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of stability stable nuclide radioactive decay alpha α beta β 2β 0v β k l capture isomeric gamma γ internal conversion spontaneous fission cluster decay neutron emission proton emission decay energy decay chain decay product radiogenic nuclide nuclear fission spontaneous products pair breaking photofission capturing processes electron 2 neutron s r proton p rp high energy processes spallation by cosmic ray photodisintegration nucleosynthesis and nuclear astrophysics nuclear fusion processes stellar big bang supernova nuclides primordial cosmogenic artificial high energy nuclear physics quark gluon plasma rhic lhc scientists alvarez becquerel bethe blackett a bohr n bohr chadwick cockcroft ir curie fr curie pi curie skłodowska curie davisson fermi hahn jensen lawrence mayer meitner oliphant oppenheimer proca purcell rabi rutherford soddy strassmann świątecki szilárd teller thomson walton wigner physics portal category v t e in nuclear physics beta decay β decay is a type of radioactive decay in which an atomic nucleus emits a beta particle fast energetic electron or positron transforming into an isobar of that nuclide for example beta decay of a neutron transforms it into a proton by the emission of an electron accompanied by an antineutrino or conversely a proton is converted into a neutron by the emission of a positron with a neutrino in what is called positron emission 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 the binding energies of all existing nuclides form what is called the nuclear band or valley of stability 1 for either electron or positron emission to be energetically possible the energy release or q value must be positive beta decay is a consequence of the weak force which is characterized by relatively long decay times nucleons are composed of up quarks and down quarks 2 and the weak force allows a quark to change its flavour by means of a virtual w boson 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 is sometimes included as a type of beta decay 3 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 edit energy level diagrams depicting nuclear transitions involving beta decay 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 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 4 beta decay conserves a quantum number known as the lepton number or the number of electrons and their associated neutrinos other leptons are the muon and tau 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 into nitrogen 14 with a half life of about 5 700 years 14 6 c 14 7 n e ν e in this form of decay the original element becomes a new chemical element in a process known as nuclear transmutation this new element has an unchanged mass number a but an atomic number z that is increased by one as in all nuclear decays the decaying element in this case 14 6 c is known as the parent nuclide while the resulting element in this case 14 7 n is known as the daughter nuclide another example is the decay of hydrogen 3 tritium into helium 3 with a half life of about 12 3 years 3 1 h 3 2 he e ν e an example of positron emission β decay is the decay of magnesium 23 into sodium 23 with a half life of about 11 3 s 23 12 mg 23 11 na e ν e β decay also results in nuclear transmutation with the daughter element having an atomic number that is decreased by one a beta spectrum showing a typical division of energy between electron and antineutrino 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 210 bi is shown in this example the total decay energy is 1 16 mev so the antineutrino has the remaining energy 1 16 mev 0 40 mev 0 76 mev 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 edit discovery and initial characterization edit radioactivity was discovered in 1896 by henri becquerel in uranium and subsequently observed by marie and pierre curie in thorium and in the newly discovered elements polonium and radium 5 54 in 1899 ernest rutherford 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 identified a still more penetrating type of radiation which rutherford termed gamma rays 6 in 1900 becquerel measured the mass to charge ratio m e for beta particles by the method of j j thomson used to study cathode rays and identify the electron he found that m e for a beta particle is the same as for thomson s electron and therefore suggested that the beta particle is in fact an electron 7 in 1901 rutherford and frederick soddy 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 independently proposed their radioactive displacement law which states that beta i e β emission from one element produces another element one place to the right in the periodic table while alpha emission produces an element two places to the left 8 9 neutrinos edit the study of beta decay provided the first physical evidence for the existence of the neutrino in both alpha and gamma decay the resulting alpha or gamma particle has a narrow energy distribution 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 and otto hahn 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 10 in 1914 james chadwick used a magnetic spectrometer with one of hans geiger s new counters to make more accurate measurements which showed that the spectrum was continuous 10 11 the results which