TheInfoList

In
nuclear physics Nuclear physics is the field of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in ot ...
, beta decay (''β''-decay) is a type of
radioactive decay 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 consi ...

in which a
beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β ...
(fast energetic
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

or
positron The positron or antielectron is the antiparticle s (left) and antiparticles (right). From top to bottom; electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is ...

) is emitted from an
atomic nucleus The atomic nucleus is the small, dense region consisting of s and s at the center of an , discovered in 1911 by based on the 1909 . After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickl ...
, transforming the original
nuclide A nuclide (or nucleide, from nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic c ...

to an isobar of that nuclide. For example, beta decay of a
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

transforms it into a
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

by the emission of an electron accompanied by an
antineutrino A neutrino ( or ) (denoted by the Greek letter Nu (letter), ) is a fermion (an elementary particle with spin-1/2, spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electric charge, electri ...
; or, conversely a proton is converted into a neutron by the emission of a positron with a
neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

in so-called ''
positron emission Positron emission, beta plus decay, or β+ decay is a subtype of radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic ...
''. Neither the beta particle nor its associated (anti-)neutrino exist within the nucleus prior to beta decay, but are created in the decay process. By this process, unstable atoms obtain a more stable ratio of protons to neutrons. The probability of a nuclide decaying due to beta and other forms of decay is determined by its
nuclear binding energy Nuclear binding energy in experimental physics is the minimum energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge ...
. The binding energies of all existing nuclides form what is called the nuclear band or
valley of stability In nuclear physics, the valley of stability (also called the belt of stability, nuclear valley, energy valley, or beta stability valley) is a characterization of the stability of nuclides to radioactivity based on their binding energy. Nuclides ...
. For either electron or positron emission to be energetically possible, the energy release (
see below See or SEE may refer to: Arts, entertainment, and media * Music: ** See (album), ''See'' (album), studio album by rock band The Rascals *** "See", song by The Rascals, on the album ''See'' ** See (Tycho song), "See" (Tycho song), song by Tycho * T ...
) or ''Q'' value must be positive. Beta decay is a consequence of the
weak force Weak may refer to: Songs * "Weak" (AJR song), 2016 * "Weak" (Melanie C song), 2011 * "Weak" (SWV song), 1993 * "Weak" (Skunk Anansie song), 1995 * "Weak", a song by Seether from '' Seether: 2002-2013'' Television episodes * "Weak" (''Fear t ...
, which is characterized by relatively lengthy decay times. Nucleons are composed of
up quark The up quark or u quark (symbol: u) is the lightest of all quark A quark () is a type of elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other partic ...

s and
down quark The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up qu ...

s, and the weak force allows a
quark A quark () is a type of elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fundam ...

to change its
flavour Flavor, or flavour, is the perceptual Perception (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area aro ...
by emission of a
W boson In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles force carrier, mediate the weak interaction; the respective symbo ...

leading to creation of an electron/antineutrino or positron/neutrino pair. For example, a neutron, composed of two down quarks and an up quark, decays to a proton composed of a down quark and two up quarks.
Electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom An atom is the smallest unit of ordinary matter In classical physics ...

is sometimes included as a type of beta decay, because the basic nuclear process, mediated by the weak force, is the same. In electron capture, an inner atomic electron is captured by a proton in the nucleus, transforming it into a neutron, and an electron neutrino is released.

# Description

The two types of beta decay are known as ''beta minus'' and ''beta plus''. In beta minus (β) decay, a neutron is converted to a proton, and the process creates an electron and an
electron antineutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin (physics), spin of . Together with the electron, it forms the first generation (physics), generation of Lepton, leptons, hence the name electron neutrino ...
; while in beta plus (β+) decay, a proton is converted to a neutron and the process creates a positron and an electron neutrino. β+ decay is also known as
positron emission Positron emission, beta plus decay, or β+ decay is a subtype of radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic ...
. Beta decay conserves a quantum number known as the
lepton number In particle physics Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various types of very small objects (e.g. , gas particl ...
, or the number of electrons and their associated neutrinos (other leptons are the
muon The muon (; from the Greek alphabet, Greek letter mu (letter), mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 ''e'' and a spin-½, spin of 1/2, but with a much greater ma ...

