View source for Neutron

{{Short description|Subatomic particle with no charge}}
{{About|the subatomic particle|other uses|Neutron (disambiguation)}}
{{Distinguish|Neuron|Neutrino}}
{{pp-move}}
{{Infobox particle
| classification  = [[Baryon]]
| name            = Neutron
| image           = [[File:Quark_structure_neutron.svg|250px]]
| caption         = The [[quark]] content of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by [[gluons]].
| group           = [[Hadron]]
| num_types       =
| composition     = 1 [[up quark]], 2 [[down quark]]s
| statistics      = [[Fermionic]]
| generation      =
| interaction     = [[Gravity]], [[weak interaction|weak]], [[strong Interaction|strong]], [[electromagnetic interaction|electromagnetic]]
| antiparticle    = [[Antineutron]]
| theorized       = [[Ernest Rutherford]]<ref name="chemed.chem.purdue.edu">[http://chemed.chem.purdue.edu/genchem/history/rutherford.html Ernest Rutherford] {{Webarchive|url=https://web.archive.org/web/20110803083616/http://chemed.chem.purdue.edu/genchem/history/rutherford.html |date=2011-08-03 }}. Chemed.chem.purdue.edu. Retrieved on 2012-08-16.</ref> (1920)
| discovered      = [[James Chadwick]]<ref name="1935 Nobel Prize in Physics">[http://nobelprize.org/nobel_prizes/physics/laureates/1935/ 1935 Nobel Prize in Physics] {{Webarchive|url=https://web.archive.org/web/20171003030602/http://nobelprize.org/nobel_prizes/physics/laureates/1935/ |date=2017-10-03 }}. Nobelprize.org. Retrieved on 2012-08-16.</ref> (1932)
| symbol          = {{SubatomicParticle|Neutron}}, {{SubatomicParticle|Neutron0}}, {{SubatomicParticle|Nucleon0}}
| mass            ={{val|1.67492749804|(95)|e=-27|ul=kg}}<ref name="CODATA2018">"2018 CODATA recommended values" https://physics.nist.gov/cuu/Constants/index.html {{Webarchive|url=https://web.archive.org/web/20180122130230/https://physics.nist.gov/cuu/Constants/index.html |date=2018-01-22 }}</ref><br /><!--
   -->{{val|939.56542052|(54)|ul=MeV/c2}}<ref name="CODATA2018"/><br /><!--
   -->{{val|1.00866491588|(49)|ul=Da}}<ref name="2014 CODATA">Mohr, P.J.; Taylor, B.N. and Newell, D.B. (2014), [http://physics.nist.gov/constants "The 2014 CODATA Recommended Values of the Fundamental Physical Constants"] {{Webarchive|url=https://web.archive.org/web/20131009032049/http://physics.nist.gov/constants |date=2013-10-09 }} (Web Version 7.0). The database was developed by J. Baker, M. Douma, and [[Svetlana Kotochigova|S. Kotochigova]]. (2014). National Institute of Standards and Technology, Gaithersburg, Maryland 20899.</ref>
| mean_lifetime   = {{val|878.4|(5)|u=s}} ([[Free neutron decay|free]])<ref name="PDG Live: 2020 Review of Particle Physics">{{cite web |last1=Zyla |first1=P. A. |title=n MEAN LIFE |url=https://pdglive.lbl.gov/DataBlock.action?node=S017T |website=PDG Live: 2020 Review of Particle Physics |publisher=Particle Data Group |access-date=25 February 2021 |date=2020 |archive-date=17 January 2021 |archive-url=https://web.archive.org/web/20210117164505/https://pdglive.lbl.gov/DataBlock.action?node=S017T |url-status=live }}</ref>
| electric_charge = {{val|0|u=[[elementary charge|''e'']]}}<br />
{{val|-2|±8|e=-22|u=[[elementary charge|''e'']]}} (experimental limits)<ref name="PDGLIVE"/>
| electric_dipole_moment  = < {{val|1.8|e=-26|u=''e''⋅cm}} (experimental upper limit)
| electric_polarizability = {{val|1.16|(15)|e=-3|u=fm<sup>3</sup>}}
| magnetic_moment = [[Neutron magnetic moment|{{val|-0.96623650|(23)|e=-26}}]]&nbsp;[[Joule|J]]·[[Tesla (unit)|T]]<sup>−1</sup><ref name="2014 CODATA"/><br /><!--
   -->{{val|-1.04187563|(25)|e=-3|u=[[Bohr magneton|''μ''<sub>B</sub>]]}}<ref name="2014 CODATA"/><br /><!--
   -->{{val|-1.91304273|(45)|u=[[Nuclear magneton|''μ''<sub>N</sub>]]}}<ref name="2014 CODATA"/>
| magnetic_polarizability = {{val|3.7|(20)|e=-4|u=fm<sup>3</sup>}}
| spin            = {{sfrac|1|2}}&nbsp;[[reduced Planck constant|''ħ'']]
| isospin         = −{{sfrac|1|2}}
| parity          = +1
| condensed_symmetries    = ''[[Isospin|I]]''(''[[Total angular momentum|J]]''<sup>''[[Intrinsic parity|P]]''</sup>)&nbsp;=&nbsp;{{sfrac|1|2}}({{sfrac|1|2}}<sup>+</sup>)
}}

The '''neutron''' is a [[subatomic particle]], symbol {{SubatomicParticle|Neutron}} or {{SubatomicParticle|Neutron0}}, that has no electric charge, and a [[mass]] slightly greater than that of a [[proton]]. Protons and neutrons constitute the [[atomic nucleus|nuclei]] of [[atom]]s. Since protons and neutrons behave similarly within the nucleus, they are both referred to as [[nucleon]]s. Nucleons have a mass of approximately one atomic mass unit, or [[dalton (unit)|dalton]] (symbol: Da).  Their properties and interactions are described by [[nuclear physics]].  Protons and neutrons are not [[elementary particle]]s; each is composed of three [[quark]]s.

The [[chemical properties]] of an atom are mostly determined by the configuration of [[electron]]s that orbit the atom's heavy nucleus. The electron configuration is determined by the charge of the nucleus, which is determined by the number of protons, or [[atomic number]]. The number of neutrons is the [[neutron number]]. Neutrons do not affect the electron configuration.

Atoms of a [[chemical element]] that differ only in neutron number are called [[isotopes]]. For example, [[carbon]], with atomic number 6, has an abundant isotope [[carbon-12]] with 6 neutrons and a rare isotope [[carbon-13]] with 7 neutrons.  Some elements occur in nature with only one [[stable nuclide|stable isotope]], such as [[fluorine]]. Other elements occur with many stable isotopes, such as [[tin]] with ten stable isotopes, or with no stable isotope, such as [[technetium]].

The properties of an atomic nucleus depend on both atomic and neutron numbers. With their positive charge, the protons within the nucleus are repelled by the long-range [[electromagnetic force]], but the much stronger, but short-range, [[nuclear force]] binds the nucleons closely together. Neutrons are required for the stability of nuclei, with the exception of the single-proton [[hydrogen]] nucleus. Neutrons are produced copiously in [[nuclear fission]] and [[nuclear fusion|fusion]]. They are a primary contributor to the [[nucleosynthesis]] of chemical elements within [[star]]s through fission, fusion, and [[neutron capture]] processes.

The neutron is essential to the production of nuclear power. In the decade after the [[Discovery of the neutron|neutron was discovered]] by [[James Chadwick]] in 1932, neutrons were used to induce many different types of [[nuclear transmutation]]s. With the discovery of [[nuclear fission]] in 1938, 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]]. These events and findings led to the first self-sustaining [[nuclear reactor]] ([[Chicago Pile-1]], 1942) and the first [[nuclear weapon]] ([[Trinity (nuclear test)|Trinity]], 1945).

Dedicated [[neutron source]]s like [[neutron generator]]s, [[research reactor]]s and [[spallation|spallation sources]] produce free neutrons for use in [[irradiation]] and in [[neutron scattering]] experiments. A free neutron spontaneously decays to a proton, an [[electron]], and an [[antineutrino]], with a [[Exponential decay#Mean lifetime|mean lifetime]] of about 15 minutes. Free neutrons do not directly ionize atoms, but they do indirectly cause [[ionizing radiation#Neutrons|ionizing radiation]], so they can be a biological hazard, depending on dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by [[cosmic ray showers]], and by the natural radioactivity of spontaneously fissionable elements in the [[Crust (geology)#Earth's crust|Earth's crust]].

== Neutrons in an atomic nucleus ==
{{Nuclear physics}}
An [[atomic nucleus]] is formed by a number of protons, ''Z'' (the [[atomic number]]), and a number of neutrons, ''N'' (the [[neutron number]]), bound together by the [[nuclear force]]. Protons and neutrons each have a mass of approximately one [[dalton (unit)|dalton]]. The atomic number determines the [[chemical element|chemical properties]] of the atom, and the neutron number determines the [[isotope]] or [[nuclide]].<ref name="ENW">{{Citation |editor1-last= Glasstone |editor1-first= Samuel |editor2-last= Dolan |editor2-first= Philip J. |title= The Effects of Nuclear Weapons |edition=3rd |publisher= U.S. Dept. of Defense and Energy Research and Development Administration, U.S. Government Printing Office |date= 1977 |isbn= 978-1-60322-016-3}}</ref>{{rp|4}} The terms isotope and nuclide are often used [[synonym]]ously, but they refer to chemical and nuclear properties, respectively.<ref name="ENW"/>{{rp|4}} Isotopes are nuclides with the same atomic number, but different neutron number. Nuclides with the same neutron number, but different atomic number, are called [[isotone]]s.<ref name="Brucer">{{Cite journal| last=Brucer| first=Marshall| year=1978| title=Nuclear Medicine Begins with a Boa Constrictor| journal=J. Nuclear Medicine| volume=19| issue=6| pages=581–598| pmid=351151| url=http://jnm.snmjournals.org/content/19/6/581.full.pdf| access-date=2024-05-01| archive-date=2019-05-09| archive-url=https://web.archive.org/web/20190509183540/http://jnm.snmjournals.org/content/19/6/581.full.pdf| url-status=live}}</ref> The [[atomic mass number]], ''A'', is equal to the sum of atomic and neutron numbers. Nuclides with the same atomic mass number, but different atomic and neutron numbers, are called [[isobar (nuclide)|isobars]].<ref name=Brucer/>  The mass of a nucleus is always slightly less than the sum of its proton and neutron masses: the difference in mass represents the [[Mass–energy equivalence|mass equivalent]] to nuclear binding energy, the energy which would need to be added to take the nucleus apart.<ref>{{Cite book |last=Giancoli |first=Douglas C. |url=https://archive.org/details/generalphysics00gian |title=General physics |date=1984 |publisher=Prentice-Hall |isbn=978-0-13-350884-0 |location=Englewood Cliffs, N.J |oclc=1033640549}}</ref>{{rp|822}}

The nucleus of the most common [[isotope]] of the [[hydrogen atom]] (with the [[chemical symbol]] <sup>1</sup>H) is a lone proton.<ref name="ENW"/>{{rp|20}} The nuclei of the heavy hydrogen isotopes [[deuterium]] (D or <sup>2</sup>H) and [[tritium]] (T or <sup>3</sup>H) contain one proton bound to one and two neutrons, respectively.<ref name="ENW"/>{{rp|20}} All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons. The most common nuclide of the common chemical element [[lead]], <sup>208</sup>Pb, has 82 protons and 126 neutrons, for example.<ref name="Stone">{{cite journal |last=Stone |first=R. |year=1997 |title=An Element of Stability |journal=[[Science (journal)|Science]] |volume=278 |issue=5338 |pages=571–572 |doi=10.1126/science.278.5338.571|bibcode=1997Sci...278..571S |s2cid=117946028 }}</ref> The [[table of nuclides]] comprises all the known nuclides. Even though it is not a chemical element, the neutron is included in this table.<ref>[http://www.nndc.bnl.gov/nudat2 Nudat 2] {{Webarchive|url=https://web.archive.org/web/20090817135107/http://www1.nndc.bnl.gov/nudat2/ |date=2009-08-17 }}. Nndc.bnl.gov. Retrieved on 2010-12-04.</ref>

[[File:Nuclear fission.svg|thumb|right|Nuclear fission caused by absorption of a neutron by uranium-235. The heavy nuclide fragments into lighter components and additional neutrons.]]