appeared to be in contradiction to the law of conservation of energy were validated by means of calorimetric measurements in 1929 by lise meitner and wilhelm orthmann 12 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 13 for beta decay however the observed electrons had a broad distribution of energies 14 160 a second problem is related to the conservation of angular momentum dubious discuss molecular band spectra showed that the nuclear spin of nitrogen 14 is 1 i e equal to the reduced planck constant and more generally that the spin is integral for nuclei of even mass number and half integral for nuclei of odd mass number this was later explained by the proton neutron model of the nucleus 13 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 citation needed from 1920 to 1927 charles drummond ellis along with chadwick and colleagues further established that the beta decay spectrum is continuous in 1933 ellis and nevill mott obtained strong evidence that the beta spectrum has an effective upper bound in energy niels bohr had suggested that the beta spectrum could be explained if conservation of energy was true only in a statistical sense thus this principle might be violated in any given decay 13 27 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 citation needed in a famous letter written in 1930 wolfgang pauli 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 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 the american physicists clyde cowan and frederick reines in the cowan reines neutrino experiment 15 the properties of neutrinos were with a few minor modifications as predicted by pauli and fermi β decay and electron capture edit in 1934 frédéric and irène joliot curie bombarded aluminium with alpha particles to effect the nuclear reaction 4 2 he 27 13 al 30 15 p 1 0 n and observed that the product isotope 30 15 p emits a positron identical to those found in cosmic rays discovered by carl david anderson in 1932 this was the first example of β decay positron emission which they termed artificial radioactivity since 30 15 p 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 in 1935 16 the theory of electron capture was first discussed by gian carlo wick in a 1934 paper and then developed by hideki yukawa and others k electron capture was first observed in 1937 by luis alvarez in the nuclide 48 v 17 18 19 alvarez went on to study electron capture in 67 ga and other nuclides 17 20 21 non conservation of parity edit main article wu experiment in 1956 tsung dao lee and chen ning yang 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 22 later that year chien shiung wu and coworkers showed experimentally that an asymmetrical beta emission from 60 co proved that parity is not conserved in beta decay 23 24 25 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 in 1957 26 however wu who was female was not awarded the nobel prize 27 β decay edit the leading order feynman diagram for β decay of a neutron into a proton electron and electron antineutrino via a virtual w boson for higher order diagrams see 28 29 in β decay the weak interaction converts an atomic nucleus into a nucleus with atomic number increased by one while emitting an electron e and an electron antineutrino ν e β decay generally occurs in neutron rich nuclei 30 the generic equation is a z x a z 1 x e ν e 1 where a and z are the mass number and atomic number of the decaying nucleus and x and x are the initial and final elements respectively another example is when the free neutron 1 0 n decays by β decay into a proton p n p e ν e at the fundamental level as depicted in the feynman diagram on the right this is caused by the conversion of the negatively charged 1 3 e down quark to the positively charged 2 3 e up quark which is promoted by a virtual w boson the w boson subsequently decays into an electron and an electron antineutrino d u e ν e β decay edit main article positron emission the leading order feynman diagram for β decay of a proton into a neutron positron and electron neutrino via an intermediate virtual w boson in β decay or positron emission the weak interaction converts an atomic nucleus into a nucleus with atomic number decreased by one while emitting a positron e and an electron neutrino ν e β decay generally occurs in proton rich nuclei the generic equation is a z x a z 1 x e ν e 1 this may be considered as the decay of a proton inside the nucleus to a neutron p n e ν e 1 however β decay cannot occur in an isolated proton because it requires energy due to the mass of the neutron being greater than the mass of the proton β decay can only happen inside nuclei when the daughter nucleus has a greater binding energy 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 w or the absorption of a w when a w boson is emitted it decays into a positron and an electron neutrino u d e ν e electron capture k capture l capture edit main article electron capture the leading order feynman diagrams for electron capture decay an electron interacts with an up quark in the nucleus via a w boson to create a down quark and electron neutrino two diagrams comprise the leading second order though as a virtual particle the type and charge of the w boson is indisti...
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