primordial nuclide]]s found in the [[Earth]] are residues from ancient [[supernova nucleosynthesis|supernova]] explosions that occurred before the formation of the [[Solar System]]. They are the fraction of radionuclides that survived from that time, through the formation of the primordial solar [[nebula]], through planet [[accretion (astrophysics)|accretion]], and up to the present time. The naturally occurring short-lived [[radiogenic]] radionuclides found in today's [[rock (geology)|rock]]s, are the daughters of those radioactive primordial nuclides. Another minor source of naturally occurring radioactive nuclides are [[cosmogenic nuclide]]s, that are formed by cosmic ray bombardment of material in the Earth's [[atmosphere]] or [[crust (geology)|crust]]. The decay of the radionuclides in rocks of the Earth's [[mantle (geology)|mantle]] and [[crust (geology)|crust]] contribute significantly to [[Earth's internal heat budget]].

==Aggregate processes==
While the underlying process of radioactive decay is subatomic, historically and in most practical cases it is encountered in bulk materials with very large numbers of atoms. This section discusses models that connect events at the atomic level to observations in aggregate.

===Terminology{{anchor|Rates}}===
<!-- "Neutron activation analysis" links here -->
The ''decay rate'', or ''activity'', of a radioactive substance is characterized by the following time-independent parameters:
* The ''[[half-life]]'', {{math|''t''<sub>1/2</sub>}}, is the time taken for the activity of a given amount of a [[list of nuclides|radioactive substance]] to decay to half of its initial value.
* The ''[[decay constant]]'', {{math|''λ''}} "[[lambda]]", the reciprocal of the mean lifetime (in {{math|s<sup>−1</sup>}}), sometimes referred to as simply ''decay rate''.
* The ''[[mean lifetime]]'', {{math|''τ''}} "[[tau]]", the average lifetime (1/[[E (mathematical constant)|e]] life) of a radioactive particle before decay.

Although these are constants, they are associated with the [[statistical mechanics|statistical behavior of populations]] of atoms. In consequence, predictions using these constants are less accurate for minuscule samples of atoms.

In principle a half-life, a third-life, or even a (1/√2)-life, could be used in exactly the same way as half-life; but the mean life and half-life {{math|''t''<sub>1/2</sub>}} have been adopted as standard times associated with exponential decay.

Those parameters can be related to the following time-dependent parameters:
* ''[[Total activity]]'' (or just ''activity''), {{math|''A''}}, is the number of decays per unit time of a radioactive sample.
* ''[[Number of particles]]'', {{math|''N''}}, in the sample.
* ''[[Specific activity]]'', {{math|''a''}}, is the number of decays per unit time per amount of substance of the sample at time set to zero ({{math|''t'' {{=}} 0}}). "Amount of substance" can be the mass, volume or moles of the initial sample.

These are related as follows:
:<math>\begin{align}
   t_{1/2} &= \frac{\ln(2)}{\lambda} = \tau \ln(2) \\[2pt]
         A &= - \frac{\mathrm{d}N}{\mathrm{d}t} = \lambda N = \frac{\ln(2)}{t_{1/2}}N \\[2pt]
   S_A a_0 &= - \frac{\mathrm{d}N}{\mathrm{d}t}\bigg|_{t=0} = \lambda N_0
\end{align}</math>
where ''N''<sub>0</sub> is the initial amount of active substance&nbsp;— substance that has the same percentage of unstable particles as when the substance was formed.

===Assumptions===

The mathematics of radioactive decay depend on a key assumption that a nucleus of a radionuclide has no "memory" or way of translating its history into its present behavior. A nucleus does not "age" with the passage of time. Thus, the probability of its breaking down does not increase with time but stays constant, no matter how long the nucleus has existed. This constant probability may differ greatly between one type of nucleus and another, leading to the many different observed decay rates. However, whatever the probability is, it does not change over time. This is in marked contrast to complex objects that do show aging, such as automobiles and humans. These aging systems do have a chance of breakdown per unit of time that increases from the moment they begin their existence.

Aggregate processes, like the radioactive decay of a lump of atoms, for which the single-event probability of realization is very small but in which the number of time-slices is so large that there is nevertheless a reasonable rate of events, are modelled by the [[Poisson distribution]], which is discrete. Radioactive decay and [[Nuclear reaction|nuclear particle reactions]] are two examples of such aggregate processes.<ref name="leofish">{{cite book |last1=Leo |first1=William R. |title=Statistics and the treatment of experimental data |date=1992 |publisher=Springer-Verlag |edition=Techniques for Nuclear and Particle Physics Experiments |url=https://ned.ipac.caltech.edu/level5/Leo/Stats2_2.html |chapter=Ch. 4}}</ref> The mathematics of [[Poisson processes]] reduce to the law of [[exponential decay]], which describes the statistical behaviour of a large number of nuclei, rather than one individual nucleus. In the following formalism, the number of nuclei or the nuclei population ''N'', is of course a discrete variable (a [[natural number]])—but for any physical sample ''N'' is so large that it can be treated as a continuous variable. [[Differential calculus]] is used to model the behaviour of nuclear decay.