Protons and neutrons behave almost identically under the influence of the nuclear force within the nucleus. They are therefore both referred to collectively as [[nucleon]]s.<ref name="Nuc">{{Citation |author1-last= Thomas |author1-first= A.W. |author2-last= Weise |author2-first= W. |title= The Structure of the Nucleon |publisher= Wiley-WCH, Berlin |date= 2001 |isbn= 978-3-527-40297-7}}</ref>  The concept of [[isospin]], in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces.<ref name=Greiner>{{cite book|last1=Greiner|first1=W.|last2=Müller|first2=B.|author-link1=Walter Greiner|title=Quantum Mechanics: Symmetries|year=1994|edition=2nd|isbn=978-3540580805|publisher=Springer|url=https://archive.org/details/quantummechanics0001grei|url-access=registration|page=[https://archive.org/details/quantummechanics0001grei/page/279 279]}}</ref>{{rp|141}}

===Nuclear energy===
Because of the strength of the nuclear force at short distances, the nuclear [[binding energy|energy binding]] nucleons is many orders of magnitude greater than the electromagnetic energy binding electrons in atoms.<ref name="ENW"/>{{rp|4}} In [[nuclear fission]], the absorption of a neutron by some heavy nuclides (such as [[uranium-235]]) can cause the nuclide to become unstable and break into lighter nuclides and additional neutrons.<ref name="ENW"/> The positively charged light nuclides, or "fission fragments", then repel, releasing electromagnetic [[potential energy]].<ref name="Nuclear Energy">{{cite web |last1= |title=Nuclear Energy |url=http://electron6.phys.utk.edu/phys250/modules/module%205/nuclear_energy.htm |website=Physics 250: Modern Physics |publisher=The University of Tennessee Department of Physics and Astronomy |access-date=1 May 2024 |archive-date=20 February 2020 |archive-url=https://web.archive.org/web/20200220205637/http://electron6.phys.utk.edu/phys250/modules/module%205/nuclear_energy.htm |url-status=live }}</ref> If this reaction occurs within a mass of [[fissile material]], the additional neutrons cause additional fission events, inducing a cascade known as a [[nuclear chain reaction]].<ref name="ENW"/>{{rp|12–13}}  For a given mass of fissile material, such [[nuclear reaction]]s release energy that is approximately ten million times that from an equivalent mass of a conventional chemical [[explosive]].<ref name="ENW"/>{{rp|13}}<ref>A 0.57 kg mass of fissionable material, such as uranium-235, can release an amount of energy equivalent to 10 metric kilotons of TNT. Fissionable material therefore has an energy density approximately 10<sup>7</sup> greater than this conventional explosive.</ref> Ultimately, the ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible; most of the energy released from fission is the kinetic energy of the fission fragments.<ref name="Nuclear Energy"/><ref name="ENW"/>{{rp|12}}

[[File:Beta-minus Decay.svg|thumb|A [[schematic]] of the [[atomic nucleus|nucleus of an atom]] indicating {{SubatomicParticle|Beta-}} radiation, the emission of a fast electron from the nucleus. The decay also creates an antineutrino (omitted) and 
converts a neutron to a proton within the nucleus. <br/>
The '''inset''' shows beta decay of a free neutron; an electron and antineutrino are created in this process.]]

===Beta decay===
{{Main|Beta decay}}
Neutrons and protons within a nucleus behave similarly and can exchange their identities by similar reactions. These reactions are a form of [[radioactive decay]] known as [[beta decay]].<ref>{{Cite book |last1=Basdevant |first1=J.-L. |last2=Rich |first2=J. |last3=Spiro |first3=M. |year=2005 |title=Fundamentals in Nuclear Physics: From Nuclear Structure to Cosmology |publisher=[[Springer (publisher)|Springer]] |isbn=978-0-387-01672-6 }}</ref> Beta decay, in which neutrons decay to protons, or vice versa, is governed by the [[weak interaction|weak force]], and it requires the emission or absorption of electrons and neutrinos, or their antiparticles.<ref name=Loveland>{{cite book |last=Loveland |first=W. D. |year=2005 |title=Modern Nuclear Chemistry |url=https://books.google.com/books?id=ZAHJkrJlwbYC&pg=PA199 |page=199 |publisher=[[John Wiley & Sons|Wiley]] |isbn=978-0-471-11532-8 |access-date=2024-05-01 |archive-date=2024-05-01 |archive-url=https://web.archive.org/web/20240501202310/https://books.google.com/books?id=ZAHJkrJlwbYC&pg=PA199 |url-status=live }}</ref> The neutron and proton decay reactions are:
: {{math|{{SubatomicParticle|Neutron0}} → {{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron}} + {{SubatomicParticle|Electron antineutrino}}}}
where {{SubatomicParticle|Proton+}}, {{SubatomicParticle|Electron}}, and {{math|{{SubatomicParticle|Electron antineutrino}}}} denote the proton, electron and electron anti-[[neutrino]] decay products,<ref>[http://pdg.lbl.gov/2007/tables/bxxx.pdf Particle Data Group Summary Data Table on Baryons] {{Webarchive|url=https://web.archive.org/web/20110910125729/http://pdg.lbl.gov/2007/tables/bxxx.pdf |date=2011-09-10 }}. lbl.gov (2007). Retrieved on 2012-08-16.</ref> and
: {{math|{{SubatomicParticle|Proton+}} → {{SubatomicParticle|Neutron0}} + {{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron neutrino}}}}

where {{SubatomicParticle|Neutron0}}, {{SubatomicParticle|Positron}}, and {{math|{{SubatomicParticle|Electron neutrino}}}} denote the neutron, positron and electron neutrino decay products.

The electron and positron produced in these reactions are historically known as [[beta particles]], denoted β<sup>−</sup> or β<sup>+</sup> respectively, lending the name to the decay process.<ref name=Loveland/>  In these reactions, the original particle is not ''composed'' of the product particles; rather, the product particles are ''created'' at the instant of the reaction.<ref name="Pais1993">{{cite book|author=Abraham Pais|title=Niels Bohr's Times: In Physics, Philosophy, and Polity|url=https://archive.org/details/nielsbohrstimesi0000pais|url-access=registration|year=1991|publisher=Oxford University Press|isbn=0-19-852049-2}}</ref>{{rp|369–370}}

== The "free" neutron ==
{{main|Free neutron decay}}

"Free" neutrons or protons are nucleons that exist independently, free of any nucleus.

The free neutron has a mass of {{val|939565413.3|ul=eV/c2}}, or {{val|939.5654133|ul=MeV/c2}}. This mass is equal to {{val|1.674927471|e=-27|ul=kg}}, or {{val|1.00866491588|ul=Da}}.<ref name="2014 CODATA"/> The neutron has a mean-square [[radius]] of about {{val|0.8|e=-15|ul=m}}, or {{val|0.8|ul=fm}},<ref name="Povh">{{cite book |last1=Povh |first1=B. |last2=Rith |first2=K. |last3=Scholz |first3=C. |last4=Zetsche |first4=F. |title=Particles and Nuclei: An Introduction to the Physical Concepts |location=Berlin |publisher=Springer-Verlag |pages=73 |year=2002 |isbn=978-3-540-43823-6}}</ref> and it is a [[spin-½]] [[fermion]].<ref name=Basdevant2>
{{cite book
|author1=Basdevant, J.-L. |author2=Rich, J. |author3=Spiro, M. |year=2005
|title=Fundamentals in Nuclear Physics
|page=155
|publisher=[[Springer (publisher)|Springer]]
|isbn=978-0-387-01672-6
}}</ref> The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by [[electric field]]s, whereas the neutron is unaffected by electric fields.<ref name="Ari">{{cite journal |last1=Arimoto |first1=Y. |last2=Geltenbort |first2=S. |year=2012 |title=Demonstration of focusing by a neutron accelerator |journal=[[Physical Review A]] |volume=86 |issue=2 |pages=023843 |url=http://www.rri.kyoto-u.ac.jp/news-en/4964 |doi=10.1103/PhysRevA.86.023843 |access-date=May 9, 2015 |bibcode=2012PhRvA..86b3843A |display-authors=etal |url-access=subscription |archive-date=January 18, 2015 |archive-url=https://web.archive.org/web/20150118105137/http://www.rri.kyoto-u.ac.jp/news-en/4964 |url-status=live }}</ref> The neutron has a [[neutron magnetic moment|magnetic moment]], however, so it is influenced by [[magnetic field]]s.<ref name="Oku">{{cite journal |last1=Oku |first1=T. |last2=Suzuki |first2=J.|year=2007 |title=Highly polarized cold neutron beam obtained by using a quadrupole magnet |journal=[[Physica B]] |volume=397 |issue=1–2 |pages=188–191 |doi=10.1016/j.physb.2007.02.055 |bibcode = 2007PhyB..397..188O |display-authors=etal}}</ref> The specific properties of the neutron are described below in the [[#Intrinsic properties|Intrinsic properties section]].

