Editing Neutron (section)

== Discovery ==
{{Main|Discovery of the neutron}}
The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century, leading ultimately to the atomic bomb in 1945. The name derives from the [[Latin]] root for ''neutralis'' (neuter) and the [[Greek language|Greek]] suffix ''-on'' (a suffix used in the names of subatomic particles, e.g. ''electron'' and ''proton'')<ref>{{cite book|doi= 10.1007/978-3-540-78801-0_3 |title= Wolfgang Pauli |series= Sources in the History of Mathematics and Physical Sciences |year= 1985 |isbn= 978-3-540-13609-5 |volume= 6 |pages= 105–144|last1= Pauli |first1= Wolfgang |last2= Hermann |first2= A. |last3= Meyenn |first3= K.v |last4= Weisskopff |first4= V.F |chapter= Das Jahr 1932 die Entdeckung des Neutrons }}</ref><ref name="CPT">{{cite book|editor1-last= Hendry |editor1-first= John |title= Cambridge Physics in the Thirties |publisher= Adam Hilger |location=Bristol |date= 1984 |isbn= 978-0852747612}}</ref>
and references to the word ''neutron'' can be found in the literature as early as 1899 in connection with discussion on the nature of the atom.<ref name="FeathHist"/>

In the 1911 [[Rutherford model]], the atom consisted of a small positively charged massive nucleus surrounded by a much larger cloud of negatively charged electrons. In 1920, [[Ernest Rutherford]] suggested that the nucleus consisted of positive protons and neutrally charged particles, suggested to be a proton and an electron bound in some way.<ref name="BakLec">
{{cite journal
|author= Rutherford, E.
|year= 1920
|title= Nuclear Constitution of Atoms
|journal= [[Proceedings of the Royal Society A]]|volume= 97 |pages= 374–400
|doi= 10.1098/rspa.1920.0040
|bibcode= 1920RSPSA..97..374R
|issue= 686|doi-access= free
}}</ref> Electrons were assumed to reside within the nucleus because it was known that [[beta particle|beta radiation]] consisted of electrons emitted from the nucleus.<ref name="BakLec"/><ref name="FeatherOnChadwick-1974">{{Cite journal |last=Feather |first=N. |date=1974-11-01 |title=Chadwick's neutron |url=https://www.tandfonline.com/doi/abs/10.1080/00107517408217489 |journal=Contemporary Physics |volume=15 |issue=6 |pages=565–572 |doi=10.1080/00107517408217489 |bibcode=1974ConPh..15..565F |issn=0010-7514}}</ref> About the time Rutherford suggested the neutral proton-electron composite, several other publications appeared making similar suggestions, and in 1921 the American chemist [[William Draper Harkins|W. D. Harkins]] first named the hypothetical particle a "neutron".<ref>{{cite journal|last1=Harkins|first1=William|title=The constitution and stability of atomic nuclei. (A contribution to the subject of inorganic evolution.)|journal=Philos. Mag.|date=1921|volume=42|issue=249|page=305|doi=10.1080/14786442108633770}}</ref><ref name="FeathHist">{{cite journal
|author= Feather, N.
|year= 1960
|title= A history of neutrons and nuclei. Part 1
|journal= [[Contemporary Physics]]|volume= 1 |pages= 191–203
|issue= 3
|doi=10.1080/00107516008202611|bibcode= 1960ConPh...1..191F
}}</ref> 