====One-decay process====
{{hatnote|For the mathematical details of exponential decay in general context, see [[exponential decay]].}}
Consider the case of a nuclide {{math|''A''}} that decays into another {{math|''B''}} by some process {{math|''A'' → ''B''}} (emission of other particles, like [[electron neutrino]]s {{SubatomicParticle|Electron antineutrino}} and [[electron]]s e<sup>−</sup> as in [[beta decay]], are irrelevant in what follows). The decay of an unstable nucleus is entirely random in time so it is impossible to predict when a particular atom will decay. However, it is equally likely to decay at any instant in time. Therefore, given a sample of a particular radioisotope, the number of decay events {{math|−d''N''}} expected to occur in a small interval of time {{math|d''t''}} is proportional to the number of atoms present {{math|''N''}}, that is<ref name="Patel">{{cite book |last=Patel |first=S.B. |title=Nuclear physics: an introduction |year=2000 |publisher=New Age International |location=New Delhi |isbn=978-81-224-0125-7 |pages=62–72}}</ref>

:<math> - \frac{\mathrm{d}N}{\mathrm{d}t} \propto N </math>

Particular radionuclides decay at different rates, so each has its own decay constant {{math|λ}}. The expected decay {{math|−d''N''/''N''}} is proportional to an increment of time, {{math|d''t''}}:

{{Equation box 1
|indent=:
|title=
|equation=<math>-\frac{\mathrm{d}N}{N} = \lambda \mathrm{d}t</math>
|cellpadding
|border
|border colour = #50C878
|background colour = #ECFCF4
}}

The negative sign indicates that {{math|''N''}} decreases as time increases, as the decay events follow one after another. The solution to this first-order [[differential equation]] is the [[mathematical function|function]]:

:<math>N(t) = N_0\,e^{-{\lambda}t}</math>

where {{math|''N''<sub>0</sub>}} is the value of {{math|''N''}} at time {{math|''t''}} = 0, with the decay constant expressed as {{math|λ}}<ref name="Patel" />

We have for all time {{math|''t''}}:

:<math> N_A + N_B = N_\text{total} = N_{A0}, </math>

where {{math|''N''{{sub|total}}}} is the constant number of particles throughout the decay process, which is equal to the initial number of {{math|''A''}} nuclides since this is the initial substance.

If the number of non-decayed {{math|''A''}} nuclei is:

:<math>N_A = N_{A0}e^{-\lambda t} </math>

then the number of nuclei of {{math|''B''}} (i.e. the number of decayed {{math|''A''}} nuclei) is

:<math> N_B = N_{A0} - N_A = N_{A0} - N_{A0}e^{-\lambda t} = N_{A0} \left( 1 - e^{-\lambda t} \right).</math>

The number of decays observed over a given interval obeys [[Poisson statistics]]. If the average number of decays is {{math|{{angbr|''N''}}}}, the probability of a given number of decays {{math|''N''}} is<ref name="Patel" />
:<math> P(N) = \frac{\langle N \rangle^N \exp(-\langle N\rangle)}{N!} .</math>

====Chain-decay processes====
{{hatnote|For the analogous mathematics in 1st order chemical reactions, see [[rate equation#Consecutive reactions|consecutive reactions]].}}

=====Chain of two decays=====

Now consider the case of a chain of two decays: one nuclide {{math|''A''}} decaying into another {{math|''B''}} by one process, then {{math|''B''}} decaying into another {{math|''C''}} by a second process, i.e. ''{{math|A → B → C}}''. The previous equation cannot be applied to the decay chain, but can be generalized as follows. Since {{math|''A''}} decays into {{math|''B''}}, ''then'' {{math|''B''}} decays into {{math|''C''}}, the activity of {{math|''A''}} adds to the total number of {{math|''B''}} nuclides in the present sample, ''before'' those {{math|''B''}} nuclides decay and reduce the number of nuclides leading to the later sample. In other words, the number of second generation nuclei {{math|''B''}} increases as a result of the first generation nuclei decay of {{math|''A''}}, and decreases as a result of its own decay into the third generation nuclei {{math|''C''}}.<ref name="Introductory Nuclear Physics 1988">Introductory Nuclear Physics, K.S. Krane, 1988, John Wiley & Sons Inc, {{ISBN|978-0-471-80553-3}}</ref> The sum of these two terms gives the law for a decay chain for two nuclides:

:<math>\frac{\mathrm{d}N_B}{\mathrm{d}t} = -\lambda_B N_B + \lambda_A N_A.</math>

The rate of change of {{math|''N<sub>B</sub>''}}, that is {{math|d''N{{sub|B}}''/d''t''}}, is related to the changes in the amounts of {{math|''A''}} and {{math|''B''}}, {{math|''N<sub>B</sub>''}} can increase as {{math|''B''}} is produced from {{math|''A''}} and decrease as {{math|''B''}} produces {{math|''C''}}.