and
tau Tau (uppercase Τ, lowercase τ; el, ταυ ) is the 19th letter of the Greek alphabet The Greek alphabet has been used to write the Greek language since the late ninth or early eighth century BC. It is derived from the earlier Phoenician ...
particles). These particles have lepton number +1, while their antiparticles have lepton number −1. Since a proton or neutron has lepton number zero, β+ decay (a positron, or antielectron) must be accompanied with an electron neutrino, while β decay (an electron) must be accompanied by an electron antineutrino. An example of electron emission (β decay) is the decay of
carbon-14 Carbon-14 (14C), or radiocarbon, is a radioactive isotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ...

into
nitrogen-14 Natural nitrogen Nitrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms tha ...

with a
half-life Half-life (symbol ''t''1⁄2) is the time required for a quantity to reduce to half of its initial value. The term is commonly used in nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies ...
of about 5,730 years: : → + + In this form of decay, the original element becomes a new chemical element in a process known as
nuclear transmutation Nuclear transmutation is the conversion of one chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of a ...
. This new element has an unchanged
mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of s and s (together known as s) in an . It is approximately equal to the of the expre ...
, but an
atomic number The atomic number or proton number (symbol ''Z'') of a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. ...
that is increased by one. As in all nuclear decays, the decaying element (in this case ) is known as the ''parent nuclide'' while the resulting element (in this case ) is known as the ''daughter nuclide''. Another example is the decay of hydrogen-3 (
tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucl ...

) into
helium-3 Helium-3 (3He see also helion) is a light, stable isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons A proton is a subatomic particle, symbol or , with a positive electric charge Ele ...

with a half-life of about 12.3 years: : → + + An example of positron emission (β+ decay) is the decay of
magnesium-23 Magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natur ...
into
sodium-23 There are 21 recognized isotopes of sodium Sodium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nucl ...

with a half-life of about 11.3 s: : → + + β+ decay also results in nuclear transmutation, with the resulting element having an atomic number that is decreased by one. The beta spectrum, or distribution of energy values for the beta particles, is continuous. The total energy of the decay process is divided between the electron, the antineutrino, and the recoiling nuclide. In the figure to the right, an example of an electron with 0.40 MeV energy from the beta decay of 210Bi is shown. In this example, the total decay energy is 1.16 MeV, so the antineutrino has the remaining energy: . An electron at the far right of the curve would have the maximum possible kinetic energy, leaving the energy of the neutrino to be only its small rest mass.

# History

## Discovery and initial characterization

Radioactivity was discovered in 1896 by
Henri Becquerel Antoine Henri Becquerel (; 15 December 1852 – 25 August 1908) was a French engineer Engineers, as practitioners of engineering Engineering is the use of scientific principles to design and build machines, structures, and other i ...

in
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

, and subsequently observed by and
Pierre Curie Pierre Curie ( , ; 15 May 1859 – 19 April 1906) was a French physicist, a pioneer in crystallography, magnetism, piezoelectricity, and radioactivity. In 1903, he received the Nobel Prize in Physics with his wife, Marie Curie (née Skłodow ...
in
thorium Thorium is a weakly radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus The atomic nucleus is the sma ...

and in the new elements
polonium Polonium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical eleme ...

and
radium Radium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

. In 1899,
Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist A physicist is a scientist A scientist is a person who conducts scientific research The sci ...
separated radioactive emissions into two types: alpha and beta (now beta minus), based on penetration of objects and ability to cause ionization. Alpha rays could be stopped by thin sheets of paper or aluminium, whereas beta rays could penetrate several millimetres of aluminium. In 1900,
Paul Villard Paul Ulrich Villard (28 September 1860 – 13 January 1934) was a French chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a ...

identified a still more penetrating type of radiation, which Rutherford identified as a fundamentally new type in 1903 and termed
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s. Alpha, beta, and gamma are the first three letters of the
Greek alphabet The Greek alphabet has been used to write the Greek language since the late ninth or early eighth century BC. It is derived from the earlier Phoenician alphabet, and was the first alphabetic script in history to have distinct letters for vowels ...

. In 1900, Becquerel measured the
mass-to-charge ratio The mass-to-charge ratio (''m''/''Q'') is a physical quantity A physical quantity is a physical property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity can be expressed as a ''value' ...
() for beta particles by the method of J.J. Thomson used to study cathode rays and identify the electron. He found that for a beta particle is the same as for Thomson's electron, and therefore suggested that the beta particle is in fact an electron. In 1901, Rutherford and
Frederick Soddy Frederick Soddy FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resourc ...

showed that alpha and beta radioactivity involves the transmutation of atoms into atoms of other chemical elements. In 1913, after the products of more radioactive decays were known, Soddy and
Kazimierz Fajans Kazimierz Fajans (Kasimir Fajans in many American publications; 27 May 1887 – 18 May 1975) was a Polish American Polish Americans ( pl, Polonia amerykańska) are Americans Americans are the and of the .; ; ''Ricketts v. Attorney Gen ...

independently proposed their radioactive displacement law, which states that beta (i.e., ) emission from one element produces another element one place to the right in the
periodic table The periodic table, also known as the periodic table of (the) chemical elements, is a tabular display of the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is ...