Outside the nucleus, free neutrons undergo beta decay with a [[mean lifetime]] of about 14 minutes, 38 seconds,<ref>R.L. Workman et al. (Particle Data Group), Prog.Theor.Exp.Phys. 2022, 083C01 (2022) and 2023 update. https://pdg.lbl.gov/2023/listings/rpp2023-list-n.pdf {{Webarchive|url=https://web.archive.org/web/20230925091703/https://pdg.lbl.gov/2023/listings/rpp2023-list-n.pdf |date=2023-09-25 }}. Gives value of 878.4 ± 0.5s; half-life is not given.</ref> corresponding to a [[half-life]] of about 10 minutes, 11 s. The mass of the neutron is greater than that of the proton by {{val|1.29332||ul=MeV/c2}},<ref name=ByrneOverview/> hence the neutron's mass provides energy sufficient for the creation of the proton, electron, and anti-neutrino. In the decay process, the proton, electron, and electron anti-neutrino conserve the energy, charge, and [[lepton number]] of the neutron.<ref name=LifetimeReview2011>{{Cite journal |last1=Wietfeldt |first1=Fred E. |last2=Greene |first2=Geoffrey L. |date=2011-11-03 |title=Colloquium : The neutron lifetime |url=https://link.aps.org/doi/10.1103/RevModPhys.83.1173 |journal=Reviews of Modern Physics |language=en |volume=83 |issue=4 |pages=1173–1192 |doi=10.1103/RevModPhys.83.1173 |bibcode=2011RvMP...83.1173W |issn=0034-6861}}</ref> The electron can acquire a kinetic energy up to {{val|0.782|0.013|u=MeV}}.<ref name=ByrneOverview>{{Cite book |last=Byrne |first=J |title=Quark-Mixing, CKM-Unitarity |date=2003-12-09 |editor-last=Abele |editor-first=Hartmut |chapter=An Overview of Neutron Decay |arxiv=hep-ph/0312124 |editor-last2=Mund |editor-first2=Daniela }}</ref>

Still unexplained, different experimental methods for measuring the neutron's lifetime, the "bottle" and "beam" methods, produce different values for it.<ref name=Wolchover-2018-02-13-Quanta>{{cite web |last=Wolchover |first=Natalie |date=13 February 2018 |title=Neutron lifetime puzzle deepens, but no dark matter seen |magazine=[[Quanta Magazine]] |access-date=31 July 2018 |url=https://www.quantamagazine.org/neutron-lifetime-puzzle-deepens-but-no-dark-matter-seen-20180213/ |archive-date=30 July 2018 |archive-url=https://web.archive.org/web/20180730080707/https://www.quantamagazine.org/neutron-lifetime-puzzle-deepens-but-no-dark-matter-seen-20180213/ |url-status=live }}</ref> The "bottle" method employs "cold" neutrons trapped in a bottle, while the "beam" method employs energetic neutrons in a particle beam. The measurements by the two methods have not been converging with time. The lifetime from the bottle method is presently 877.75 s<ref>{{Cite web|date=2021-10-13|title=How Long Does a Neutron Live?|url=https://www.caltech.edu/about/news/how-long-does-a-neutron-live|access-date=2021-10-14|website=California Institute of Technology|language=en|archive-date=2021-10-13|archive-url=https://web.archive.org/web/20211013190528/https://www.caltech.edu/about/news/how-long-does-a-neutron-live|url-status=live}}</ref><ref name=Gonzalez-2021>{{Cite journal|last1=UCNτ Collaboration|last2=Gonzalez|first2=F. M.|last3=Fries|first3=E. M.|last4=Cude-Woods|first4=C.|last5=Bailey|first5=T.|last6=Blatnik|first6=M.|last7=Broussard|first7=L. J.|last8=Callahan|first8=N. B.|last9=Choi|first9=J. H.|last10=Clayton|first10=S. M.|last11=Currie|first11=S. A.|date=2021-10-13|title=Improved Neutron Lifetime Measurement with UCNτ|url=https://par.nsf.gov/servlets/purl/10304438|journal=Physical Review Letters|volume=127|issue=16|page=162501|arxiv=2106.10375|doi=10.1103/PhysRevLett.127.162501|pmid=34723594|bibcode=2021PhRvL.127p2501G|s2cid=235490073|access-date=2024-04-01|archive-date=2024-04-01|archive-url=https://web.archive.org/web/20240401134040/https://par.nsf.gov/servlets/purl/10304438|url-status=live}}</ref> which is 10 seconds below the value from the beam method of 887.7 s<ref>{{Cite journal|last=Anonymous|date=2013-11-27|title=Discrepancy in Neutron Lifetime Still Unresolved|url=https://physics.aps.org/articles/v6/s150|journal=Physics|language=en|volume=6|doi=10.1103/Physics.6.s150|bibcode=2013PhyOJ...6S.150.|access-date=2024-04-01|archive-date=2023-08-18|archive-url=https://web.archive.org/web/20230818212949/https://physics.aps.org/articles/v6/s150|url-status=live}}</ref>

A small fraction (about one per thousand) of free neutrons decay with the same products, but add an extra particle in the form of an emitted gamma ray:<ref name=Fisher-2005>{{Cite journal|last1=Fisher|first1=BM|display-authors=etal |title=Detecting the Radiative Decay Mode of the Neutron|journal=J. Res. Natl. Inst. Stand. Technol.|volume=110|year=2005|issue=4 |pages=421–425|doi=10.6028/jres.110.064|pmid=27308161 |pmc=4852828 }}</ref>

: {{math|{{SubatomicParticle|Neutron0}} → {{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron}} + {{SubatomicParticle|Electron antineutrino}} + {{SubatomicParticle|gamma}}}}

Called a "radiative decay mode" of the neutron, the gamma ray may be thought of as resulting from an "internal [[bremsstrahlung]]" that arises from the electromagnetic interaction of the emitted beta particle with the proton.<ref name=Fisher-2005/>

A smaller fraction (about four per million) of free neutrons decay in so-called "two-body (neutron) decays", in which a proton, electron and antineutrino are produced as usual, but the electron fails to gain the {{val|13.6|ul=eV}} necessary energy to escape the proton (the [[ionization energy]] of [[hydrogen]]), and therefore simply remains bound to it, forming a neutral [[hydrogen atom]] (one of the "two bodies"). In this type of free neutron decay, almost all of the neutron [[decay energy]] is carried off by the antineutrino (the other "body"). (The hydrogen atom recoils with a speed of only about (decay energy)/(hydrogen rest energy) times the speed of light, or {{val|250|ul=km/s}}.)

== Neutrons and protons bound in a nucleus ==
{{Main|Atomic nucleus|Nuclear physics}}
{{See also|Valley of stability|Beta-decay stable isobars|Neutron emission}}
Neutrons are a necessary constituent of any atomic nucleus that contains more than one proton. As a result of their positive charges, interacting protons have a mutual [[electromagnetic interaction|electromagnetic repulsion]] that is stronger than their attractive [[nuclear force|nuclear interaction]], so proton-only nuclei are unstable (see [[diproton]] and [[neutron–proton ratio]]).<ref>[http://ansnuclearcafe.org/2011/10/19/pioneers102011/ Sir James Chadwick's Discovery of Neutrons] {{Webarchive|url=https://web.archive.org/web/20111026055138/http://ansnuclearcafe.org/2011/10/19/pioneers102011/ |date=2011-10-26 }}. ANS Nuclear Cafe. Retrieved on 2012-08-16.</ref> Neutrons bind with protons and one another in the nucleus via the [[nuclear force]], effectively moderating the repulsive forces between the protons and stabilizing the nucleus.<ref name="Pais1993"/>{{rp|461}} Heavy nuclei carry a large positive charge, hence they require "extra" neutrons to be stable.<ref name="Pais1993"/>{{rp|461}}

While a free neutron is unstable and a free proton is stable, within nuclei neutrons are often stable and protons are sometimes unstable. When bound within a nucleus, nucleons can [[radioactive decay|decay]] by the beta decay process. The neutrons and protons in a nucleus form a [[introduction to quantum mechanics|quantum mechanical system]] according to the [[nuclear shell model]]. Protons and neutrons of a [[nuclide]] are organized into discrete hierarchical [[energy level]]s with unique [[quantum numbers]]. Nucleon decay within a nucleus can occur if allowed by basic energy conservation and quantum mechanical constraints. The decay products, that is, the emitted particles, carry away the energy excess as a nucleon falls from one quantum state to one with less energy, while the neutron (or proton) changes to a proton (or neutron).

For a neutron to decay, the resulting proton requires an available state at lower energy than the initial neutron state. In stable nuclei the possible lower energy states are all filled, meaning each state is occupied by a pair of protons, one with [[Spin (physics)|spin]] up, another with spin down. When all available proton states are filled, the [[Pauli exclusion principle]] disallows the decay of a neutron to a proton.<ref name="Byrne_NNM">Byrne, J. ''Neutrons, Nuclei, and Matter'', Dover Publications, Mineola, New York, 2011, {{ISBN|0486482383}}</ref>{{rp|§3.3}} The situation is similar to electrons of an atom, where electrons that occupy distinct [[atomic orbital]]s are prevented by the exclusion principle from decaying to lower, already-occupied, energy states.<ref name="Byrne_NNM"/>{{rp|§3.3}} The [[stability of matter]] is a consequence of these constraints.<ref name="DysonI">{{Cite journal|last1=Dyson| first1=F. J. |last2=Lenard| first2=A. |title= Stability of Matter. I|journal= Journal of Mathematical Physics|volume=8|pages=423–434|year=1967| issue=3 | doi=10.1063/1.1705209 | bibcode=1967JMP.....8..423D }}</ref><ref name="DysonII">{{Cite journal|last1=Dyson| first1=F. J. |last2=Lenard| first2=A. |title= Stability of Matter. II|journal= Journal of Mathematical Physics|volume=9|pages=698–711|year=1968| issue=5 | doi=10.1063/1.1664631 | bibcode=1968JMP.....9..698L }}</ref><ref name="Ball">{{Cite journal | last=Ball | first=Philip | date=17 February 2021 | title=Why is matter stable? | journal=Chemistry World | url=https://www.chemistryworld.com/opinion/why-is-matter-stable/4013146.article | access-date=8 May 2024 | archive-date=8 May 2024 | archive-url=https://web.archive.org/web/20240508191013/https://www.chemistryworld.com/opinion/why-is-matter-stable/4013146.article | url-status=live }}</ref>

The decay of a neutron within a nuclide is illustrated by the decay of the [[carbon]] isotope [[carbon-14]], which has 6 protons and 8 neutrons. With its excess of neutrons, this isotope decays by beta decay to [[nitrogen-14]] (7 protons, 7 neutrons), a process with a half-life of about {{val|5730|u=years|fmt=commas}}.<ref name="McKie">{{Cite journal| last=McKie | first=Robin | date=10 August 2019 | title= 'Perhaps the most important isotope': how carbon-14 revolutionised science| journal=The Guardian|url=https://www.theguardian.com/science/2019/aug/10/most-important-isotope-how-carbon-14-revolutionised-science|access-date=8 May 2024}}</ref> Nitrogen-14 is stable.<ref name="Nova">{{Cite journal | date=10 August 2019 | title= Close Encounters (of the Cosmic Kind)| journal=PBS: Nova Online|url=https://www.pbs.org/wgbh/nova/first/radiocarbonce.html|access-date=8 May 2024}}</ref>