Re-writing using the previous results:

{{Equation box 1
|indent=:
|title=
|equation=<math> \frac{\mathrm{d}N_B}{\mathrm{d}t} = - \lambda_B N_B + \lambda_A N_{A0} e^{-\lambda_A t} </math>
|cellpadding
|border
|border colour = #50C878
|background colour = #ECFCF4
}}

The subscripts simply refer to the respective nuclides, i.e. {{math|''N{{sub|A}}''}} is the number of nuclides of type {{math|''A''}}; {{math|''N''{{sub|''A''0}}}} is the initial number of nuclides of type {{math|''A''}}; {{math|''λ{{sub|A}}''}} is the decay constant for {{math|''A''}} – and similarly for nuclide {{math|''B''}}. Solving this equation for {{math|''N<sub>B</sub>''}} gives:

:<math> N_B = \frac{N_{A0}\lambda_A}{\lambda_B - \lambda_A} \left( e^{-\lambda_A t} - e^{-\lambda_B t}\right) . </math>

In the case where {{math|''B''}} is a stable nuclide ({{math|''λ{{sub|B}}''}} = 0), this equation reduces to the previous solution:

:<math>
  \lim_{\lambda_B\rightarrow 0} \left[ \frac{N_{A0}\lambda_A}{\lambda_B - \lambda_A} \left( e^{-\lambda_A t} - e^{-\lambda_B t} \right) \right] =
  \frac{N_{A0}\lambda_A}{0 - \lambda_A} \left( e^{-\lambda_A t} - 1 \right) =
  N_{A0} \left( 1- e^{-\lambda_A t} \right),
</math>

as shown above for one decay. The solution can be found by the [[integrating factor|integration factor]] method, where the integrating factor is {{math|''e''{{sup|λ{{sub|''B''}}''t''}}}}. This case is perhaps the most useful since it can derive both the one-decay equation (above) and the equation for multi-decay chains (below) more directly.

=====Chain of any number of decays=====

For the general case of any number of consecutive decays in a decay chain, i.e. {{math|A<sub>1</sub> → A<sub>2</sub> ··· → A<sub>''i''</sub> ··· → A<sub>''D''</sub>}}, where {{math|''D''}} is the number of decays and {{math|''i''}} is a dummy index ({{math|''i'' {{=}} 1, 2, 3, ..., ''D''}}), each nuclide population can be found in terms of the previous population. In this case {{math|''N''<sub>2</sub> {{=}} 0}}, {{math|''N''<sub>3</sub> {{=}} 0}}, ..., {{math|''N<sub>D</sub>'' {{=}} 0}}. Using the above result in a recursive form:

:<math> \frac{\mathrm{d}N_j}{\mathrm{d}t} = - \lambda_j N_j + \lambda_{j-1} N_{(j-1)0} e^{-\lambda_{j-1} t}. </math>

The general solution to the recursive problem is given by '''Bateman's equations''':<ref name="general solution of Bateman">{{cite journal|last=Cetnar|first=Jerzy|title=General solution of Bateman equations for nuclear transmutations|journal=Annals of Nuclear Energy|date=May 2006|volume=33|issue=7|pages=640–645|doi=10.1016/j.anucene.2006.02.004|bibcode=2006AnNuE..33..640C }}</ref>

{{Equation box 1
|indent=:
|title='Bateman's equations'
|equation=<math>\begin{align}
  N_D &= \frac{N_1(0)}{\lambda_D} \sum_{i=1}^D \lambda_i c_i e^{-\lambda_i t} \\[3pt]
  c_i &= \prod_{j=1, i\neq j}^D \frac{\lambda_j}{\lambda_j - \lambda_i}
\end{align}</math>
|cellpadding
|border
|border colour = #0073CF
|background colour=#F5FFFA
}}

====Multiple products====

In all of the above examples, the initial nuclide decays into just one product.<ref>{{cite book|title=Introductory Nuclear Physics|author=K.S. Krane|year=1988|publisher=John Wiley & Sons Inc|page=164|isbn=978-0-471-80553-3}}</ref> Consider the case of one initial nuclide that can decay into either of two products, that is ''{{math|A → B}}'' and ''{{math|A → C}}'' in parallel. For example, in a sample of [[potassium-40]], 89.3% of the nuclei decay to [[calcium-40]] and 10.7% to [[argon-40]]. We have for all time {{math|''t''}}:

:<math> N = N_A + N_B + N_C </math>

which is constant, since the total number of nuclides remains constant. Differentiating with respect to time:

:<math> \begin{align}
\frac{\mathrm{d}N_A}{\mathrm{d}t} & = - \left(\frac{\mathrm{d}N_B}{\mathrm{d}t} + \frac{\mathrm{d}N_C}{\mathrm{d}t} \right) \\
- \lambda N_A & = - N_A \left ( \lambda_B + \lambda_C \right ) \\
\end{align}</math>

defining the ''total decay constant'' {{math|λ}} in terms of the sum of ''partial decay constants'' {{math|λ<sub>''B''</sub>}} and {{math|λ<sub>''C''</sub>}}:

:<math> \lambda = \lambda_B + \lambda_C . </math>

Solving this equation for {{math|''N{{sub|A}}''}}:

:<math> N_A = N_{A0} e^{-\lambda t} .</math>

where {{math|''N''<sub>''A''0</sub>}} is the initial number of nuclide A. When measuring the production of one nuclide, one can only observe the total decay constant {{math|''λ''}}. The decay constants {{math|''λ{{sub|B}}''}} and {{math|''λ{{sub|C}}''}} determine the probability for the decay to result in products {{math|''B''}} or {{math|''C''}} as follows:

:<math> N_B = \frac{\lambda_B}{\lambda} N_{A0} \left ( 1 - e^{-\lambda t} \right ),</math>

:<math> N_C = \frac{\lambda_C}{\lambda} N_{A0} \left ( 1 - e^{-\lambda t} \right ).</math>

because the fraction {{math|''λ{{sub|B}}''/''λ''}} of nuclei decay into {{math|''B''}} while the fraction {{math|''λ{{sub|C}}''/''λ''}} of nuclei decay into {{math|''C''}}.

