Editing Beta decay (section)

==History==

===Discovery and initial characterization===
Radioactivity was discovered in 1896 by [[Henri Becquerel]] in [[uranium]], and subsequently observed by [[Marie Curie|Marie]] and [[Pierre Curie]] in [[thorium]] and in the newly discovered elements [[polonium]] and [[radium]]. 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 identified as a fundamentally new type in 1903 and termed [[gamma ray]]s. Alpha, beta, and gamma are the first three letters of the [[Greek alphabet]].

In 1900, Becquerel measured the [[mass-to-charge ratio]] ({{math|''m''/''e''}}) for beta particles by the method of [[J.J.&nbsp;Thomson]] used to study cathode rays and identify the electron. He found that {{math|''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.<ref name="Handbook">{{cite book|last1=L'Annunziata|first1=Michael|title=Handbook of Radioactivity Analysis|date=2012|publisher=Elsevier Inc.|page=3|edition=Third|url=https://books.google.com/books?id=S4FrejzJy0cC&pg=PA3|access-date=4 October 2017|isbn=978-0-12-384874-1}}</ref>

In 1901, Rutherford and [[Frederick Soddy]] showed that alpha and beta radioactivity involves the [[Nuclear transmutation|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 of Fajans and Soddy|radioactive displacement law]], which states that beta (i.e., {{SubatomicParticle|Beta-}})&nbsp;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.

===Neutrinos===

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 [[Frequency distribution|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.<ref name="Jensen">{{cite book
 |last1=Jensen |first1=C.
 |year=2000
 |title=Controversy and Consensus: Nuclear Beta Decay 1911-1934
 |url=https://www.springer.com/birkhauser/physics/book/978-3-7643-5313-1
 |publisher=[[Birkhäuser Verlag]]
 |isbn=978-3-7643-5313-1
}}</ref> In 1914, [[James Chadwick]] used a magnetic [[spectrometer]] with one of [[Hans Geiger|Hans Geiger's]] new [[Geiger counter|counters]] to make more accurate measurements which showed that the spectrum was continuous.<ref name="Jensen" /><ref>{{cite journal |last=Chadwick |first=J. |year=1914 |title=Intensitätsverteilung im magnetischen Spektren der β-Strahlen von Radium B + C |journal=[[Verhandlungen der Deutschen Physikalischen Gesellschaft]] |language=de |volume=16 |pages=383–391}}</ref> 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]].<ref>{{Cite journal |last1=Meitner |first1=Lise |last2=Orthmann |first2=Wilhelm |date=1930-03-01 |title=Über eine absolute Bestimmung der Energie der primären β-Strahlen von Radium E |url=https://link.springer.com/article/10.1007/BF01339819 |journal=Zeitschrift für Physik |language=de |volume=60 |issue=3 |pages=143–155 |doi=10.1007/BF01339819 |bibcode=1930ZPhy...60..143M |issn=0044-3328}}</ref> 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.<ref name=Brown>{{cite journal
 |last1=Brown |first1=L. M.
 |year=1978
 |title=The idea of the neutrino
 |journal=[[Physics Today]]
 |volume=31 |issue=9 |pages=23–8
 |bibcode=1978PhT....31i..23B
 |doi=10.1063/1.2995181
}}</ref> 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]]. 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 [[Discovery of the neutron#Proton–neutron model of the nucleus|proton-neutron model of the nucleus]].<ref name=Brown/> 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 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 [[Laws of science|principle]] might be violated in any given decay.<ref name=Brown/>{{rp|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.

In a [[Electron neutrino#Pauli's letter|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 [[Fermi's interaction|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]].<ref>{{cite journal
 |last1=Cowan |first1=C. L. Jr.
 |last2=Reines |first2=F.
 |last3=Harrison |first3=F. B.
 |last4=Kruse |first4=H. W.
 |last5=McGuire |first5=A. D.
 |year=1956
 |title=Detection of the Free Neutrino: a Confirmation
 |journal=[[Science (journal)|Science]]
 |volume=124 |issue=3212 |pages=103–104
 |bibcode=1956Sci...124..103C
 |doi=10.1126/science.124.3212.103
 |pmid=17796274
}}</ref> The properties of neutrinos were (with a few minor modifications) as predicted by Pauli and Fermi.