, while alpha emission produces an element two places to the left.

## Neutrinos

The study of beta decay provided the first physical evidence for the existence of the
neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

. In both alpha and gamma decay, the resulting alpha or gamma particle has a narrow energy
distributionDistribution may refer to: Mathematics *Distribution (mathematics) Distributions, also known as Schwartz distributions or generalized functions, are objects that generalize the classical notion of functions in mathematical analysis. Distr ...

, since the particle carries the energy from the difference between the initial and final nuclear states. However, the kinetic energy distribution, or spectrum, of beta particles measured by
Lise Meitner Elise Meitner ( , ; 7 November 1878 – 27 October 1968) was a leading Austrian-Swedish physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, ...

and
Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a classical language ...

in 1911 and by Jean Danysz in 1913 showed multiple lines on a diffuse background. These measurements offered the first hint that beta particles have a continuous spectrum. In 1914,
James Chadwick Sir James Chadwick, (20 October 1891 – 24 July 1974) was a British physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical m ...

used a magnetic
spectrometer A spectrometer () is a scientific instrument used to separate and measure Spectrum, spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon ...

with one of new to make more accurate measurements which showed that the spectrum was continuous. The distribution of beta particle energies was in apparent contradiction to the
law of conservation of energy In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through ...
. If beta decay were simply electron emission as assumed at the time, then the energy of the emitted electron should have a particular, well-defined value. For beta decay, however, the observed broad distribution of energies suggested that energy is lost in the beta decay process. This spectrum was puzzling for many years. A second problem is related to the
conservation of angular momentum In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. " ...
. Molecular band spectra showed that the
nuclear spin In nuclear physics, atomic physics, and nuclear chemistry, the nuclear shell model is a nuclear model, model of the atomic nucleus which uses the Pauli exclusion principle to describe the structure of the nucleus in terms of energy levels. The f ...
of
nitrogen-14 Natural nitrogen Nitrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms tha ...

is 1 (i.e., equal to the
reduced Planck constant The Planck constant, or Planck's constant, is the quantum of electromagnetic action that relates a photon's energy to its frequency. The Planck constant multiplied by a photon's frequency is equal to a photon's energy. The Planck constant i ...
) and more generally that the spin is integral for nuclei of even
mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of s and s (together known as s) in an . It is approximately equal to the of the expre ...
and half-integral for nuclei of odd mass number. This was later explained by the proton-neutron model of the nucleus. Beta decay leaves the mass number unchanged, so the change of nuclear spin must be an integer. However, the electron spin is 1/2, hence angular momentum would not be conserved if beta decay were simply electron emission. From 1920 to 1927,
Charles Drummond EllisSir Charles Drummond Ellis (b. Hampstead, 11 August 1895; died Cookham 10 January 1980) was an English physicist and scientific administrator. His work on the magnetic spectrum of the beta-rays helped to develop a better understanding of nuclear s ...
(along with Chadwick and colleagues) further established that the beta decay spectrum is continuous. In 1933, Ellis and
Nevill Mott Sir Nevill Francis Mott (30 September 1905 – 8 August 1996) was a British physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical me ...
obtained strong evidence that the beta spectrum has an effective upper bound in energy.
Niels Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish Danish may refer to: * Something of, from, or related to the country of Denmark * A national or citizen of Denmark, also called a "Dane", see Demographics of D ...

had suggested that the beta spectrum could be explained if
conservation of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...
was true only in a statistical sense, thus this
principle A principle is a proposition or value that is a guide for behavior or evaluation. In law, it is a Legal rule, rule that has to be or usually is to be followed. It can be desirably followed, or it can be an inevitable consequence of something, suc ...
might be violated in any given decay. However, the upper bound in beta energies determined by Ellis and Mott ruled out that notion. Now, the problem of how to account for the variability of energy in known beta decay products, as well as for conservation of momentum and angular momentum in the process, became acute. In a famous letter written in 1930,
Wolfgang Pauli Wolfgang Ernst Pauli (; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and one of the pioneers of quantum physics Quantum mechanics is a fundamental theory in physics that provides a description of the physica ...