===Corollaries of laws===

The above equations can also be written using quantities related to the number of nuclide particles {{math|''N''}} in a sample;
* The activity: {{math|''A'' {{=}} ''λN''}}.
* The [[amount of substance]]: {{math|''n'' {{=}} ''N''/''N''<sub>A</sub>}}.
* The [[mass]]: {{math|1= ''m'' = ''Mn'' = ''MN''/''N''<sub>A</sub>}}.

where {{math|''N''<sub>A</sub>}} = {{physconst|NA}} is the [[Avogadro constant]], {{math|''M''}} is the [[molar mass]] of the substance in kg/mol, and the amount of the substance {{math|''n''}} is in [[Mole (unit)|moles]].

===Decay timing: definitions and relations===

====Time constant and mean-life====

For the one-decay solution ''{{math|A → B}}'':

:<math>N = N_0\,e^{-{\lambda}t} = N_0\,e^{-t/ \tau}, \,\!</math>

the equation indicates that the decay constant {{math|''λ''}} has units of {{math|''t''<sup>−1</sup>}}, and can thus also be represented as 1/{{math|''τ''}}, where {{math|''τ''}} is a characteristic time of the process called the ''[[time constant]]''.

In a radioactive decay process, this time constant is also the [[mean lifetime]] for decaying atoms. Each atom "lives" for a finite amount of time before it decays, and it may be shown that this mean lifetime is the [[arithmetic mean]] of all the atoms' lifetimes, and that it is {{math|''τ''}}, which again is related to the decay constant as follows:

:<math>\tau = \frac{1}{\lambda}.</math>

This form is also true for two-decay processes simultaneously ''{{math|A → B + C}}'', inserting the equivalent values of decay constants (as given above)

:<math> \lambda = \lambda_B + \lambda_C \,</math>

into the decay solution leads to:

:<math>\frac{1}{\tau} = \lambda = \lambda_B + \lambda_C = \frac{1}{\tau_B} + \frac{1}{\tau_C}\,</math>

[[File:Halflife-sim.gif|thumb|right|Simulation of many identical atoms undergoing radioactive decay, starting with either 4 atoms (left) or 400 (right). The number at the top indicates how many half-lives have elapsed.]]

====Half-life====
{{hatnote|For related derivations with some further details, see [[half-life]].}}
A more commonly used parameter is the half-life {{math|''T''<sub>1/2</sub>}}. Given a sample of a particular radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. For the case of one-decay nuclear reactions:

:<math>N = N_0\,e^{-{\lambda}t} = N_0\,e^{-t/\tau}, \,\!</math>

the half-life is related to the decay constant as follows: set {{math|1=N = ''N''{{sub|0}}/2}} and {{math|''t''}} = {{math|''T''<sub>1/2</sub>}} to obtain

:<math>t_{1/2} = \frac{\ln 2}{\lambda} = \tau \ln 2.</math>

This relationship between the half-life and the decay constant shows that highly radioactive substances are quickly spent, while those that radiate weakly endure longer. [[List of radioactive nuclides by half-life|Half-lives of known radionuclides]] vary by almost 54 orders of magnitude, from more than {{val|2.25|e=24|(9)}} years ({{val|6.9|e=31}} sec) for the very nearly stable nuclide [[isotopes of tellurium|<sup>128</sup>Te]], to {{val|8.6|e=-23|(6)}} seconds for the highly unstable nuclide [[Isotopes of hydrogen|<sup>5</sup>H]].<ref name=NUBASE2020/>

The factor of {{math|ln(2)}} in the above relations results from the fact that the concept of "half-life" is merely a way of selecting a different base other than the natural base {{math|''e''}} for the lifetime expression. The time constant {{math|''τ''}} is the {{subSup|''e''||−1}}-life, the time until only 1/''e'' remains, about 36.8%, rather than the 50% in the half-life of a radionuclide. Thus, {{math|''τ''}} is longer than {{math|''t''{{sub|1/2}}}}. The following equation can be shown to be valid:

:<math>N(t) = N_0\,e^{-t/\tau} = N_0\,2^{-t/t_{1/2}}. \,\!</math>

Since radioactive decay is exponential with a constant probability, each process could as easily be described with a different constant time period that (for example) gave its "(1/3)-life" (how long until only 1/3 is left) or "(1/10)-life" (a time period until only 10% is left), and so on. Thus, the choice of {{math|''τ''}} and {{math|''t''{{sub|1/2}}}} for marker-times, are only for convenience, and from convention. They reflect a fundamental principle only in so much as they show that the ''same proportion'' of a given radioactive substance will decay, during any time-period that one chooses.

Mathematically, the {{math|''n''{{sup|th}}}} life for the above situation would be found in the same way as above{{mdash}}by setting ''{{math|1=N = N{{sub|0}}/n}}'', {{math|1=''t'' = ''T''{{sub|1/''n''}}}} and substituting into the decay solution to obtain

:<math>t_{1/n} = \frac{\ln n}{\lambda} = \tau \ln n.</math>

===Example for carbon-14===

[[Carbon-14]] has a half-life of {{val|5700|(±30)}} years<ref name=NUBASE2020/> and a decay rate of 14 disintegrations per minute (dpm) per gram of natural carbon.