"Beta decay" reactions can also occur by the capture of a [[lepton]] by the nucleon.  The transformation of a proton to a neutron inside of a nucleus is possible through [[electron capture]]:<ref>{{cite book |last1=Cottingham |first1=W.N. |last2=Greenwood  |first2=D.A. |year=1986|title=An introduction to nuclear physics|page=[https://archive.org/details/introductiontonu0000cott/page/40 40] |publisher=[[Cambridge University Press]] |isbn=978-0-521-31960-7 |url=https://archive.org/details/introductiontonu0000cott/page/40 }}</ref>
:{{math|{{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron}} → {{SubatomicParticle|Neutron0}} + {{SubatomicParticle|Electron neutrino}}}}
A rarer reaction, [[inverse beta decay]], involves the capture of a neutrino by a nucleon.<ref>{{cite journal |year=1997 |title=The Reines-Cowan Experiments: Detecting the Poltergeist |url=http://library.lanl.gov/cgi-bin/getfile?25-02.pdf |journal=[[Los Alamos Science]] |volume=25 |page=3 |access-date=2024-05-09 |archive-date=2013-02-21 |archive-url=https://web.archive.org/web/20130221123519/http://library.lanl.gov/cgi-bin/getfile?25-02.pdf |url-status=live }}</ref>
Rarer still, positron capture by neutrons can occur in the high-temperature environment of stars.<ref name="Fowler">{{cite journal|last=Fowler|first=W.A.|title=The quest for the origin of the elements|journal=Science|volume=226|year=1984|issue=4677 |pages=922–935 |doi=10.1126/science.226.4677.922 |pmid=17737334 |bibcode=1984Sci...226..922F }}</ref>

=== Competition of beta decay types ===
Three types of beta decay in competition are illustrated by the single isotope [[copper-64]] (29 protons, 35 neutrons), which has a half-life of about 12.7 hours.<ref name="Cu-64">[http://www.lnhb.fr/nuclides/Cu-64_tables.pdf Atomic and Nuclear Data: Chapter 12 Cu-64 ] {{Webarchive|url=https://web.archive.org/web/20240502181344/http://www.lnhb.fr/nuclides/Cu-64_tables.pdf |date=2024-05-02 }} Laboratoire National Henri Becquerel, 2011. Retrieved on 2024-05-01.</ref> This isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay.<ref name="Copper-64">{{cite web |last1=Gilbert |first1=Thomas R. |title=Problem 20: Copper-64 is an unusual radionuclide |url=https://www.vaia.com/en-us/textbooks/chemistry/chemistry-the-science-in-context-5-edition/chapter-19/problem-20-copper-64-is-an-unusual-radionuclide-in-that-it-m/ |website=Chemistry The Science in Context |publisher=Vaia |access-date=2 May 2024 |archive-date=2 May 2024 |archive-url=https://web.archive.org/web/20240502182148/https://www.vaia.com/en-us/textbooks/chemistry/chemistry-the-science-in-context-5-edition/chapter-19/problem-20-copper-64-is-an-unusual-radionuclide-in-that-it-m/ |url-status=live }}</ref> This particular nuclide is almost equally likely to undergo proton decay (by [[positron emission]], 18% or by [[electron capture]], 43%; both forming [[Isotopes of nickel|{{SimpleNuclide|Nickel|64}}]]) or neutron decay (by electron emission, 39%; forming [[Isotopes of zinc|{{SimpleNuclide|Zinc|64}}]]).<ref name="Cu-64"/><ref name="Copper-64"/>

== The neutron in elementary particle physics - the Standard Model ==
[[File:Beta Negative Decay.svg|thumb|200px|The principal [[Feynman diagram]] for {{SubatomicParticle|Beta-}}&nbsp;decay of a neutron into a proton, electron, and [[electron antineutrino]] via an intermediate heavy [[W boson|{{SubatomicParticle|W boson-}} boson]]]]
[[File:Electron Capture Decay.svg|thumb|200px|The principal Feynman diagram for {{SubatomicParticle|Beta+}}&nbsp;decay of a proton into a neutron, positron, and [[electron neutrino]] via an intermediate heavy {{SubatomicParticle|W boson+}} boson]]
{{Main|Standard Model}}

Within the theoretical framework of the Standard Model for particle physics, a neutron comprises two [[down quark]]s with charge {{nowrap|−{{sfrac|1|3}}[[elementary charge|''e'']]}} and one [[up quark]] with charge {{nowrap|+{{sfrac|2|3}}''e''}}. The neutron is therefore a [[composite particle]] classified as a ''[[hadron]]''. The neutron is also classified as a ''[[baryon]]'', because it is composed of three [[valence quark]]s.<ref>
{{cite book
|author=Adair, R.K.
|year=1989
|title=The Great Design: Particles, Fields, and Creation
|page=214
|publisher=[[Oxford University Press]]
|bibcode=1988gdpf.book.....A
}}</ref> The finite size of the neutron and its magnetic moment both indicate that the neutron is a [[composite particle|composite]], rather than [[elementary particle|elementary]], particle.

The quarks of the neutron are held together by the [[strong interaction|strong force]], mediated by [[gluon]]s.<ref name=Cottingham>
{{cite book
|author1=Cottingham, W.N. |author2=Greenwood, D.A. |year=1986
|title=An Introduction to Nuclear Physics
|publisher=[[Cambridge University Press]]
|isbn=9780521657334
}}</ref> The nuclear force results from [[Nuclear force#The nuclear force as a residual of the strong force|secondary effects of the more fundamental strong force]].

The only possible decay mode for the neutron that obeys the [[conservation law]] for the [[baryon number]] is for one of the neutron's quarks to change [[flavour (physics)|flavour]] (through a [[Cabibbo–Kobayashi–Maskawa matrix]]) via the [[weak interaction]]. The decay of one of the neutron's down quarks into a lighter up quark can be achieved by the emission of a [[W boson]]. By this process, the Standard Model description of beta decay, the neutron decays into a proton (which contains one down and two up quarks), an electron, and an [[electron neutrino|electron antineutrino]].

The decay of the proton to a neutron occurs similarly through the weak force. The decay of one of the proton's up quarks into a down quark can be achieved by the emission of a W boson. The proton decays into a neutron, a positron, and an electron neutrino. This reaction can only occur within an atomic nucleus which has a quantum state at lower energy available for the created neutron.

== 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. 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"/> 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> 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, i.e. ''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> References to the word ''neutron'' in connection with the atom can be found in the literature as early as 1899, however.<ref name="FeathHist"/>

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> but this raised obvious problems. It was difficult to reconcile the proton–electron model of the nucleus with the [[Heisenberg uncertainty relation]] of quantum mechanics.<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 [[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, presented further quantum mechanical objections to the notion of an electron confined within a nucleus.<ref name="Stuewer"/> 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.

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 (at the time 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"/>

== Properties ==

=== Mass ===
The mass of a neutron cannot be directly determined by [[mass spectrometry]] since it has no electric charge. But since the masses of a proton and of a [[deuteron]] can be measured with a mass spectrometer, the mass of a neutron can be deduced by subtracting proton mass from deuteron mass, with the difference being the mass of the neutron plus the [[binding energy]] of deuterium (expressed as a positive emitted energy). The latter can be directly measured by measuring the energy (<math>B_d</math>) of the single {{val|2.224|u=MeV}} gamma photon emitted when a deuteron is formed by a proton capturing a neutron (this is exothermic and happens with zero-energy neutrons). The small recoil kinetic energy (<math>E_{rd}</math>) of the deuteron (about 0.06% of the total energy) must also be accounted for.
: <math>m_n= m_d - m_p + B_d - E_{rd}</math>

The energy of the gamma ray can be measured to high precision by X-ray diffraction techniques, as was first done by Bell and Elliot in 1948. The best modern (1986) values for neutron mass by this technique are provided by Greene, et al.<ref>{{cite journal |last1= Greene |first1= GL |display-authors= etal |year= 1986 |title= New determination of the deuteron binding energy and the neutron mass |journal= [[Physical Review Letters]]|volume= 56 |issue= 8|pages= 819–822 |doi=10.1103/PhysRevLett.56.819 |bibcode=1986PhRvL..56..819G |pmid=10033294}}</ref> These give a neutron mass of:
: ''m''<sub>neutron</sub> = {{val|1.008644904|(14)|ul=Da}}

The value for the neutron mass in MeV is less accurately known, due to less accuracy in the known conversion of [[Dalton (unit)|Da]] to MeV/''c''<sup>2</sup>:<ref name="Byrne_NNM"/>{{rp|18–19}}
: ''m''<sub>neutron</sub> = {{val|939.56563|(28)|ul=MeV/c2}}.

Another method to determine the mass of a neutron starts from the beta decay of the neutron, when the momenta of the resulting proton and electron are measured.

=== Spin ===
The neutron is a spin&nbsp;{{small|{{sfrac|1|2}}}} particle, that is, it is a [[fermion]] with intrinsic angular momentum equal to {{small|{{sfrac|1|2}}}}&nbsp;{{mvar|ħ}}, where {{mvar|ħ}} is the [[reduced Planck constant]]. For many years after the discovery of the neutron, its exact spin was ambiguous. Although it was assumed to be a spin&nbsp;{{small|{{sfrac|1|2}}}} [[Dirac particle]], the possibility that the neutron was a spin&nbsp;{{small|{{sfrac|3|2}}}} particle lingered. The interactions of the neutron's magnetic moment with an external magnetic field were exploited to finally determine the spin of the neutron.<ref name="Byrne">{{cite book |title=Neutrons, Nuclei and Matter: An exploration of the physics of slow neutrons |author=J. Byrne |isbn= 978-0486482385 |year=2011 |location=Mineola, NY |publisher=Dover Publications |pages=28–31}}</ref> In 1949, Hughes and Burgy measured neutrons reflected from a ferromagnetic mirror and found that the angular distribution of the reflections was consistent with spin&nbsp;{{small|{{sfrac|1|2}}}}.<ref>{{cite journal |last1=Hughes |first1=D.J. |last2=Burgy |first2=M.T. |year=1949 |title=Reflection and polarization of neutrons by magnetized mirrors |journal=[[Physical Review]] |volume=76 |issue=9 |pages=1413–1414 |doi=10.1103/PhysRev.76.1413 |bibcode=1949PhRv...76.1413H |url=http://physics.princeton.edu/~mcdonald/examples/EP/hughes_pr_76_1413_49.pdf |access-date=2016-06-26 |url-status=dead |archive-url=https://web.archive.org/web/20160813204055/http://physics.princeton.edu/~mcdonald/examples/EP/hughes_pr_76_1413_49.pdf |archive-date=2016-08-13}}</ref> In 1954, Sherwood, Stephenson, and Bernstein employed neutrons in a [[Stern–Gerlach experiment]] that used a magnetic field to separate the neutron spin states. They recorded two such spin states, consistent with a spin&nbsp;{{small|{{sfrac|1|2}}}} particle.<ref name="Byrne"/><ref name="Sherwood">{{cite journal |last1=Sherwood |first1=J.E. |last2=Stephenson |first2=T.E. |first3=S. |last3=Bernstein |year=1954 |title=Stern-Gerlach experiment on polarized neutrons |journal=[[Physical Review]] |volume=96 |issue=6 |pages=1546–1548 |doi=10.1103/PhysRev.96.1546 |bibcode=1954PhRv...96.1546S }}</ref>