==={{SubatomicParticle|Beta+}}&nbsp;decay and electron capture===
In 1934, [[Frédéric Joliot-Curie|Frédéric]] and [[Irène Joliot-Curie]] bombarded aluminium with alpha particles to effect the nuclear reaction {{nuclide|Helium|4}}&nbsp;+&nbsp;{{nuclide|Aluminium|27}}&nbsp;→ {{nuclide|Phosphorus|30}}&nbsp;+&nbsp;{{nuclide|neutronium|1}}, and observed that the product isotope {{nuclide|Phosphorus|30}} emits a positron identical to those found in cosmic rays (discovered by [[Carl David Anderson]] in 1932). This was the first example of {{SubatomicParticle|Beta+}}&nbsp;decay ([[positron emission]]), which they termed [[artificial radioactivity]] since {{nuclide|Phosphorus|30}} 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.<ref>{{Cite web|url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1935/|title=The Nobel Prize in Chemistry 1935|website=www.nobelprize.org|access-date=2018-04-25}}</ref>

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 Walter Alvarez|Luis Alvarez]], in the nuclide <sup>48</sup>V.<ref name=k>{{cite book
 |last=Segré
 |first=E.
 |year=1987
 |chapter=K-Electron Capture by Nuclei
 |editor1-last=Trower
 |editor1-first=P. W.
 |title=Discovering Alvarez: Selected Works of Luis W. Alvarez
 |pages=[https://archive.org/details/discoveringalvar0000alva/page/11 11–12]
 |publisher=[[University of Chicago Press]]
 |isbn=978-0-226-81304-2
 |chapter-url=https://archive.org/details/discoveringalvar0000alva/page/11
 }}</ref><ref>{{cite web |title=The Nobel Prize in Physics 1968: Luis Alvarez |url=http://nobelprize.org/nobel_prizes/physics/laureates/1968/alvarez-bio.html |publisher=[[The Nobel Foundation]] |access-date=2009-10-07 }}</ref><ref>{{cite journal
 |last1=Alvarez |first1=L. W.
 |year=1937
 |title=Nuclear K Electron Capture
 |journal=[[Physical Review]]
 |volume=52 |issue=2 |pages=134–135
 |bibcode=1937PhRv...52..134A
 |doi=10.1103/PhysRev.52.134
}}</ref> Alvarez went on to study electron capture in <sup>67</sup>Ga and other nuclides.<ref name=k /><ref>{{cite journal
 |last1=Alvarez |first1=L. W.
 |year=1938
 |title=Electron Capture and Internal Conversion in Gallium 67
 |journal=[[Physical Review]]
 |volume=53 |issue=7 |page=606
 |bibcode=1938PhRv...53..606A
 |doi=10.1103/PhysRev.53.606
}}</ref><ref>{{cite journal
 |last1=Alvarez|first1=L. W.
 |year=1938
 |title=The Capture of Orbital Electrons by Nuclei
 |journal=[[Physical Review]]
 |volume=54 |issue=7 |pages=486–497
 |bibcode=1938PhRv...54..486A
 |doi=10.1103/PhysRev.54.486
}}</ref>

===Non-conservation of parity===
In 1956, [[Tsung-Dao Lee]] and [[Chen Ning Yang]] noticed that there was no evidence that [[parity (physics)|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.<ref>{{cite journal
 |last1=Lee |first1=T. D.
 |last2=Yang |first2=C. N.
 |year=1956
 |title=Question of Parity Conservation in Weak Interactions
 |journal=[[Physical Review]]
 |volume=104 |issue=1
 |pages=254–258
 |bibcode=1956PhRv..104..254L
 |doi=10.1103/PhysRev.104.254
|doi-access=free
 }}</ref> Later that year, [[Chien-Shiung Wu]] and coworkers conducted the [[Wu experiment]] showing an asymmetrical beta decay of [[cobalt-60|{{SimpleNuclide|cobalt|60}}]] at cold temperatures that proved that parity is not conserved in beta decay.<ref name="Wu1957" >{{cite journal
 |last1=Wu |first1=C.-S.
 |last2=Ambler |first2=E.
 |last3=Hayward |first3=R. W.
 |last4=Hoppes |first4=D. D.
 |last5=Hudson |first5=R. P.
 |year=1957
 |title=Experimental Test of Parity Conservation in Beta Decay
 |journal=[[Physical Review]]
 |volume=105 |issue=4
 |pages=1413–1415
 |bibcode=1957PhRv..105.1413W
 |doi=10.1103/PhysRev.105.1413
|doi-access=free
 }}</ref><ref>{{cite web|url=http://blogs.scientificamerican.com/guest-blog/2013/10/15/channeling-ada-lovelace-chien-shiung-wu-courageous-hero-of-physics/|title=Channeling Ada Lovelace: Chien-Shiung Wu, Courageous Hero of Physics| first=Maia|last=Weinstock | website=scientificamerican.com}}</ref> 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.<ref>{{cite web |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1957/ | title=The Nobel Prize in Physics 1957 |publisher=[[The Nobel Foundation]] |access-date=March 24, 2015}}</ref> However Wu, who was female, was not awarded the Nobel prize.<ref>{{cite web|last=Webb| first=Richard| title=Chien-Shiung Wu {{!}} Particle physicist denied a Nobel prize| website=newscientist.com| url=https://www.newscientist.com/people/chien-shiung-wu/| access-date=February 18, 2025}}</ref>