If an artifact is found to have radioactivity of 4 dpm per gram of its present C, we can find the approximate age of the object using the above equation:
:<math> N = N_0\,e^{-t/ \tau}, </math>
where:
:<math>\begin{align}
  \frac{N}{N_0} &= 4/14 \approx 0.286, \\
           \tau &= \frac{T_{1/2}}{\ln 2} \approx 8267\text{ years}, \\
              t &= -\tau\,\ln\frac{N}{ N_0} \approx 10356\text{ years}.
\end{align}</math>

===Changing rates===
The radioactive decay modes of electron capture and internal conversion are known to be slightly sensitive to chemical and environmental effects that change the electronic structure of the atom, which in turn affects the presence of '''1s''' and '''2s''' electrons that participate in the decay process. A small number of nuclides are affected.<ref>{{cite journal |last1=Emery |first1=G T |title=Perturbation of Nuclear Decay Rates |journal=Annual Review of Nuclear Science |date=December 1972 |volume=22 |issue=1 |pages=165–202 |doi=10.1146/annurev.ns.22.120172.001121 |bibcode=1972ARNPS..22..165E |url=https://www.annualreviews.org/doi/abs/10.1146/annurev.ns.22.120172.001121 |access-date=23 February 2022 |language=en |issn=0066-4243|url-access=subscription }}</ref> For example, [[chemical bonds]] can affect the rate of electron capture to a small degree (in general, less than 1%) depending on the proximity of electrons to the nucleus. In <sup>7</sup>Be, a difference of 0.9% has been observed between half-lives in metallic and insulating environments.<ref name="WangYan2006">{{cite journal |last1=Wang |first1=B. |display-authors=etal |title=Change of the 7Be electron capture half-life in metallic environments |journal=The European Physical Journal A |volume=28 |issue=3 |year=2006 |pages=375–377 |issn=1434-6001 |doi=10.1140/epja/i2006-10068-x |bibcode=2006EPJA...28..375W|s2cid=121883028 }}</ref> This relatively large effect is because beryllium is a small atom whose valence electrons are in '''2s''' [[atomic orbital]]s, which are subject to electron capture in <sup>7</sup>Be because (like all '''s''' atomic orbitals in all atoms) they naturally penetrate into the nucleus.

In 1992, Jung et al. of the Darmstadt Heavy-Ion Research group observed an accelerated β<sup>−</sup>&nbsp;decay of <sup>163</sup>Dy<sup>66+</sup>. Although neutral <sup>163</sup>Dy is a stable isotope, the fully ionized <sup>163</sup>Dy<sup>66+</sup> undergoes β<sup>−</sup>&nbsp;decay [[Bound-state beta decay|into the K and L shells]] to <sup>163</sup>Ho<sup>66+</sup> with a half-life of 47&nbsp;days.<ref name="JungBosch1992">{{cite journal |last1=Jung |first1=M. |display-authors=etal |title=First observation of bound-state β<sup>−</sup> decay |journal=Physical Review Letters |volume=69 |issue=15 |year=1992 |pages=2164–2167 |issn=0031-9007 |doi=10.1103/PhysRevLett.69.2164 |pmid=10046415 |bibcode=1992PhRvL..69.2164J}}</ref>

[[Rhenium-187]] is another spectacular example. <sup>187</sup>Re normally undergoes beta decay to <sup>187</sup>Os with a half-life of 41.6 billion&nbsp;years,<ref>{{cite journal |doi=10.1126/science.271.5252.1099 |last1=Smoliar |first1=M.I. |last2=Walker |first2=R.J. |last3=Morgan |first3=J.W. |year=1996 |title=Re-Os ages of group IIA, IIIA, IVA, and IVB iron meteorites |journal=Science |volume=271 |pages=1099–1102 |bibcode=1996Sci...271.1099S |issue=5252|s2cid=96376008 }}</ref> but studies using fully ionised <sup>187</sup>[[rhenium|Re]] atoms (bare nuclei) have found that this can decrease to only 32.9&nbsp;years.<ref>
{{cite journal
 |last1=Bosch |first1=F.
 |display-authors=etal
 |year=1996
 |title=Observation of bound-state beta minus decay of fully ionized <sup>187</sup>Re: <sup>187</sup>Re–<sup>187</sup>Os Cosmochronometry
 |journal=[[Physical Review Letters]]
 |volume=77 |issue=26 |pages=5190–5193
 |bibcode=1996PhRvL..77.5190B
 |doi=10.1103/PhysRevLett.77.5190
 |pmid=10062738
}}</ref> This is attributed to "[[Beta decay#Bound-state β− decay|bound-state β<sup>−</sup> decay]]" of the fully ionised atom – the electron is emitted into the "K-shell" ('''1s''' atomic orbital), which cannot occur for neutral atoms in which all low-lying bound states are occupied.<ref name="Bosch1996">{{cite journal |doi=10.1103/PhysRevLett.77.5190 |title=Observation of bound-state β– decay of fully ionized <sup>187</sup>Re:<sup>187</sup>Re-<sup>187</sup>Os Cosmochronometry |year=1996 |journal=Physical Review Letters |volume=77 |issue=26 |pages=5190–5193 |pmid=10062738 |first1=F. |last1=Bosch |display-authors=etal |bibcode=1996PhRvL..77.5190B}}</ref>