As a fermion, the neutron is subject to the [[Pauli exclusion principle]]; two neutrons cannot have the same quantum numbers. This is the source of the [[Neutron degeneracy|degeneracy pressure]] which counteracts gravity in [[neutron star]]s and prevents them from forming black holes.<ref name=Bombaci>{{cite journal |first=I. |last=Bombaci |date=1996 |title=The Maximum Mass of a Neutron Star |journal=[[Astronomy and Astrophysics]] |volume=305 | pages=871–877 |bibcode=1996A&A...305..871B}}</ref>

{{See also|Delta baryon}}

=== Magnetic moment ===
{{Main|Nucleon magnetic moment}}
Even though the neutron is a neutral particle, the magnetic moment of a neutron is not zero. The neutron is not affected by electric fields, but it is affected by magnetic fields. The value for the neutron's magnetic moment was first directly measured by [[Luis Walter Alvarez|Luis Alvarez]] and [[Felix Bloch]] at [[Berkeley, California]], in 1940.<ref name="Alvarez">{{cite journal |last1=Alvarez |first1=L.W |last2=Bloch |first2=F.
|year=1940 |title=A quantitative determination of the neutron magnetic moment in absolute nuclear magnetons
|journal=[[Physical Review]]|volume=57 |issue=2 |pages=111–122 |doi=10.1103/physrev.57.111|bibcode=1940PhRv...57..111A}}</ref> Alvarez and Bloch determined the magnetic moment of the neutron to be {{nowrap|1=''μ''<sub>n</sub>= {{val|-1.93|(2)|u=''μ''<sub>N</sub>}}}}, where ''μ''<sub>N</sub> is the [[nuclear magneton]]. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin.<ref name=Llewellyn>{{cite book |title=Modern Physics |author1=Tipler, Paul Allen |author2=Llewellyn, Ralph A. |url=https://books.google.com/books?id=tpU18JqcSNkC&pg=PA310 |page=310 |isbn=978-0-7167-4345-3 |year=2002 |edition=4 |publisher=[[Macmillan Publishers (United States)|Macmillan]] |access-date=2020-08-27 |archive-date=2022-04-07 |archive-url=https://web.archive.org/web/20220407104048/https://books.google.com/books?id=tpU18JqcSNkC&pg=PA310 |url-status=live }}</ref>

The magnetic moment of the neutron is an indication of its quark substructure and internal charge distribution.<ref name="ReferenceA">{{cite journal
|last1=Gell |first1=Y.
|last2=Lichtenberg |first2=D.B.
|year=1969
|title=Quark model and the magnetic moments of proton and neutron
|journal=[[Il Nuovo Cimento A]]|series=Series 10 |volume=61 |issue=1
|pages=27–40
|doi=10.1007/BF02760010
|bibcode= 1969NCimA..61...27G|s2cid=123822660
}}</ref> In the [[quark model]] for [[hadrons]], the neutron is composed of one up quark (charge +2/3&nbsp;''e'') and two down quarks (charge −1/3&nbsp;''e'').<ref name="ReferenceA"/> The magnetic moment of the neutron can be modeled as a sum of the magnetic moments of the constituent quarks.<ref name="Perk">{{cite book |author1-last= Perkins |author1-first= Donald H. |title= Introduction to High Energy Physics |pages= [https://archive.org/details/introductiontohi0000perk/page/201 201–202] |publisher= Addison Wesley, Reading, Massachusetts |date= 1982 |isbn= 978-0-201-05757-7 |url= https://archive.org/details/introductiontohi0000perk/page/201 }}</ref> The calculation assumes that the quarks behave like point-like Dirac particles, each having their own magnetic moment. Simplistically, the magnetic moment of the neutron can be viewed as resulting from the vector sum of the three quark magnetic moments, plus the orbital magnetic moments caused by the movement of the three charged quarks within the neutron.

In one of the early successes of the Standard Model, in 1964 Mirza A.B. Beg, [[Benjamin W. Lee]], and [[Abraham Pais]] calculated the ratio of proton to neutron magnetic moments to be −3/2 (or a ratio of −1.5), which agrees with the experimental value to within 3%.<ref name="Greenberg">
{{citation
|last=Greenberg |first=O.W.
|chapter=Color Charge Degree of Freedom in Particle Physics
|year=2009
|title=Compendium of Quantum Physics
|publisher=Springer Berlin Heidelberg
|pages=109–111
|doi=10.1007/978-3-540-70626-7_32
|arxiv=0805.0289
|isbn=978-3-540-70622-9
|s2cid=17512393
}}</ref><ref name="Beg">{{cite journal |last1=Beg |first1=M.A.B. |last2=Lee |first2=B.W.|last3=Pais |first3=A.
|year=1964 |title=SU(6) and electromagnetic interactions
|journal=[[Physical Review Letters]]|volume=13 |issue=16 |pages=514–517, erratum 650 |doi=10.1103/physrevlett.13.514|bibcode= 1964PhRvL..13..514B}}</ref><ref name="Sakita">{{cite journal |last1=Sakita |first1=B.
|year=1964 |title=Electromagnetic properties of baryons in the supermultiplet scheme of elementary particles
|journal=[[Physical Review Letters]]|volume=13 |issue=21
|pages=643–646 |doi=10.1103/physrevlett.13.643|bibcode= 1964PhRvL..13..643S}}</ref> The measured value for this ratio is {{val|-1.45989805|(34)}}.<ref name="2014 CODATA" />

The above treatment compares neutrons with protons, allowing the complex behavior of quarks to be subtracted out between models, and merely exploring what the effects would be of differing quark charges (or quark type). Such calculations are enough to show that the interior of neutrons is very much like that of protons, save for the difference in quark composition with a down quark in the neutron replacing an up quark in the proton.

The neutron magnetic moment can be roughly computed by assuming a simple [[special relativity|nonrelativistic]], quantum mechanical [[wavefunction]] for [[baryon]]s composed of three quarks. A straightforward calculation gives fairly accurate estimates for the magnetic moments of neutrons, protons, and other baryons.<ref name="Perk"/> For a neutron, the result of this calculation is that the magnetic moment of the neutron is given by {{nowrap|1=''μ''<sub>n</sub>= 4/3 ''μ''<sub>d</sub> − 1/3 ''μ''<sub>u</sub>}}, where ''μ''<sub>d</sub> and ''μ''<sub>u</sub> are the magnetic moments for the down and up quarks, respectively. This result combines the intrinsic magnetic moments of the quarks with their orbital magnetic moments, and assumes the three quarks are in a particular, dominant quantum state.

{| class="wikitable" style="text-align:center;"
|-
! Baryon
! Magnetic moment<br/>of quark model
! Computed<br/>(<math>\mu_\mathrm{N}</math>)
! Observed<br/>(<math>\mu_\mathrm{N}</math>)
|-
| p
| 4/3 ''μ''<sub>u</sub> − 1/3 ''μ''<sub>d</sub>
| 2.79
| 2.793
|-
| n
| 4/3 ''μ''<sub>d</sub> − 1/3 ''μ''<sub>u</sub>
| −1.86
| −1.913
|}

The results of this calculation are encouraging, but the masses of the up or down quarks were assumed to be 1/3 the mass of a nucleon.<ref name="Perk"/> The masses of the quarks are actually only about 1% that of a nucleon.<ref name="Mass">{{cite web |url=https://www.science.org/content/article/mass-common-quark-finally-nailed-down |title=Mass of the Common Quark Finally Nailed Down |last1=Cho |first1=Adrian |date=2 April 2010 |website=Science |publisher=American Association for the Advancement of Science |access-date=27 September 2014 |archive-date=27 August 2015 |archive-url=https://web.archive.org/web/20150827120227/http://news.sciencemag.org/physics/2010/04/mass-common-quark-finally-nailed-down |url-status=live }}</ref> The discrepancy stems from the complexity of the Standard Model for nucleons, where most of their mass originates in the [[gluon]] fields, virtual particles, and their associated energy that are essential aspects of the [[strong force]].<ref name="Mass"/><ref name="Wilczek">{{cite journal |last1=Wilczek |first1=F. |year=2003 |title=The Origin of Mass |journal=[[MIT Physics Annual]] |pages=24–35 |url=http://web.mit.edu/physics/news/physicsatmit/physicsatmit_03_wilczek_originofmass.pdf |archive-date=June 20, 2015 |archive-url=https://web.archive.org/web/20150620011542/http://web.mit.edu/physics/news/physicsatmit/physicsatmit_03_wilczek_originofmass.pdf |url-status=live }}</ref> Furthermore, the complex system of quarks and gluons that constitute a neutron requires a relativistic treatment.<ref>
{{cite journal
|last1=Ji |first1=Xiangdong
|year=1995
|title=A QCD Analysis of the Mass Structure of the Nucleon
|journal=[[Physical Review Letters]]|volume=74 |issue=7
|pages=1071–1074
|doi=10.1103/PhysRevLett.74.1071
|pmid=10058927
|arxiv= hep-ph/9410274 |bibcode= 1995PhRvL..74.1071J|s2cid=15148740
}}</ref> But the nucleon magnetic moment has been successfully computed numerically from [[first principle]]s, including all of the effects mentioned and using more realistic values for the quark masses. The calculation gave results that were in fair agreement with measurement, but it required significant computing resources.<ref>{{cite journal
|last1=Martinelli
|first1=G.
|last2=Parisi
|first2=G.
|last3=Petronzio
|first3=R.
|last4=Rapuano
|first4=F.
|year=1982
|title=The proton and neutron magnetic moments in lattice QCD
|journal=[[Physics Letters B]]
|volume=116
|issue=6
|pages=434–436
|doi=10.1016/0370-2693(82)90162-9
|bibcode=1982PhLB..116..434M
|url=https://cds.cern.ch/record/138281/files/198207343.pdf
|access-date=2019-08-25
|archive-date=2020-04-20
|archive-url=https://web.archive.org/web/20200420144400/https://cds.cern.ch/record/138281/files/198207343.pdf
|url-status=live
}}</ref><ref name="MagMom">{{cite web |url=http://phys.org/news/2015-02-magnetic-moments-nuclear.html |title=Pinpointing the magnetic moments of nuclear matter |last1=Kincade |first1=Kathy |date=2 February 2015 |website=[[Phys.org]] |access-date=May 8, 2015 |archive-date=2 May 2015 |archive-url=https://web.archive.org/web/20150502123656/http://phys.org/news/2015-02-magnetic-moments-nuclear.html |url-status=live }}</ref>

=== Electric charge ===
The total electric charge of the neutron is {{val|0|u=[[elementary charge|''e'']]}}. This zero value has been tested experimentally, and the present experimental limit for the charge of the neutron is {{val|-2|(8)|e=-22|u=[[elementary charge|''e'']]}},<ref name="PDGLIVE">
{{cite journal
|last1=Olive
|first1=K.A.
|author2=(Particle Data Group)
|year=2014
|title=Review of Particle Physics
|journal=[[Chinese Physics C]]
|volume=38
|issue=9
|pages=1–708
|doi=10.1088/1674-1137/38/9/090001
|pmid=10020536
|display-authors=etal
|bibcode=2014ChPhC..38i0001O
|url=http://scipp.ucsc.edu/%7Ehaber/pubs/Review_of_Particle_Physics_2014.pdf
|arxiv=1412.1408
|s2cid=118395784
|access-date=2017-10-26
|archive-date=2020-06-01
|archive-url=https://web.archive.org/web/20200601115825/http://scipp.ucsc.edu/%7Ehaber/pubs/Review_of_Particle_Physics_2014.pdf
|url-status=live
}}</ref> or {{val|-3|(13)|e=-41|ul=C}}. This value is consistent with zero, given the experimental [[uncertainty#In measurements|uncertainties]] (indicated in parentheses). By comparison, the charge of the proton is {{val|+1|u=[[elementary charge|''e'']]}}.