attempted to resolve the beta-particle energy conundrum by suggesting that, in addition to electrons and protons, atomic nuclei also contained an extremely light neutral particle, which he called the neutron. He suggested that this "neutron" was also emitted during beta decay (thus accounting for the known missing energy, momentum, and angular momentum), but it had simply not yet been observed. In 1931,
Enrico Fermi Enrico Fermi (; 29 September 1901 - 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...

renamed Pauli's "neutron" the "neutrino" ('little neutral one' in Italian). In 1933, Fermi published his landmark theory for beta decay, where he applied the principles of quantum mechanics to matter particles, supposing that they can be created and annihilated, just as the light quanta in atomic transitions. Thus, according to Fermi, neutrinos are created in the beta-decay process, rather than contained in the nucleus; the same happens to electrons. The neutrino interaction with matter was so weak that detecting it proved a severe experimental challenge. Further indirect evidence of the existence of the neutrino was obtained by observing the recoil of nuclei that emitted such a particle after absorbing an electron. Neutrinos were finally detected directly in 1956 by
Clyde Cowan Clyde Lorrain Cowan Jr (December 6, 1919 – May 24, 1974) was an American physicist, the co-discoverer of the neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion (an elementary particle In particle physics Particle ...

and
Frederick Reines Frederick Reines ( ; March 16, 1918 – August 26, 1998) was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment. He may be the only scientist in ...

in the
Cowan–Reines neutrino experimentThe Cowan–Reines neutrino experiment was conducted by Washington University in St. Louis alumnus Clyde L. Cowan and Stevens Institute of Technology and New York University alumnus Frederick Reines in 1956. The experiment confirmed the existence ...
. The properties of neutrinos were (with a few minor modifications) as predicted by Pauli and Fermi.

## decay and electron capture

In 1934,
Frédéric Frédéric and Frédérick are the French French (french: français(e), link=no) may refer to: * Something of, from, or related to France France (), officially the French Republic (french: link=no, République française), is a country prim ...

and
Irène Joliot-Curie Irène Joliot-Curie (; 12 September 1897 – 17 March 1956) was a French chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a person who conducts Scie ...
bombarded aluminium with alpha particles to effect the nuclear reaction  +  →  + , and observed that the product isotope emits a positron identical to those found in cosmic rays (discovered by
Carl David Anderson Carl David Anderson (September 3, 1905 – January 11, 1991) was an American physicist. He is best known for his discovery of the positron in 1932, an achievement for which he received the 1936 Nobel Prize in Physics, and of the muon in 1936. Bio ...

in 1932). This was the first example of  decay (
positron emission Positron emission, beta plus decay, or β+ decay is a subtype of radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic ...
), which they termed artificial radioactivity since is a short-lived nuclide which does not exist in nature. In recognition of their discovery the couple were awarded the
Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
in 1935. The theory of
electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom An atom is the smallest unit of ordinary matter In classical physics ...

was first discussed by
Gian-Carlo Wick Gian Carlo Wick (October 15, 1909 – April 20, 1992) was an Italian theoretical physicist who made important contributions to quantum field theory. The Wick rotation, Wick contraction, Wick's theorem, and the Wick product are named after him.
in a 1934 paper, and then developed by
Hideki Yukawa was a Japanese theoretical physicist and the first Japanese Nobel laureate Nobel laureates of 2012 Alvin E. Roth, Brian Kobilka, Robert J. Lefkowitz">Brian_Kobilka.html" ;"title="Alvin E. Roth, Brian Kobilka">Alvin E. Roth, Brian Kobilka, R ...
and others. K-electron capture was first observed in 1937 by Luis Alvarez, in the nuclide 48V. Alvarez went on to study electron capture in 67Ga and other nuclides.

## Non-conservation of parity

In 1956,
Tsung-Dao Lee Tsung-Dao Lee (; born November 24, 1926) is a Chinese-American physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of ...
and
Chen Ning Yang Yang Chen-Ning or Chen-Ning Yang (; born 1 October, 1922), also known as C. N. Yang or by the English name Frank Yang, is a Chinese theoretical physicist who made significant contributions to statistical mechanics In physics, statistical ...
noticed that there was no evidence that parity was conserved in weak interactions, and so they postulated that this symmetry may not be preserved by the weak force. They sketched the design for an experiment for testing conservation of parity in the laboratory. Later that year,
Chien-Shiung Wu ) , spouse = , residence = , nationality = ChineseAmerican , field = Physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge whi ...
and coworkers conducted the
Wu experiment The Wu experiment was a particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which can be ascribed several physical property, physica ...

showing an asymmetrical beta decay of
cobalt-60 Cobalt-60 (60Co) is a synthetic isotope, synthetic radioactive Isotopes of cobalt, isotope of cobalt with a half-life of 5.2713 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activat ...

at cold temperatures that proved that parity is not conserved in beta decay. This surprising result overturned long-held assumptions about parity and the weak force. In recognition of their theoretical work, Lee and Yang were awarded the
Nobel Prize for Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
in 1957. However Wu, who was female, was not awarded the Nobel prize.