[[File:DecayRate vs Solar Time.png|thumb|upright=1.3|Example of diurnal and seasonal variations in gamma ray detector response.]]
A number of experiments have found that decay rates of other modes of artificial and naturally occurring radioisotopes are, to a high degree of precision, unaffected by external conditions such as temperature, pressure, the chemical environment, and electric, magnetic, or gravitational fields.<ref>{{cite journal |last1=Emery |first1=G.T. |year=1972 |title=Perturbation of Nuclear Decay Rates |journal=[[Annual Review of Nuclear Science]] |volume=22 |issue=1 |pages=165–202 |doi=10.1146/annurev.ns.22.120172.001121|doi-access=free |bibcode=1972ARNPS..22..165E}}</ref> Comparison of laboratory experiments over the last century, studies of the Oklo [[Natural nuclear fission reactor|natural nuclear reactor]] (which exemplified the effects of thermal neutrons on nuclear decay), and astrophysical observations of the luminosity decays of distant supernovae (which occurred far away so the light has taken a great deal of time to reach us), for example, strongly indicate that unperturbed decay rates have been constant (at least to within the limitations of small experimental errors) as a function of time as well.{{citation needed|reason=Entire paragraph needs sourcing|date=October 2014}}

Recent results suggest the possibility that decay rates might have a weak dependence on environmental factors. It has been suggested that measurements of decay rates of [[silicon-32]], [[manganese-54]], and [[radium-226]] exhibit small seasonal variations (of the order of 0.1%).<ref>{{cite web |title=The mystery of varying nuclear decay |work=Physics World |date=2 October 2008 |url=http://physicsworld.com/cws/article/news/36108}}</ref><ref>{{cite journal |title=Perturbation of Nuclear Decay Rates During the Solar Flare of 13 December 2006 |journal=Astroparticle Physics |volume=31 |issue=6 |year=2009 |pages=407–411 |arxiv=0808.3156 |bibcode=2009APh....31..407J |doi=10.1016/j.astropartphys.2009.04.005 |last1=Jenkins |first1=Jere H. |last2=Fischbach |first2=Ephraim|s2cid=118863334 }}</ref><ref>{{cite journal |last1=Jenkins |first1=J.H. |last2=Fischbach |first2=Ephraim |last3=Buncher |first3=John B. |last4=Gruenwald |first4=John T. |last5=Krause |first5=Dennis E. |last6=Mattes |first6=Joshua J. |title=Evidence of correlations between nuclear decay rates and Earth–Sun distance |journal=Astroparticle Physics |volume=32 |issue=1 |pages=42–46 |year=2009 |arxiv=0808.3283 |bibcode=2009APh....32...42J |doi=10.1016/j.astropartphys.2009.05.004|s2cid=119113836 }}</ref>  However, such measurements are highly susceptible to systematic errors, and a subsequent paper<ref>{{cite journal|last1=Norman|first1=E.B.|last2=Browne|first2=Edgardo|last3=Shugart|first3=Howard A.|last4=Joshi|first4=Tenzing H.|last5=Firestone|first5=Richard B.|year=2009|title=Evidence against correlations between nuclear decay rates and Earth–Sun distance|url=http://donuts.berkeley.edu/papers/EarthSun.pdf|journal=Astroparticle Physics|volume=31|issue=2|pages=135–137|arxiv=0810.3265|bibcode=2009APh....31..135N|doi=10.1016/j.astropartphys.2008.12.004|s2cid=7051382|access-date=23 September 2009|archive-url=https://web.archive.org/web/20100629033415/http://donuts.berkeley.edu/papers/EarthSun.pdf|archive-date=29 June 2010|url-status=dead}}</ref> has found no evidence for such correlations in seven other isotopes (<sup>22</sup>Na, <sup>44</sup>Ti, <sup>108</sup>Ag, <sup>121</sup>Sn, <sup>133</sup>Ba, <sup>241</sup>Am, <sup>238</sup>Pu), and sets upper limits on the size of any such effects. The decay of [[radon-222]] was once reported to exhibit large 4% peak-to-peak seasonal variations (see plot),<ref name="SturrockSteinitz2012">{{cite journal|last1=Sturrock|first1=P.A.|last2=Steinitz|first2=G.|last3=Fischbach|first3=E.|last4=Javorsek|first4=D.|last5=Jenkins|first5=J.H.|title=Analysis of gamma radiation from a radon source: Indications of a solar influence|journal=Astroparticle Physics|volume=36|issue=1|year=2012|pages=18–25|issn=0927-6505|doi=10.1016/j.astropartphys.2012.04.009 |arxiv=1205.0205|bibcode=2012APh....36...18S|s2cid=119163371}}</ref>  which were proposed to be related to either [[solar flare]] activity or the distance from the Sun, but detailed analysis of the experiment's design flaws, along with comparisons to other, much more stringent and systematically controlled, experiments refute this claim.<ref>{{Cite journal|date=2018-01-01|title=On the claim of modulations in radon decay and their association with solar rotation|journal=Astroparticle Physics|language=en|volume=97|pages=38–45|doi=10.1016/j.astropartphys.2017.10.011|issn=0927-6505|bibcode=2018APh....97...38P|last1=Pommé|first1=S.|last2=Lutter|first2=G.|last3=Marouli|first3=M.|last4=Kossert|first4=K.|last5=Nähle|first5=O.|doi-access=free}}</ref>