=== Structure and geometry of charge distribution ===
An article published in 2007 featuring a model-independent analysis concluded that the neutron has a negatively charged exterior, a positively charged middle, and a negative core.<ref>
{{cite journal
|author=Miller, G.A.
|year=2007
|title=Charge Densities of the Neutron and Proton
|journal=[[Physical Review Letters]]|volume=99 |page=112001
|doi=10.1103/PhysRevLett.99.112001
|pmid=17930428
|bibcode=2007PhRvL..99k2001M
|issue=11
|arxiv=0705.2409
|s2cid=119120565
}}</ref> In a simplified classical view, the negative "skin" of the neutron assists it to be attracted to the protons with which it interacts in the nucleus; but the main attraction between neutrons and protons is via the [[nuclear force]], which does not involve electric charge.

The simplified classical view of the neutron's charge distribution also "explains" the fact that the neutron magnetic dipole points in the opposite direction from its spin angular momentum vector (as compared to the proton). This gives the neutron, in effect, a magnetic moment which resembles a negatively charged particle. This can be reconciled classically with a neutral neutron composed of a charge distribution in which the negative sub-parts of the neutron have a larger average radius of distribution, and therefore contribute more to the particle's magnetic dipole moment, than do the positive parts that are, on average, nearer the core.

=== Electric dipole moment ===
{{Main|Neutron electric dipole moment}}

The Standard Model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanent [[electric dipole moment]].<ref name="sussex">{{cite press release
|date= 20 February 2006
|title= Pear-shaped particles probe big-bang mystery
|url= http://www.sussex.ac.uk/press_office/media/media537.shtml
|publisher= [[University of Sussex]]
|access-date= 2009-12-14
|archive-date= 2011-06-07
|archive-url= https://web.archive.org/web/20110607140456/http://www.sussex.ac.uk/press_office/media/media537.shtml
|url-status= live
}}</ref> But the predicted value is well below the current sensitivity of experiments. From several [[list of unsolved problems in physics#High-energy physics/particle physics|unsolved puzzles in particle physics]], it is clear that the Standard Model is not the final and full description of all particles and their interactions. New theories going [[beyond the Standard Model]] generally lead to much larger predictions for the electric dipole moment of the neutron. Currently, there are at least four experiments trying to measure for the first time a finite neutron electric dipole moment, including:
* [[Cryogenic neutron EDM experiment]] being set up at the [[Institut Laue–Langevin]]<ref>[http://hepwww.rl.ac.uk/EDM/index_files/CryoEDM.htm A cryogenic experiment to search for the EDM of the neutron] {{Webarchive|url=https://web.archive.org/web/20120216171059/http://hepwww.rl.ac.uk/EDM/index_files/CryoEDM.htm |date=2012-02-16 }}. Hepwww.rl.ac.uk. Retrieved on 2012-08-16.</ref>
* n2EDM experiment under construction at the UCN source at the [[Paul Scherrer Institute]]<ref>[http://nedm.web.psi.ch/ Search for the neutron electric dipole moment at PSI: The n2EDM Project of the nEDM collaboration] {{Webarchive|url=https://web.archive.org/web/20150925151115/http://nedm.web.psi.ch/ |date=2015-09-25 }}. Nedm.web.psi.ch (2001-09-12). Retrieved on 2012-08-16.</ref>
* nEDM experiment being envisaged at the [[Spallation Neutron Source]]<ref>[http://www.phy.ornl.gov/nedm/ US nEDM ORNL experiment public page] {{Webarchive|url=https://web.archive.org/web/20170430055915/http://www.phy.ornl.gov/nedm |date=2017-04-30 }}. Retrieved on 2017-02-08.</ref><ref>[http://p25ext.lanl.gov/edm/edm.html SNS Neutron EDM Experiment] {{Webarchive|url=https://web.archive.org/web/20110210021529/http://p25ext.lanl.gov/edm/edm.html |date=2011-02-10 }}. P25ext.lanl.gov. Retrieved on 2012-08-16.</ref>
* nEDM experiment being built at the [[Institut Laue–Langevin]]<ref>[http://nrd.pnpi.spb.ru/LabSereb/neutronedm.htm Measurement of the Neutron Electric Dipole Moment] {{Webarchive|url=https://web.archive.org/web/20110823110347/http://nrd.pnpi.spb.ru/LabSereb/neutronedm.htm |date=2011-08-23 }}. Nrd.pnpi.spb.ru. Retrieved on 2012-08-16.</ref>

=== Antineutron ===
{{Main|Antineutron}}

The antineutron is the [[antiparticle]] of the neutron. It was discovered by [[Bruce Cork]] in 1956, a year after the [[antiproton]] was discovered. [[CPT-symmetry]] puts strong constraints on the relative properties of particles and antiparticles, so studying antineutrons provides stringent tests on CPT-symmetry. The fractional difference in the masses of the neutron and antineutron is {{val|9|6|e=-5}}. Since the difference is only about two [[standard deviation]]s away from zero, this does not give any convincing evidence of CPT-violation.<ref name="RPP">{{cite journal|doi=10.1088/0954-3899/37/7A/075021|title=Review of Particle Physics|year=2010|last1=Nakamura|first1=K|journal=[[Journal of Physics G]]|volume=37|issue=7A|pages=1–708|pmid=10020536|bibcode= 2010JPhG...37g5021N|doi-access=free|hdl=10481/34593|hdl-access=free}} [http://pdg.lbl.gov/2011/listings/rpp2011-list-n.pdf PDF with 2011 partial update for the 2012 edition] {{Webarchive|url=https://web.archive.org/web/20120920183244/http://pdg.lbl.gov/2011/listings/rpp2011-list-n.pdf |date=2012-09-20 }}.
The exact value of the mean lifetime is still uncertain, due to conflicting results from experiments.
The Particle Data Group reports values up to six seconds apart (more than four standard deviations), commenting that "our 2006, 2008, and 2010 Reviews stayed with 885.7±0.8 s; but we noted that in light of SEREBROV 05 our value should be regarded as suspect until further experiments clarified matters. Since our 2010 Review, PICHLMAIER 10 has obtained a mean life of 880.7±1.8 s, closer to the value of SEREBROV 05 than to our average. And SEREBROV 10B[...] claims their values should be lowered by about 6 s, which would bring them into line with the two lower values. But those re-evaluations have not received an enthusiastic response from the experimenters in question; and in any case the Particle Data Group would have to await published changes (by those experimenters) of published values.

At this point, we can think of nothing better to do than to average the seven best but discordant measurements, getting {{val|881.5|1.5|u=s}}. Note that the error includes a scale factor of 2.7. This is a jump of 4.2 old (and 2.8 new) standard deviations. This state of affairs is a particularly unhappy one, because the value is so important. We again call upon the experimenters to clear this up."</ref>

== Neutron compounds ==

=== Dineutrons and tetraneutrons ===
{{Main|Dineutron|Tetraneutron}}

The [[dineutron]] is considered an unbound isotope with lifetimes around 10<sup>−22</sup> seconds. The first evidence for this state was reported by Haddock et al. in 1965.<ref name=ThoennessenReview>{{Cite book |last=Thoennessen |first=Michael |title=The Discovery of Isotopes |chapter-url=http://link.springer.com/10.1007/978-3-319-31763-2_16 |chapter=Unbound Isotopes |date=2016 |publisher=Springer International Publishing |isbn=978-3-319-31761-8 |location=Cham |pages=275–291 |language=en |doi=10.1007/978-3-319-31763-2_16 |access-date=2024-01-05 |archive-date=2024-05-12 |archive-url=https://web.archive.org/web/20240512232101/https://link.springer.com/chapter/10.1007/978-3-319-31763-2_16 |url-status=live }}</ref>{{rp|275}} In 2012, [[Artemis Spyrou]] from Michigan State University and coworkers reported that they observed, for the first time, direct dineutron emission in the decay of <sup>16</sup>Be. The dineutron character is evidenced by a small emission angle between the two neutrons. The authors measured the two-neutron separation energy to be 1.35(10) MeV, in good agreement with shell model calculations, using standard interactions for this mass region.<ref name="Spyrou">{{cite journal |last= Spyrou |first= A. |year= 2012 |title= First Observation of Ground State Dineutron Decay: 16Be |journal= [[Physical Review Letters]]|volume= 108 |pages= 102501 |doi= 10.1103/PhysRevLett.108.102501|bibcode= 2012PhRvL.108j2501S |pmid=22463404 |issue=10|display-authors=etal|doi-access= free }}</ref>

Evidence for unbound clusters of 4 neutrons, or [[tetraneutron]] as resonances in the disintegration of [[beryllium]]-14 nuclei,<ref>{{Cite journal |last1=Marqués |first1=F. M. |last2=Labiche |first2=M. |last3=Orr |first3=N. A. |last4=Angélique |first4=J. C. |last5=Axelsson |first5=L. |last6=Benoit |first6=B. |last7=Bergmann |first7=U. C. |last8=Borge |first8=M. J. G. |last9=Catford |first9=W. N. |last10=Chappell |first10=S. P. G. |last11=Clarke |first11=N. M. |last12=Costa |first12=G. |last13=Curtis |first13=N. |last14=D’Arrigo |first14=A. |last15=de Góes Brennand |first15=E. |date=2002-04-01 |title=Detection of neutron clusters |url=https://link.aps.org/doi/10.1103/PhysRevC.65.044006 |journal=Physical Review C |language=en |volume=65 |issue=4 |page=044006 |doi=10.1103/PhysRevC.65.044006 |issn=0556-2813 |arxiv=nucl-ex/0111001 |bibcode=2002PhRvC..65d4006M |s2cid=37431352 |access-date=2024-01-05 |archive-date=2024-05-12 |archive-url=https://web.archive.org/web/20240512232054/https://journals.aps.org/prc/abstract/10.1103/PhysRevC.65.044006 |url-status=live }}</ref> in <sup>8</sup>He-<sup>8</sup>Be interactions,<ref name="Kisamori">{{cite journal |last= Kisamori |first= K. |display-authors= etal |year= 2016 |title= Candidate Resonant Tetraneutron State Populated by the He4(He8,Be8) Reaction |journal= [[Physical Review Letters]]|volume= 116 |issue= 5 |pages= 052501 |doi= 10.1103/PhysRevLett.116.052501|pmid= 26894705 |bibcode= 2016PhRvL.116e2501K }}</ref> and collisions of <sup>4</sup>He nuclei give an estimated lifetime around 10<sup>−22</sup> seconds.<ref>{{Cite journal |last1=Duer |first1=M. |last2=Aumann |first2=T. |last3=Gernhäuser |first3=R. |last4=Panin |first4=V. |last5=Paschalis |first5=S. |last6=Rossi |first6=D. M. |last7=Achouri |first7=N. L. |last8=Ahn |first8=D. |last9=Baba |first9=H. |last10=Bertulani |first10=C. A. |last11=Böhmer |first11=M. |last12=Boretzky |first12=K. |last13=Caesar |first13=C. |last14=Chiga |first14=N. |last15=Corsi |first15=A. |date=2022-06-23 |title=Observation of a correlated free four-neutron system |journal=Nature |language=en |volume=606 |issue=7915 |pages=678–682 |doi=10.1038/s41586-022-04827-6 |issn=0028-0836 |pmc=9217746 |pmid=35732764|bibcode=2022Natur.606..678D }}</ref>
These discoveries should deepen our understanding of the nuclear forces.<ref name="sciencealert">{{cite web|url=https://www.sciencenews.org/article/physicists-find-signs-four-neutron-nucleus|title=Physicists find signs of four-neutron nucleus|date=2016-02-24|access-date=2017-06-27|archive-date=2017-07-29|archive-url=https://web.archive.org/web/20170729030228/https://www.sciencenews.org/article/physicists-find-signs-four-neutron-nucleus|url-status=live}}</ref><ref>{{cite journal|first=Nigel|last=Orr|title=Can Four Neutrons Tango?|journal=[[Physics (American Physical Society magazine)|Physics]]|volume=9|date=2016-02-03|pages=14|doi=10.1103/Physics.9.14|bibcode=2016PhyOJ...9...14O|doi-access=free}}</ref>