# β− decay

In  decay, the
weak interaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
converts an
atomic nucleus The atomic nucleus is the small, dense region consisting of s and s at the center of an , discovered in 1911 by based on the 1909 . After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickl ...
into a nucleus with
atomic number The atomic number or proton number (symbol ''Z'') of a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. ...
increased by one, while emitting an electron () and an electron
antineutrino A neutrino ( or ) (denoted by the Greek letter Nu (letter), ) is a fermion (an elementary particle with spin-1/2, spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electric charge, electri ...
().  decay generally occurs in neutron-rich nuclei. The generic equation is: : → + + where and are the
mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of s and s (together known as s) in an . It is approximately equal to the of the expre ...
and
atomic number The atomic number or proton number (symbol ''Z'') of a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. ...
of the decaying nucleus, and X and X′ are the initial and final elements, respectively. Another example is when the
free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...

() decays by  decay into a proton (): : → + + . At the
fundamental Fundamental may refer to: * Foundation of reality * Fundamental frequency, as in music or phonetics, often referred to as simply a "fundamental" * Fundamentalism, the belief in, and usually the strict adherence to, the simple or "fundamental" ideas ...
level (as depicted in the
Feynman diagram In theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict List of natural phenomena, natural phenomena. This is in ...
on the right), this is caused by the conversion of the negatively charged (−  e) down quark to the positively charged (+ e) up quark by emission of a ; the boson subsequently decays into an electron and an electron antineutrino: : → + + .

# β+ decay

In  decay, or positron emission, the weak interaction converts an atomic nucleus into a nucleus with atomic number decreased by one, while emitting a positron () and an
electron neutrino The electron neutrino () is an elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fu ...

().  decay generally occurs in proton-rich nuclei. The generic equation is: : → + + This may be considered as the decay of a proton inside the nucleus to a neutron: :p → n + + However,  decay cannot occur in an isolated proton because it requires energy, due to the
mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value ...
of the neutron being greater than the mass of the proton.  decay can only happen inside nuclei when the daughter nucleus has a greater
binding energy In physics and chemistry, binding energy is the smallest amount of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of know ...
(and therefore a lower total energy) than the mother nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron and a neutrino and into the kinetic energy of these particles. This process is opposite to negative beta decay, in that the weak interaction converts a proton into a neutron by converting an up quark into a down quark resulting in the emission of a or the absorption of a . When a boson is emitted, it decays into a
positron The positron or antielectron is the antiparticle s (left) and antiparticles (right). From top to bottom; electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is ...

and an
electron neutrino The electron neutrino () is an elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fu ...

: : → + + .

# Electron capture (K-capture)

In all cases where  decay (positron emission) of a nucleus is allowed energetically, so too is
electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom An atom is the smallest unit of ordinary matter In classical physics ...

allowed. This is a process during which a nucleus captures one of its atomic electrons, resulting in the emission of a neutrino: : + → + An example of electron capture is one of the decay modes of krypton-81 into bromine-81: : + → + All emitted neutrinos are of the same energy. In proton-rich nuclei where the energy difference between the initial and final states is less than ,  decay is not energetically possible, and electron capture is the sole decay mode. If the captured electron comes from the innermost shell of the atom, the K-shell, which has the highest probability to interact with the nucleus, the process is called K-capture. If it comes from the L-shell, the process is called L-capture, etc. Electron capture is a competing (simultaneous) decay process for all nuclei that can undergo β+ decay. The converse, however, is not true: electron capture is the ''only'' type of decay that is allowed in proton-rich nuclides that do not have sufficient energy to emit a positron and neutrino.