Throughout the 1920s, physicists assumed that the atomic nucleus was composed of protons and "nuclear electrons".<ref>{{cite journal|doi=10.1063/1.2995181|title=The idea of the neutrino|year=1978|last1=Brown|first1=Laurie M.|journal=[[Physics Today]]|volume=31|issue=9|pages=23–28|bibcode= 1978PhT....31i..23B|s2cid=121080564 }}</ref><ref name=FK>Friedlander G., Kennedy J.W. and Miller J.M. (1964) ''Nuclear and Radiochemistry'' (2nd edition), Wiley, pp. 22–23 and 38–39</ref> Beginning in 1928, it became clear that this model was inconsistent with the then-new quantum theory. Confined to a volume the size of an nucleus, an electron consistent with the [[Heisenberg uncertainty relation]] of quantum mechanics would have an energy exceeding the binding energy of the nucleus.<ref name="Stuewer">{{cite book |last=Stuewer |first=Roger H. |editor1-last=French |editor1-first=A.P. |editor2-last=Kennedy |editor2-first=P.J. |title=Niels Bohr: A Centenary Volume |publisher=Harvard University Press |date=1985 |pages=[https://archive.org/details/nielsbohrcentena00fren/page/197 197–220] |chapter=Niels Bohr and Nuclear Physics |isbn=978-0674624160 |chapter-url=https://archive.org/details/nielsbohrcentena00fren/page/197 }}</ref><ref name="Pais">{{cite book |last=Pais |first=Abraham |date=1986 |title=Inward Bound |url=https://archive.org/details/inwardboundofmat00pais_0 |url-access=registration |location=Oxford |publisher=Oxford University Press |page=[https://archive.org/details/inwardboundofmat00pais_0/page/299 299] |isbn= 978-0198519973}}</ref> The energy was so large that according to the [[Klein paradox]],<ref>{{cite journal|last1=Klein|first1=O.|title=Die Reflexion von Elektronen an einem Potentialsprung nach der relativistischen Dynamik von Dirac|journal=[[Zeitschrift für Physik]]|volume=53|pages=157–165|year=1929|doi=10.1007/BF01339716|bibcode= 1929ZPhy...53..157K|issue=3–4|s2cid=121771000}}</ref> discovered by [[Oskar Klein]] in 1928, an electron would escape the confinement of a nucleus.<ref name="Stuewer"/> Furthermore, the observed properties of atoms and molecules were inconsistent with the nuclear spin expected from the proton–electron hypothesis. Protons and electrons both carry an intrinsic spin of {{sfrac|2}}''ħ'', and the isotopes of the same species were found to have either integer or fractional spin. By the hypothesis, isotopes would be composed of the same number of protons, but differing numbers of neutral bound proton+electron "particles". This physical picture was a contradiction, since there is no way to arrange the spins of an electron and a proton in a bound state to get a fractional spin.<ref name="Stuewer"/>

In 1931, [[Walther Bothe]] and [[Herbert Becker (physicist)|Herbert Becker]] found that if [[alpha particle]] radiation from [[polonium]] fell on [[beryllium]], [[boron]], or [[lithium]], an unusually penetrating radiation was produced. The radiation was not influenced by an electric field, so Bothe and Becker assumed it was [[gamma radiation]].<ref>{{cite journal |doi= 10.1007/BF01390908 |title= Künstliche Erregung von Kern-γ-Strahlen |trans-title= Artificial excitation of nuclear γ-radiation |year= 1930 |last1= Bothe |first1= W. |last2= Becker |first2= H. |journal= [[Zeitschrift für Physik]]|volume= 66 |issue= 5–6 |pages= 289–306|bibcode= 1930ZPhy...66..289B|s2cid= 122888356 }}</ref><ref>{{cite journal |doi= 10.1007/BF01336726 |title= Die in Bor und Beryllium erregten γ-Strahlen |trans-title= Γ-rays excited in boron and beryllium|year= 1932 |last1= Becker |first1= H. |last2= Bothe |first2= W. |journal= [[Zeitschrift für Physik]]|volume= 76 |issue= 7–8 |pages= 421–438|bibcode= 1932ZPhy...76..421B|s2cid= 121188471 }}</ref> The following year [[Irène Joliot-Curie]] and [[Frédéric Joliot-Curie]] in Paris showed that if this "gamma" radiation fell on [[paraffin wax|paraffin]], or any other [[hydrogen]]-containing compound, it ejected protons of very high energy.<ref>{{cite journal |author1=Joliot-Curie, Irène |author2=Joliot, Frédéric |name-list-style=amp |url=http://visualiseur.bnf.fr/CadresFenetre?O=NUMM-3147&I=1236 |title=Émission de protons de grande vitesse par les substances hydrogénées sous l'influence des rayons γ très pénétrants |trans-title=Emission of high-speed protons by hydrogenated substances under the influence of very penetrating γ-rays |volume=194 |page=273 |year=1932 |journal=[[Comptes Rendus]] |access-date=2012-06-16 |archive-date=2022-03-04 |archive-url=https://web.archive.org/web/20220304054301/http://visualiseur.bnf.fr/CadresFenetre?O=NUMM-3147&I=1236 |url-status=live }}</ref> Neither Rutherford nor [[James Chadwick]] at the [[Cavendish Laboratory]] in [[Cambridge]] were convinced by the gamma ray interpretation.<ref>{{cite book
|last=Brown |first=Andrew
|year=1997
|title=The Neutron and the Bomb: A Biography of Sir James Chadwick
|publisher=[[Oxford University Press]]
|isbn=978-0-19-853992-6
}}</ref> Chadwick quickly performed a series of experiments that showed that the new radiation consisted of uncharged particles with about the same mass as the proton.<ref name="Chad1932">{{cite journal|last=Chadwick|first=James|year=1932|title=Possible Existence of a Neutron|journal=[[Nature (journal)|Nature]]|volume=129|page=312|doi=10.1038/129312a0|bibcode=1932Natur.129Q.312C|issue=3252|s2cid=4076465|url=https://web.mit.edu/22.54/resources/Chadwick.pdf|access-date=2023-12-13|archive-date=2024-02-08|archive-url=https://web.archive.org/web/20240208065844/https://web.mit.edu/22.54/resources/Chadwick.pdf|url-status=live}}</ref><ref name="AM">{{cite web |url=http://www.aip.org/history/exhibits/rutherford/sections/atop-physics-wave.html |title=Atop the Physics Wave: Rutherford Back in Cambridge, 1919–1937 |publisher=American Institute of Physics |date=2011–2014 |website=Rutherford's Nuclear World |access-date=19 August 2014 |archive-date=21 October 2014 |archive-url=https://web.archive.org/web/20141021094704/http://www.aip.org/history/exhibits/rutherford/sections/atop-physics-wave.html |url-status=dead }}</ref><ref>{{cite journal |doi= 10.1098/rspa.1933.0152 |title= Bakerian Lecture. The Neutron |year= 1933 |last1= Chadwick |first1= J. |journal= [[Proceedings of the Royal Society A]]|volume= 142 |issue= 846 |pages= 1–25|bibcode= 1933RSPSA.142....1C|doi-access= free }}</ref> These properties matched Rutherford's hypothesized neutron. Chadwick won the 1935 [[Nobel Prize in Physics]] for this discovery.<ref name="1935 Nobel Prize in Physics"/>