====GSI anomaly====
{{Main|GSI anomaly}}
An unexpected series of experimental results for the rate of decay of heavy [[Highly charged ion|highly charged]] radioactive [[ion]]s circulating in a [[storage ring]] has provoked theoretical activity in an effort to find a convincing explanation. The rates of [[Weak interaction|weak]] decay of two radioactive species with half lives of about 40 s and 200 s are found to have a significant [[oscillation|oscillatory]] [[modulation]], with a period of about 7 s.<ref name="KienleBosch2013">{{cite journal |vauthors=Kienle P, Bosch F, Bühler P, Faestermanna T, ((Litvinov Yu.A.)), Winckler N, Sanjari MS, Shubina DB, Atanasov D, Geissel H, Ivanova V, Yan XL, Boutin D, Brandau C, Dillmann I, Dimopoulou C, Hess R, Hillebrand PM, Izumikawa T, Knöbel R, Kurcewicz J, Kuzminchuk N, Lestinsky M, Litvinov SA, Ma XW, Maier L, Mazzocco M, Mukha I, Nociforo C, Nolden F, Scheidenberger C, Spillmann U, Steck M, ((Stöhlker Th)), Sun BH, Suzaki F, Suzuki T, ((Torilov S.Yu)), Trassinelli M, Tu XL, Wang M, Weick H, Winters DF, Winters N, Woods PJ, Yamaguchi T, Zhang GL, Ohtsubo T|display-authors=6 |title=High-resolution measurement of the time-modulated orbital electron capture and of the β<sup>+</sup> decay of hydrogen-like <sup>142</sup>Pm<sub>60</sub><sup>+</sup> ions |journal=Physics Letters B |volume=726 |issue=4–5 |year=2013 |pages=638–645 |issn=0370-2693 |doi=10.1016/j.physletb.2013.09.033 |bibcode=2013PhLB..726..638K|arxiv=1309.7294 |s2cid=55085840 }}</ref>
The observed phenomenon is known as the [[GSI anomaly]], as the storage ring is a facility at the [[GSI Helmholtz Centre for Heavy Ion Research]] in [[Darmstadt]], [[Germany]].  As the decay process produces an [[electron neutrino]], some of the proposed explanations for the observed rate oscillation invoke neutrino properties. Initial ideas related to [[Neutrino#Flavor oscillations|flavour oscillation]] met with skepticism.<ref name="Giunti2009">{{cite journal |last1=Giunti|first1=Carlo |title=The GSI Time Anomaly: Facts and Fiction |journal=Nuclear Physics B: Proceedings Supplements |volume=188 |year=2009 |pages=43–45 |issn=0920-5632 |doi=10.1016/j.nuclphysbps.2009.02.009 |arxiv=0812.1887 |bibcode=2009NuPhS.188...43G |s2cid=10196271 }}</ref>  A more recent proposal involves mass differences between neutrino mass [[eigenstates]].<ref name="Gal2016">{{cite journal |last1=Gal|first1=Avraham |title=Neutrino Signals in Electron-Capture Storage-Ring Experiments |journal=Symmetry |volume=8 |issue=6 |year=2016 |pages=49 |issn=2073-8994 |doi=10.3390/sym8060049|arxiv=1407.1789 |bibcode=2016Symm....8...49G |s2cid=14287612 |doi-access=free }}</ref>

==Nuclear processes==
A nuclide is considered to "exist" if it has a half-life greater than 2x10<sup>−14</sup>s. This is an arbitrary boundary; shorter half-lives are considered resonances, such as a system undergoing a nuclear reaction. This time scale is characteristic of the [[strong interaction]] which creates the [[nuclear force]].  Only nuclides are considered to decay and produce radioactivity.<ref name=Pfutzner2012>{{Cite journal |last1=Pfützner |first1=M. |last2=Karny |first2=M. |last3=Grigorenko |first3=L. V. |last4=Riisager |first4=K. |date=2012-04-30 |title=Radioactive decays at limits of nuclear stability |url=https://link.aps.org/doi/10.1103/RevModPhys.84.567 |journal=Reviews of Modern Physics |language=en |volume=84 |issue=2 |pages=567–619 |doi=10.1103/RevModPhys.84.567 |issn=0034-6861|arxiv=1111.0482 |bibcode=2012RvMP...84..567P }}</ref>{{rp|568}}

Nuclides can be stable or unstable. Unstable nuclides decay, possibly in several steps, until they become stable. There are 251 known [[stable nuclide]]s. The number of unstable nuclides discovered has grown, with about 3000 known in 2006.<ref name=Pfutzner2012/>

The most common and consequently historically the most important forms of natural radioactive decay involve the emission of alpha-particles, beta-particles, and gamma rays. Each of these correspond to a [[fundamental interaction]] predominantly responsible for the radioactivity:<ref name=PaisInwardBound>
{{Cite book
|last=Pais
|first=Abraham
|title=Inward bound: of matter and forces in the physical world
|date=2002
|publisher=Clarendon Press [u.a.]
|isbn=978-0-19-851997-3
|edition=Reprint
|location=Oxford}}
</ref>{{rp|142}}
* [[alpha-decay]] -> [[strong interaction]],
* [[beta-decay]] -> [[weak interaction]],
* [[gamma-decay]] -> [[electromagnetism]].
In alpha decay, a particle containing two protons and two neutrons, equivalent to a He nucleus, breaks out of the parent nucleus. The process represents a competition between the electromagnetic repulsion between the protons in the nucleus and attractive [[nuclear force]], a residual of the strong interaction. The alpha particle is an especially strongly bound nucleus, helping it win the competition more often.<ref name=Ohanian1994>{{Cite book |last=Ohanian |first=Hans C. |title=Principles of physics |date=1994 |publisher=W.W. Norton |isbn=978-0-393-95773-0 |edition=1 |location=New York |oclc=1311044116}}</ref>{{rp|872}}  However some nuclei break up or [[nuclear fission|fission]] into larger particles and artificial nuclei decay with the emission of
single protons, double protons, and other combinations.<ref name=Pfutzner2012/>