# Nuclear transmutation

If the proton and neutron are part of an
atomic nucleus The atomic nucleus is the small, dense region consisting of s and s at the center of an , discovered in 1911 by based on the 1909 . After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickl ...
, the above described decay processes transmute one chemical element into another. For example: : Beta decay does not change the number () of
nucleon In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s in the nucleus, but changes only its
charge Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * Charge (David Ford album), ''Charge'' (David Ford album) * Charge (Machel Montano album), ''Charge'' (Mac ...
. Thus the set of all
nuclide A nuclide (or nucleide, from nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic c ...

s with the same  can be introduced; these ''isobaric'' nuclides may turn into each other via beta decay. For a given there is one that is most stable. It is said to be beta stable, because it presents a local minima of the mass excess: if such a nucleus has numbers, the neighbour nuclei and have higher mass excess and can beta decay into , but not vice versa. For all odd mass numbers , there is only one known beta-stable isobar. For even , there are up to three different beta-stable isobars experimentally known; for example, , , and are all beta-stable. There are about 350 known beta-decay stable nuclides.

## Competition of beta decay types

Usually unstable nuclides are clearly either "neutron rich" or "proton rich", with the former undergoing beta decay and the latter undergoing electron capture (or more rarely, due to the higher energy requirements, positron decay). However, in a few cases of odd-proton, odd-neutron radionuclides, it may be energetically favorable for the radionuclide to decay to an even-proton, even-neutron isobar either by undergoing beta-positive or beta-negative decay. An often-cited example is the single isotope (29 protons, 35 neutrons), which illustrates three types of beta decay in competition. Copper-64 has a half-life of about 12.7 hours. This isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay. This particular nuclide (though not all nuclides in this situation) is almost equally likely to decay through proton decay by
positron emission Positron emission, beta plus decay, or β+ decay is a subtype of radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic ...
(18%) or
electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom An atom is the smallest unit of ordinary matter In classical physics ...

(43%) to , as it is through neutron decay by electron emission (39%) to .

## Stability of naturally occurring nuclides

Most naturally occurring nuclides on earth are beta stable. Those that are not have
half-lives Half-life (symbol ''t''1⁄2) is the time required for a quantity to reduce to half of its initial value. The term is commonly used in nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies ...
ranging from under a second to periods of time significantly greater than the
age of the universe In physical cosmology Physical cosmology is a branch of cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the chro ...
. One common example of a long-lived isotope is the odd-proton odd-neutron nuclide , which undergoes all three types of beta decay (, and electron capture) with a half-life of .

# Conservation rules for beta decay

## Baryon number is conserved

:$B=\frac$ where : $n_q$ is the number of constituent quarks, and : $n_$ is the number of constituent antiquarks. Beta decay just changes
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

to
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

or, in the case of positive beta decay (
electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom An atom is the smallest unit of ordinary matter In classical physics ...

)
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

to
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

so the number of individual
quarks A quark () is a type of elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fundamen ...

doesn't change. It is only the baryon flavor that changes, here labelled as the
isospin In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time ...
. Up and down
quarks A quark () is a type of elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fundamen ...

have total isospin $I=\frac$ and isospin projections :$I_\text=\begin \frac & \text \\ -\frac & \text \end$ All other quarks have . In general :$I_\text=\frac \left(n_\text - n_\text\right)$

## Lepton number is conserved

::$L \equiv n_ - n_$ so all leptons have assigned a value of +1, antileptons −1, and non-leptonic particles 0. :$\begin & \text & \rightarrow & \text & + & \text^- & + & \bar_\text \\ L: & 0 &=& 0 & + & 1 & - & 1 \end$

## Angular momentum

For allowed decays, the net orbital angular momentum is zero, hence only spin quantum numbers are considered. The electron and antineutrino are
fermions In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the ...
, spin-1/2 objects, therefore they may couple to total $S=1$ (parallel) or $S=0$ (anti-parallel). For forbidden decays, orbital angular momentum must also be taken into consideration.

# Energy release

The value is defined as the total energy released in a given nuclear decay. In beta decay, is therefore also the sum of the kinetic energies of the emitted beta particle, neutrino, and recoiling nucleus. (Because of the large mass of the nucleus compared to that of the beta particle and neutrino, the kinetic energy of the recoiling nucleus can generally be neglected.) Beta particles can therefore be emitted with any
kinetic energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...
ranging from 0 to . A typical is around 1
MeV In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "P ...
, but can range from a few
keV In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular suc ...
to a few tens of MeV. Since the
rest mass The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** ...
of the electron is 511 keV, the most energetic beta particles are
ultrarelativisticIn physics, a particle is called ultrarelativistic when its speed is very close to the speed of light . The expression for the relativistic energy of a particle with rest mass and momentum is given by :E^2 = m^2 c^4 + p^2 c^2. The energy of an u ...
, with speeds very close to the
speed of light The speed of light in vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
.