[[File:Blausen 0342 ElectronEnergyLevels.png|thumb|360px|Models depicting the nucleus and electron energy levels in hydrogen, helium, lithium, and neon atoms. In reality, the diameter of the nucleus is about 100,000 times smaller than the diameter of the atom.]]

Models for an atomic nucleus consisting of protons and neutrons were quickly developed by [[Werner Heisenberg]]<ref>{{cite journal |last=Heisenberg |first=W. |title=Über den Bau der Atomkerne. I |journal=[[Zeitschrift für Physik]]|volume=77 |issue=1–2 |pages=1–11 |year=1932 |doi=10.1007/BF01342433|bibcode=1932ZPhy...77....1H |s2cid=186218053 }}</ref><ref>{{cite journal |last=Heisenberg |first=W. |title=Über den Bau der Atomkerne. II |journal=[[Zeitschrift für Physik]]|volume=78 |pages=156–164 |year=1932 |doi=10.1007/BF01337585 |issue=3–4|bibcode=1932ZPhy...78..156H |s2cid=186221789 }}</ref><ref>{{cite journal |last=Heisenberg |first=W. |title=Über den Bau der Atomkerne. III |journal=[[Zeitschrift für Physik]]|volume=80 |pages=587–596 |year=1933 |doi=10.1007/BF01335696 |issue=9–10|bibcode=1933ZPhy...80..587H |s2cid=126422047 }}</ref> and others.<ref>{{Cite journal |doi = 10.1038/129798d0|title = The Neutron Hypothesis|journal = [[Nature (journal)|Nature]] |volume = 129|issue = 3265|pages = 798|year = 1932|last1 = Iwanenko|first1 = D.|bibcode = 1932Natur.129..798I|s2cid = 4096734|doi-access = free}}</ref><ref>Miller A.I. (1995) ''Early Quantum Electrodynamics: A Sourcebook'', Cambridge University Press, Cambridge, {{ISBN|0521568919}}, pp. 84–88.</ref> The proton–neutron model explained the puzzle of nuclear spins. The origins of beta radiation were explained by [[Enrico Fermi]] in 1934 by the [[Fermi's interaction|process of beta decay]], in which the neutron decays to a proton by ''creating'' an electron and a then-undiscovered neutrino.<ref name="Wilson">{{cite journal |last=Wilson |first=Fred L. |title=Fermi's Theory of Beta Decay |journal=[[American Journal of Physics]]|volume=36 |issue=12 |pages=1150–1160 |year=1968 |bibcode= 1968AmJPh..36.1150W|doi= 10.1119/1.1974382}}</ref> In 1935, Chadwick and his doctoral student [[Maurice Goldhaber]] reported the first accurate measurement of the mass of the neutron.<ref>{{cite journal |author1-last=Chadwick |author1-first=J. |author2-last=Goldhaber |author2-first=M.|title=A nuclear photo-effect: disintegration of the diplon by gamma rays |journal=[[Nature (journal)|Nature]] |volume=134 |issue=3381 |pages=237–238 |year=1934 |doi=10.1038/134237a0|bibcode=1934Natur.134..237C|s2cid=4137231 |doi-access=free }}</ref><ref>{{cite journal |author1-last=Chadwick |author1-first=J. |author2-last=Goldhaber |author2-first=M.|title=A nuclear photoelectric effect |journal=[[Proceedings of the Royal Society of London A]] |volume=151 |issue=873 |pages=479–493 |year=1935|doi= 10.1098/rspa.1935.0162|bibcode=1935RSPSA.151..479C|doi-access=free }}</ref>