Beta decay transforms a neutron into proton or vice versa.  When a neutron inside a parent nuclide decays to a proton, an electron, a [[anti-neutrino]], and nuclide with high atomic number results. When a proton in a parent nuclide transforms to a neutron, a [[positron]], a [[neutrino]], and  nuclide with a lower atomic number results. These changes are a direct manifestation of the weak interaction.<ref name=Ohanian1994/>{{rp|874}}

Gamma decay resembles other kinds of electromagnetic emission: it corresponds to transitions between an excited quantum state and lower energy state. Any of the particle decay mechanisms often leave the daughter in an excited state, which then decays via gamma emission.<ref name=Ohanian1994/>{{rp|876}}

Other forms of decay include [[neutron emission]], [[electron capture]], [[internal conversion]], [[cluster decay]].<ref>{{Cite web |title=Other types of radioactive decay |url=https://www.arpansa.gov.au/understanding-radiation/what-is-radiation/ionising-radiation/radiation-decay |website= Australian Radiation Protection and Nuclear Safety Agency}}</ref>

==Hazard warning signs==
<gallery>
file:Radioactive.svg|The trefoil symbol used to warn of presence of radioactive material or ionising radiation<!-- The [[Unicode]] encoding of this symbol is U+2622 ({{unicode|☢}}) -->
File:Logo iso radiation.svg|2007 ISO radioactivity [[hazard symbol]] intended for IAEA Category 1, 2 and 3 sources defined as dangerous sources capable of death or serious injury<ref>[http://www.iaea.org/newscenter/news/2007/radiationsymbol.html IAEA news release Feb 2007]</ref>
File:Dangclass7.svg|One of several dangerous goods transport classification signs for radioactive materials
</gallery>

==See also==
{{Div col|small=no}}
* [[Actinides in the environment]]
* [[Background radiation]]
* [[Chernobyl disaster]]
* [[Crimes involving radioactive substances]]
* [[Decay correction]]
* [[Fallout shelter]]
* [[Geiger counter]]
* [[Induced radioactivity]]
* [[Lists of nuclear disasters and radioactive incidents]]
* [[National Council on Radiation Protection and Measurements]]
* [[Nuclear engineering]]
* [[Nuclear pharmacy]]
* [[Nuclear power]]
* [[Nuclear chain reaction]]
* [[Radiation therapy]]
* [[Radioactive contamination]]
* [[Radioactivity in biology]]
* [[Radiometric dating]]
* [[Transient equilibrium]]
{{Div col end}}
{{Portal inline|Nuclear technology}}
{{portal inline|Physics}}

==Notes==
{{reflist|group=note}}

==References==
{{reflist}}

==External links==
{{Wikibooks |Historical Geology|Radioactive decay}}
{{Wiktionary|radioactivity}}
* [https://web.archive.org/web/20130108122246/http://ie.lbl.gov/toi/abouttoi.htm The Lund/LBNL Nuclear Data Search] – Contains tabulated information on radioactive decay types and energies.
* [http://www.radiochemistry.org/nomenclature/ Nomenclature of nuclear chemistry]
* [http://www.iem-inc.com/prhlfr.html Specific activity and related topics].
* [http://www-nds.iaea.org/livechart The Live Chart of Nuclides – IAEA]
* [http://www.nndc.bnl.gov/chart/ Interactive Chart of Nuclides] {{Webarchive|url=https://web.archive.org/web/20181010070007/http://www.nndc.bnl.gov/chart/ |date=10 October 2018 }}
* [http://www.radiationanswers.org/ Health Physics Society Public Education Website]
* {{Cite NSRW|wstitle=Becquerel Rays}}
* [http://alsos.wlu.edu/qsearch.aspx?browse=science/Radioactivity Annotated bibliography for radioactivity from the Alsos Digital Library for Nuclear Issues] {{Webarchive|url=https://web.archive.org/web/20101007075539/http://alsos.wlu.edu/qsearch.aspx?browse=science%2FRadioactivity |date=7 October 2010 }}
* "Henri Becquerel: The Discovery of Radioactivity", Becquerel's 1896 articles online and analyzed on ''[http://www.bibnum.education.fr/physique/radioactivite/sur-les-radiations-invisibles-emises-par-les-corps-phosphorescents BibNum]'' <small>[click 'à télécharger' for English version]</small>.
* "Radioactive change", Rutherford & Soddy article (1903), online and analyzed on ''[http://www.bibnum.education.fr/physique/radioactivite/la-transformation-radioactive Bibnum]'' <small>[click 'à télécharger' for English version]</small>

{{Radiation|state=uncollapsed}}
{{Marie & Pierre Curie}}
{{Authority control}}

{{DEFAULTSORT:Radioactive Decay}}
[[Category:Radioactivity| ]]
[[Category:Exponentials]]
[[Category:Poisson point processes]]