=== Neutron stars and neutron matter ===
{{Main|Neutron matter|Neutron star}}
At extremely high pressures and temperatures, nucleons and electrons are believed to collapse into bulk neutronic matter, called [[neutron matter]]. This is presumed to happen in [[neutron star]]s.<ref>{{Cite journal |last1=Gandolfi |first1=Stefano |last2=Gezerlis |first2=Alexandros |last3=Carlson |first3=J. |date=2015-10-19 |title=Neutron Matter from Low to High Density |url=https://www.annualreviews.org/doi/10.1146/annurev-nucl-102014-021957 |journal=Annual Review of Nuclear and Particle Science |language=en |volume=65 |issue=1 |pages=303–328 |doi=10.1146/annurev-nucl-102014-021957 |issn=0163-8998 |arxiv=1501.05675 |bibcode=2015ARNPS..65..303G |access-date=2024-01-04 |archive-date=2022-06-14 |archive-url=https://web.archive.org/web/20220614180255/https://www.annualreviews.org/doi/10.1146/annurev-nucl-102014-021957 |url-status=live }}</ref>

The extreme pressure inside a neutron star may deform the neutrons into a cubic symmetry, allowing tighter packing of neutrons.<ref>{{cite journal |last1=Llanes-Estrada|arxiv=1108.1859|title=Cubic neutrons |year=2012 |first1=Felipe J. |first2=Gaspar|last2= Moreno Navarro |doi=10.1142/S0217732312500332 |volume=27 |issue=6|pages=1250033–1–1250033–7|journal=[[Modern Physics Letters A]]|bibcode=2012MPLA...2750033L|s2cid=118407306}}</ref>

== Detection ==
{{Main|Neutron detection}}
The common means of detecting a [[electric charge|charged]] [[elementary particle|particle]] by looking for a track of ionization (such as in a [[cloud chamber]]) does not work for neutrons directly. Neutrons that elastically scatter off atoms can create an ionization track that is detectable, but the experiments are not as simple to carry out; other means for detecting neutrons, consisting of allowing them to interact with atomic nuclei, are more commonly used. The commonly used methods to detect neutrons can therefore be categorized according to the nuclear processes relied upon, mainly [[neutron capture]] or [[elastic scattering]].<ref>{{cite book|chapter=Ch. 14|title=Radiation Detection and Measurement|author=Knoll, Glenn F.|publisher=John Wiley & Sons|year=1979|isbn=978-0471495451|chapter-url=https://archive.org/details/radiationdetecti00knol_0}}</ref>

=== Neutron detection by neutron capture ===
A common method for detecting neutrons involves converting the energy released from [[neutron capture]] reactions into electrical signals. Certain nuclides have a high neutron capture [[cross section (physics)|cross section]], which is the probability of absorbing a neutron. Upon neutron capture, the compound nucleus emits more easily detectable radiation, for example an alpha particle, which is then detected. The nuclides {{SimpleNuclide|Helium|3}}, {{SimpleNuclide|Lithium|6}}, {{SimpleNuclide|Boron|10}}, {{SimpleNuclide|Uranium|233}}, {{SimpleNuclide|Uranium|235}}, {{SimpleNuclide|Neptunium|237}}, and {{SimpleNuclide|Plutonium|239}} are useful for this purpose. <!-- The following needs correction, errors due to lack of reference: These nuclides are rarely found in nature, but can be accumulated through processes such as isotopic enrichment.

The cross section for the process of neutron capture is much lower at high energies than at low energies. Therefore, the detection of neutrons by neutron capture requires a preceding slowing down of neutrons. For this purpose, a [[neutron moderator]] is used, typically a thick slab of polyethylene. Neutron detectors using the moderate-and-capture approach cannot measure neutron energy, precise time of arrival, or direction of incidence, because this information is lost during moderation. -->

=== Neutron detection by elastic scattering ===
Neutrons can elastically scatter off nuclei, causing the struck nucleus to recoil. Kinematically, a neutron can transfer more energy to a light nucleus such as hydrogen or helium than to a heavier nucleus. Detectors relying on elastic scattering are called fast neutron detectors. Recoiling nuclei can ionize and excite further atoms through collisions. Charge and/or scintillation light produced in this way can be collected to produce a detected signal. A major challenge in fast neutron detection is discerning such signals from erroneous signals produced by gamma radiation in the same detector. Methods such as pulse shape discrimination can be used in distinguishing neutron signals from gamma-ray signals, although certain inorganic scintillator-based detectors have been developed <ref>{{Cite journal |last = Ghosh |first = P. |author2 = W. Fu |author3 = M. J. Harrison |author4 = P. K. Doyle |author5 = N. S. Edwards |author6 = J. A. Roberts |author7 = D. S. McGregor |year = 2018 |title = A high-efficiency, low-Ĉerenkov Micro-Layered Fast-Neutron Detector for the TREAT hodoscope |journal =   Nuclear Instruments and Methods in Physics Research Section A |volume = 904 |pages = 100–106 |doi = 10.1016/j.nima.2018.07.035 |bibcode = 2018NIMPA.904..100G |s2cid = 126130994 |doi-access = free }}</ref><ref>{{Cite book |last = Ghosh |first = P. |author2= D. M. Nichols |author3= W. Fu |author4 = J. A. Roberts |author5 = D. S. McGregor |title = 2019 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC) |chapter = Gamma-Ray Rejection of the SiPM-coupled Micro-Layered Fast-Neutron Detector |year = 2019 |pages= 1–3 |doi= 10.1109/NSS/MIC42101.2019.9059869|isbn = 978-1-7281-4164-0 |s2cid = 204877955 }}</ref> to selectively detect neutrons in mixed radiation fields inherently without any additional techniques.

Fast neutron detectors have the advantage of not requiring a moderator, and are therefore capable of measuring the neutron's energy, time of arrival, and in certain cases direction of incidence.

== Sources and production ==
{{Main|Neutron source|Neutron generator|Research reactor}}
Free neutrons are unstable, although they have the longest half-life of any unstable subatomic particle by several orders of magnitude. Their half-life is still only about 10 minutes, so they can be obtained only from sources that produce them continuously.

'''Natural neutron background.''' A small natural background flux of free neutrons exists everywhere on Earth.<ref name="NatNeu">
{{cite journal
|author1= Carson, M.J.
|year= 2004
|title= Neutron background in large-scale xenon detectors for dark matter searches
|journal= [[Astroparticle Physics (journal)|Astroparticle Physics]]
|volume= 21 |pages= 667–687
|doi= 10.1016/j.astropartphys.2004.05.001
|issue= 6 |display-authors=etal|arxiv= hep-ex/0404042|bibcode= 2004APh....21..667C|s2cid= 17887096
}}</ref> In the atmosphere and deep into the ocean, the "neutron background" is caused by [[muon]]s produced by [[cosmic ray]] interaction with the atmosphere. These high-energy muons are capable of penetration to considerable depths in water and soil. There, in striking atomic nuclei, among other reactions they induce spallation reactions in which a neutron is liberated from the nucleus. Within the Earth's crust a second source is neutrons produced primarily by spontaneous fission of uranium and thorium present in crustal minerals. The neutron background is not strong enough to be a biological hazard, but it is of importance to very high resolution particle detectors that are looking for very rare events, such as (hypothesized) interactions that might be caused by particles of [[dark matter]].<ref name="NatNeu"/> Recent research has shown that even thunderstorms can produce neutrons with energies of up to several tens of MeV.<ref name="KohnEbert">{{cite journal |last1=Köhn |first1=C. |last2=Ebert |first2=U. |author2-link=Ute Ebert |title=Calculation of beams of positrons, neutrons and protons associated with terrestrial gamma-ray flashes |journal=[[Journal of Geophysical Research: Atmospheres]] |date=2015 |volume=23 |issue=4 |doi=10.1002/2014JD022229 |pages=1620–1635 |bibcode=2015JGRD..120.1620K |url=https://ir.cwi.nl/pub/23845/23845D.pdf |doi-access=free |access-date=2019-08-25 |archive-date=2019-12-23 |archive-url=https://web.archive.org/web/20191223070457/https://ir.cwi.nl/pub/23845/23845D.pdf |url-status=live }}</ref> Recent research has shown that the fluence of these neutrons lies between 10<sup>−9</sup> and 10<sup>−13</sup> per ms and per m<sup>2</sup> depending on the detection altitude. The energy of most of these neutrons, even with initial energies of 20 MeV, decreases down to the keV range within 1 ms.<ref name="KohnHarakeh">{{cite journal|last1=Köhn |first1=C. |last2=Diniz |first2=G. |last3=Harakeh |first3=Muhsin |title=Production mechanisms of leptons, photons, and hadrons and their possible feedback close to lightning leaders |journal=[[Journal of Geophysical Research: Atmospheres]]|date=2017 |volume=122 |issue=2 |pages=1365–1383 |doi=10.1002/2016JD025445|pmid=28357174 |pmc=5349290 |bibcode=2017JGRD..122.1365K }}</ref>

Even stronger neutron background radiation is produced at the surface of Mars, where the atmosphere is thick enough to generate neutrons from cosmic ray muon production and neutron-spallation, but not thick enough to provide significant protection from the neutrons produced. These neutrons not only produce a Martian surface neutron radiation hazard from direct downward-going neutron radiation but may also produce a significant hazard from reflection of neutrons from the Martian surface, which will produce reflected neutron radiation penetrating upward into a Martian craft or habitat from the floor.<ref>{{cite journal |last1=Clowdsley |url=http://www.physicamedica.com/VOLXVII_S1/20-CLOWDSLEY%20et%20alii.pdf |archive-url=https://web.archive.org/web/20050225054811/http://www.physicamedica.com/VOLXVII_S1/20-CLOWDSLEY%20et%20alii.pdf |url-status=dead |archive-date=2005-02-25 |journal=[[Physica Medica]]|first1=MS |volume=17 |issue=Suppl 1 |last2=Wilson |pages=94–96 |first2=JW |last3=Kim |first3=MH |last4=Singleterry |first4=RC |last5=Tripathi |first5=RK |last6=Heinbockel |first6=JH |last7=Badavi |first7=FF |last8=Shinn |first8=JL |title=Neutron Environments on the Martian Surface |year=2001 |pmid=11770546 }}</ref>

'''Sources of neutrons for research.''' These include certain types of [[radioactive decay]] ([[spontaneous fission]] and [[neutron emission]]), and from certain [[nuclear reaction]]s. Convenient nuclear reactions include tabletop reactions such as natural alpha and gamma bombardment of certain nuclides, often beryllium or deuterium, and induced [[nuclear fission]], such as occurs in nuclear reactors. In addition, high-energy nuclear reactions (such as occur in cosmic radiation showers or accelerator collisions) also produce neutrons from disintegration of target nuclei. Small (tabletop) [[particle accelerator]]s optimized to produce free neutrons in this way, are called [[neutron generator]]s.