## β− decay

Consider the generic equation for beta decay : → + + . The value for this decay is :, where is the mass of the nucleus of the atom, is the mass of the electron, and is the mass of the electron antineutrino. In other words, the total energy released is the mass energy of the initial nucleus, minus the mass energy of the final nucleus, electron, and antineutrino. The mass of the nucleus is related to the standard atomic mass by :. That is, the total atomic mass is the mass of the nucleus, plus the mass of the electrons, minus the sum of all ''electron'' binding energies for the atom. This equation is rearranged to find , and is found similarly. Substituting these nuclear masses into the -value equation, while neglecting the nearly-zero antineutrino mass and the difference in electron binding energies, which is very small for high- atoms, we have : This energy is carried away as kinetic energy by the electron and neutrino. Because the reaction will proceed only when the  value is positive, β decay can occur when the mass of atom is greater than the mass of atom .

## β+ decay

The equations for β+ decay are similar, with the generic equation : → + + giving :. However, in this equation, the electron masses do not cancel, and we are left with : Because the reaction will proceed only when the  value is positive, β+ decay can occur when the mass of atom exceeds that of by at least twice the mass of the electron.

## Electron capture

The analogous calculation for electron capture must take into account the binding energy of the electrons. This is because the atom will be left in an excited state after capturing the electron, and the binding energy of the captured innermost electron is significant. Using the generic equation for electron capture : + → + we have :, which simplifies to :, where is the binding energy of the captured electron. Because the binding energy of the electron is much less than the mass of the electron, nuclei that can undergo β+ decay can always also undergo electron capture, but the reverse is not true.

# Beta emission spectrum

Beta decay can be considered as a Perturbation theory (quantum mechanics), perturbation as described in quantum mechanics, and thus Fermi's Golden Rule can be applied. This leads to an expression for the kinetic energy spectrum of emitted betas as follows: :$N\left(T\right)=C_L\left(T\right) F\left(Z,T\right) p E \left(Q-T\right)^2$ where is the kinetic energy, is a shape function that depends on the forbiddenness of the decay (it is constant for allowed decays), is the Fermi Function (see below) with ''Z'' the charge of the final-state nucleus, is the total energy, is the momentum, and is the Q value (nuclear science), Q value of the decay. The kinetic energy of the emitted neutrino is given approximately by minus the kinetic energy of the beta. As an example, the beta decay spectrum of 210Bi (originally called RaE) is shown to the right.

## Fermi function

The Fermi function that appears in the beta spectrum formula accounts for the Coulomb attraction / repulsion between the emitted beta and the final state nucleus. Approximating the associated wavefunctions to be spherically symmetric, the Fermi function can be analytically calculated to be: :$F\left(Z,T\right)=\frac \left(2 p \rho\right)^ e^ , \Gamma\left(S+i \eta\right), ^2,$ where is the final momentum, Γ the Gamma function, and (if is the fine-structure constant and the radius of the final state nucleus) , ( for electrons, for positrons), and . For non-relativistic betas (), this expression can be approximated by: :$F\left(Z,T\right) \approx \frac.$ Other approximations can be found in the literature.

## Kurie plot

A Kurie plot (also known as a Fermi–Kurie plot) is a graph used in studying beta decay developed by Franz N. D. Kurie, in which the square root of the number of beta particles whose momenta (or energy) lie within a certain narrow range, divided by the Fermi function, is plotted against beta-particle energy. It is a straight line for allowed transitions and some forbidden transitions, in accord with the Fermi beta-decay theory. The energy-axis (x-axis) intercept of a Kurie plot corresponds to the maximum energy imparted to the electron/positron (the decay's  value). With a Kurie plot one can find the limit on the effective mass of a neutrino.

# Helicity (polarization) of neutrinos, electrons and positrons emitted in beta decay

After the discovery of parity non-conservation (see Beta decay#Non-conservation of parity, History), it was found that, in beta decay, electrons are emitted mostly with negative Chirality (physics), helicity, i.e., they move, naively speaking, like left-handed screws driven into a material (they have negative longitudinal Spin polarization, polarization). Conversely, positrons have mostly positive helicity, i.e., they move like right-handed screws. Neutrinos (emitted in positron decay) have negative helicity, while antineutrinos (emitted in electron decay) have positive helicity. The higher the energy of the particles, the higher their polarization.

# Types of beta decay transitions

Beta decays can be classified according to the angular momentum (Angular momentum operator,  value) and total spin (Spin (physics),  value) of the emitted radiation. Since total angular momentum must be conserved, including orbital and spin angular momentum, beta decay occurs by a variety of quantum state transitions to various nuclear angular momentum or spin states, known as "Fermi" or "Gamow–Teller" transitions. When beta decay particles carry no angular momentum (), the decay is referred to as "allowed", otherwise it is "forbidden". Other decay modes, which are rare, are known as bound state decay and double beta decay.