By 1934, Fermi had bombarded heavier elements with neutrons to induce radioactivity in elements of high atomic number. In 1938, Fermi received the Nobel Prize in Physics "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of [[nuclear reaction]]s brought about by slow neutrons".<ref name="Cooper">{{cite book |last=Cooper |first=Dan |year=1999 |title=Enrico Fermi: And the Revolutions in Modern physics |location=New York |publisher=Oxford University Press |url=https://books.google.com/books?id=JK94sqLFsNsC |isbn=978-0-19-511762-2 |oclc=39508200}}</ref> In December 1938 [[Otto Hahn]], [[Lise Meitner]], and [[Fritz Strassmann]] discovered [[nuclear fission]], or the fractionation of uranium nuclei into lighter elements, induced by neutron bombardment.<ref>{{cite journal|doi=10.1007/BF01488241|author1=Hahn, O. |author2=Strassmann, F. |name-list-style=amp |title=Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle |trans-title=On the detection and characteristics of the alkaline earth metals formed by irradiation of uranium with neutrons|journal=[[Die Naturwissenschaften]]|volume=27|issue=1|pages=11–15|year=1939|bibcode= 1939NW.....27...11H|s2cid=5920336 }}</ref><ref name="Hahn_1958">{{cite journal |last1= Hahn |first1= O. |title= The Discovery of Fission |doi= 10.1038/scientificamerican0258-76 |journal= [[Scientific American]]|volume= 198 |issue= 2 |pages= 76–84 |year= 1958 |bibcode= 1958SciAm.198b..76H }}</ref><ref>{{cite book |author=Rife, Patricia |title=Lise Meitner and the dawn of the nuclear age |url=https://archive.org/details/lisemeitnerdawno0000rife |url-access=registration |publisher=Birkhäuser |location=Basel, Switzerland |year=1999 |isbn=978-0-8176-3732-3 }}</ref><ref>{{cite journal |author1-last=Hahn |author1-first=O. |author2-last=Strassmann |author2-first=F.|title=Proof of the Formation of Active Isotopes of Barium from Uranium and Thorium Irradiated with Neutrons; Proof of the Existence of More Active Fragments Produced by Uranium Fission |journal=[[Die Naturwissenschaften]]|volume=27 |issue=6 |pages=89–95 |date=10 February 1939|bibcode=1939NW.....27...89H |doi=10.1007/BF01488988|s2cid=33512939 }}</ref> In 1945 Hahn received the 1944 [[Nobel Prize in Chemistry]] "for his discovery of the fission of heavy atomic nuclei".<ref name=Nobel1944>{{cite web |title=The Nobel Prize in Chemistry 1944 |publisher=[[Nobel Foundation]] |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1944/index.html |access-date=2007-12-17 |archive-date=2018-12-26 |archive-url=https://web.archive.org/web/20181226102253/https://www.nobelprize.org/prizes/chemistry/1944/summary/ |url-status=live }}</ref><ref>{{cite book |author=Bernstein, Jeremy |title=Hitler's uranium club: the secret recordings at Farm Hall |publisher=Copernicus |location=New York |year=2001 |isbn=978-0-387-95089-1 |page=[https://archive.org/details/hitlersuraniumcl00bern/page/281 281] |url=https://archive.org/details/hitlersuraniumcl00bern/page/281 |author-link=Jeremy Bernstein }}</ref><ref name="NF-1944press">{{cite web |title=The Nobel Prize in Chemistry 1944: Presentation Speech |publisher=Nobel Foundation |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1944/press.html |access-date=2008-01-03 |archive-date=2007-10-25 |archive-url=https://web.archive.org/web/20071025011452/http://nobelprize.org/nobel_prizes/chemistry/laureates/1944/press.html |url-status=live }}</ref>

The discovery of nuclear fission would lead to the development of nuclear power and the atomic bomb by the end of World War II. It was quickly realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, in a cascade known as a nuclear chain reaction.<ref name="Pais1993"/>{{rp|460–461}}<ref name="ENW"/> These events and findings led Fermi to construct the [[Chicago Pile-1]] at the University of Chicago in 1942, the first self-sustaining [[nuclear reactor]].<ref name="Segre">{{cite book|author=Emilio Segrè|title=Enrico Fermi: Physicist|year=1970|publisher=University of Chicago|isbn=0-226-74472-8}}</ref> Just three years later the [[Manhattan Project]] was able to test the first [[nuclear weapon|atomic bomb]], the [[Trinity (nuclear test)|Trinity nuclear test]] in July 1945.<ref name="Segre"/>