## Fermi transitions

A Fermi transition is a beta decay in which the spins of the emitted electron (positron) and anti-neutrino (neutrino) couple to total spin $S=0$, leading to an angular momentum change $\Delta J=0$ between the initial and final states of the nucleus (assuming an allowed transition). In the non-relativistic limit, the nuclear part of the operator for a Fermi transition is given by :$\mathcal_=G_\sum_ \hat_$ with $G_V$ the weak vector coupling constant, $\tau_$ the
isospin In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time ...
Ladder operator, raising and lowering operators, and $a$ running over all protons and neutrons in the nucleus.

## Gamow–Teller transitions

A Gamow–Teller transition is a beta decay in which the spins of the emitted electron (positron) and anti-neutrino (neutrino) couple to total spin $S=1$, leading to an angular momentum change $\Delta J=0,\pm 1$ between the initial and final states of the nucleus (assuming an allowed transition). In this case, the nuclear part of the operator is given by :$\mathcal_=G_\sum_ \hat_\hat_$ with $G_$ the weak axial-vector coupling constant, and $\sigma$ the Pauli matrices, spin Pauli matrices, which can produce a spin-flip in the decaying nucleon.

## Forbidden transitions

When , the decay is referred to as "forbidden transition, forbidden". Nuclear selection rules require high  values to be accompanied by changes in
nuclear spin In nuclear physics, atomic physics, and nuclear chemistry, the nuclear shell model is a nuclear model, model of the atomic nucleus which uses the Pauli exclusion principle to describe the structure of the nucleus in terms of energy levels. The f ...
() and Parity (physics), parity (π). The selection rules for the th forbidden transitions are: :$\Delta J=-L-1, L, L+1; \Delta \pi=\left(-1\right)^L,$ where or corresponds to no parity change or parity change, respectively. The special case of a transition between isobaric analogue states, where the structure of the final state is very similar to the structure of the initial state, is referred to as "superallowed" for beta decay, and proceeds very quickly. The following table lists the Δ and Δπ values for the first few values of :

# Rare decay modes

## Bound-state β− decay

A very small minority of free neutron decays (about four per million) are so-called "two-body decays", in which the proton, electron and antineutrino are produced, but the electron fails to gain the 13.6 eV energy necessary to escape the proton, and therefore simply remains bound to it, as a neutral hydrogen atom. In this type of beta decay, in essence all of the neutron decay energy is carried off by the antineutrino. For fully ionized atoms (bare nuclei), it is possible in likewise manner for electrons to fail to escape the atom, and to be emitted from the nucleus into low-lying atomic bound states (orbitals). This cannot occur for neutral atoms with low-lying bound states which are already filled by electrons. Bound-state β decays were predicted by Raymond Daudel, Daudel, Jean, and Lecoin in 1947, and the phenomenon in fully ionized atoms was first observed for 163Dy66+ in 1992 by Jung et al. of the Darmstadt Heavy-Ion Research group. Although neutral 163Dy is a stable isotope, the fully ionized 163Dy66+ undergoes β decay into the K and L shells with a half-life of 47 days. Another possibility is that a fully ionized atom undergoes greatly accelerated β decay, as observed for 187Re by Bosch et al., also at Darmstadt. Neutral 187Re does undergo β decay with a half-life of 41.6 × 109 years, but for fully ionized 187Re75+ this is shortened to only 32.9 years. For comparison the variation of decay rates of other nuclear processes due to chemical environment is radioactive decay#Changing rates, less than 1%.

## Double beta decay

Some nuclei can undergo double beta decay (ββ decay) where the charge of the nucleus changes by two units. Double beta decay is difficult to study, as the process has an extremely long half-life. In nuclei for which both β decay and ββ decay are possible, the rarer ββ decay process is effectively impossible to observe. However, in nuclei where β decay is forbidden but ββ decay is allowed, the process can be seen and a half-life measured. Thus, ββ decay is usually studied only for beta stable nuclei. Like single beta decay, double beta decay does not change ; thus, at least one of the nuclides with some given has to be stable with regard to both single and double beta decay. "Ordinary" double beta decay results in the emission of two electrons and two antineutrinos. If neutrinos are Majorana particles (i.e., they are their own antiparticles), then a decay known as neutrinoless double beta decay will occur. Most neutrino physicists believe that neutrinoless double beta decay has never been observed.

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