In practice, the most commonly used small laboratory sources of neutrons use radioactive decay to power neutron production. One noted neutron-producing [[radioisotope]], [[californium]]-252 decays (half-life 2.65 years) by [[spontaneous fission]] 3% of the time with production of 3.7 neutrons per fission, and is used alone as a neutron source from this process. [[Nuclear reaction]] sources (that involve two materials) powered by radioisotopes use an [[alpha decay]] source plus a beryllium target, or else a source of high-energy gamma radiation from a source that undergoes [[beta decay]] followed by [[gamma decay]], which produces [[photoneutron]]s on interaction of the high-energy [[gamma ray]] with ordinary stable beryllium, or else with the [[deuterium]] in [[heavy water]]. A popular [[startup neutron source|source of the latter type]] is radioactive [[antimony-124]] plus beryllium, a system with a half-life of 60.9 days, which can be constructed from natural antimony (which is 42.8% stable antimony-123) by activating it with neutrons in a nuclear reactor, then transported to where the neutron source is needed.<ref>Byrne, J. ''Neutrons, Nuclei, and Matter'', Dover Publications, Mineola, New York, 2011, {{ISBN|0486482383}}, pp. 32–33.</ref>

[[File:Institut Laue–Langevin (ILL) in Grenoble, France.jpg|thumb|right|[[Institut Laue–Langevin]] (ILL) in Grenoble, France – a major neutron research facility]]

[[Nuclear reactor|Nuclear fission reactors]] naturally produce free neutrons; their role is to sustain the energy-producing [[chain reaction]]. The intense [[neutron radiation]] can also be used to produce various radioisotopes through the process of [[neutron activation]], which is a type of [[neutron capture]].

Experimental [[fusion power|nuclear fusion reactors]] produce free neutrons as a waste product. But it is these neutrons that possess most of the energy and converting that energy to a useful form has proved a difficult engineering challenge. Fusion reactors that generate neutrons are likely to create radioactive waste, but the waste is composed of neutron-activated lighter isotopes, which have relatively short (50–100 years) decay periods as compared to typical half-lives of 10,000 years<ref>{{Cite web|url=https://eesc.columbia.edu/courses/ees/lithosphere/labs/lab12/radioisotope_tutorial.html|title=Isotopes and Radioactivity Tutorial|access-date=2020-04-16|archive-date=2020-02-14|archive-url=https://web.archive.org/web/20200214215448/https://eesc.columbia.edu/courses/ees/lithosphere/labs/lab12/radioisotope_tutorial.html|url-status=dead}}</ref> for fission waste, which is long due primarily to the long half-life of alpha-emitting transuranic actinides.<ref>[http://news.bbc.co.uk/1/hi/sci/tech/4627237.stm Science/Nature |Q&A: Nuclear fusion reactor] {{Webarchive|url=https://web.archive.org/web/20220225092021/http://news.bbc.co.uk/1/hi/sci/tech/4627237.stm |date=2022-02-25 }}. BBC News (2006-02-06). Retrieved on 2010-12-04.</ref> Some [[nuclear fusion-fission hybrid]]s are proposed to make use of those neutrons to either maintain a [[subcritical reactor]] or to aid in [[nuclear transmutation]] of harmful long lived nuclear waste to shorter lived or stable nuclides.

=== Neutron beams and modification of beams after production ===
Free neutron beams are obtained from [[neutron source]]s by [[neutron transport]]. For access to intense neutron sources, researchers must go to a specialized [[neutron research facility|neutron facility]] that operates a [[research reactor]] or a [[spallation]] source.

The neutron's lack of total electric charge makes it difficult to steer or accelerate them. Charged particles can be accelerated, decelerated, or deflected by [[electric field|electric]] or [[magnetic field]]s. These methods have little effect on neutrons. But some effects may be attained by use of inhomogeneous magnetic fields because of the [[neutron magnetic moment|neutron's magnetic moment]]. Neutrons can be controlled by methods that include [[neutron moderator|moderation]], [[neutron reflector|reflection]], and [[neutron-velocity selector|velocity selection]]. [[Thermal neutron]]s can be polarized by transmission through [[magnet]]ic materials in a method analogous to the [[Faraday effect]] for [[photon]]s. Cold neutrons of wavelengths of 6–7 angstroms can be produced in beams of a high degree of polarization, by use of [[neutron supermirror|magnetic mirrors]] and magnetized interference filters.<ref>Byrne, J. ''Neutrons, Nuclei, and Matter'', Dover Publications, Mineola, New York, 2011, {{ISBN|0486482383}}, p. 453.</ref>

== Applications ==
{{Science with neutrons}}

The neutron plays an important role in many nuclear reactions. For example, neutron capture often results in [[neutron activation]], inducing [[radioactivity]]. In particular, knowledge of neutrons and their behavior has been important in the development of [[nuclear reactor]]s and [[nuclear weapon]]s. The [[nuclear fission|fissioning]] of elements like [[uranium-235]] and [[plutonium-239]] is caused by their absorption of neutrons.

[[Neutron temperature|''Cold'', ''thermal'', and ''hot'']] [[neutron radiation]] is commonly employed in [[neutron scattering]] facilities for [[neutron diffraction]], [[small-angle neutron scattering]], and [[neutron reflectometry]]. Slow neutron [[matter waves]] exhibit properties similar to geometrical and wave optics of light, including reflection, refraction, diffraction, and interference.<ref name="Klein Werner 1983 pp. 259–335">{{cite journal | last1=Klein | first1=A G | last2=Werner | first2=S A | title=Neutron optics | journal=Reports on Progress in Physics | publisher=IOP Publishing | volume=46 | issue=3 | date=1983-03-01 | issn=0034-4885 | doi=10.1088/0034-4885/46/3/001 | pages=259–335 | s2cid=250903152 | url=https://www.researchgate.net/publication/231072989 | access-date=2023-07-06 | archive-date=2024-05-12 | archive-url=https://web.archive.org/web/20240512232101/https://www.researchgate.net/publication/231072989_4_Neutron_Optics | url-status=live }}</ref> Neutrons are complementary to [[X-ray]]s in terms of atomic contrasts by different scattering [[cross section (physics)|cross sections]]; sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.

The development of "neutron lenses" based on total internal reflection within hollow glass capillary tubes or by reflection from dimpled aluminum plates has driven ongoing research into neutron microscopy and neutron/gamma ray tomography.<ref>{{cite journal |last=Kumakhov |first=M.A. |author2=Sharov, V.A. |year=1992 |title=A neutron lens |journal=[[Nature (journal)|Nature]] |volume=357 |issue= 6377|pages=390–391 |doi=10.1038/357390a0 |bibcode= 1992Natur.357..390K|s2cid=37062511 }}</ref><ref>[http://www.physorg.com/news599.html Physorg.com, "New Way of 'Seeing': A 'Neutron Microscope'"] {{Webarchive|url=https://web.archive.org/web/20120124122838/http://www.physorg.com/news599.html |date=2012-01-24 }}. Physorg.com (2004-07-30). Retrieved on 2012-08-16.</ref><ref>[http://www.nasa.gov/vision/earth/technologies/nuggets.html "NASA Develops a Nugget to Search for Life in Space"] {{Webarchive|url=https://web.archive.org/web/20140308200231/http://www.nasa.gov/vision/earth/technologies/nuggets.html |date=2014-03-08 }}. NASA.gov (2007-11-30). Retrieved on 2012-08-16.</ref><ref>{{Cite journal|last1=Ioffe|first1=A.|last2=Dabagov|first2=S.|last3=Kumakhov|first3=M.|date=1995-01-01|title=Effective neutron bending at large angles|url=https://doi.org/10.1080/10448639508217696|journal=Neutron News|volume=6|issue=3|pages=20–21|doi=10.1080/10448639508217696|issn=1044-8632}}</ref>

A major use of neutrons is to excite delayed and prompt [[gamma ray]]s from elements in materials. This forms the basis of [[neutron activation analysis]] (NAA) and [[prompt gamma neutron activation analysis]] (PGNAA). NAA is most often used to analyze small samples of materials in a [[nuclear reactor]] whilst PGNAA is most often used to analyze subterranean rocks around [[bore hole]]s and industrial bulk materials on conveyor belts.

Another use of neutron emitters is the detection of light nuclei, in particular the hydrogen found in water molecules. When a fast neutron collides with a light nucleus, it loses a large fraction of its energy. By measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei, a [[neutron probe]] may determine the water content in soil.

== Medical therapies ==
{{Main|Fast neutron therapy|Neutron capture therapy of cancer}}
Because neutron radiation is both penetrating and ionizing, it can be exploited for medical treatments. However, neutron radiation can have the unfortunate side-effect of leaving the affected area radioactive. [[Neutron tomography]] is therefore not a viable medical application.

Fast neutron therapy uses high-energy neutrons typically greater than 20 MeV to treat cancer. [[Radiation therapy]] of cancers is based upon the biological response of cells to ionizing radiation. If radiation is delivered in small sessions to damage cancerous areas, normal tissue will have time to repair itself, while tumor cells often cannot.<ref>{{Cite book |last=Hall |first=Eric J. |url=https://www.worldcat.org/oclc/43854159 |title=Radiobiology for the radiologist |date=2000 |publisher=Lippincott Williams & Wilkins |isbn=0-7817-2649-2 |edition=5th |location=Philadelphia |oclc=43854159 |access-date=2023-03-11 |archive-date=2024-05-12 |archive-url=https://web.archive.org/web/20240512232112/https://search.worldcat.org/title/43854159 |url-status=live }}</ref> Neutron radiation can deliver energy to a cancerous region at a rate an order of magnitude larger than [[gamma radiation]].<ref>Johns HE and Cunningham JR (1978). ''The Physics of Radiology''. Charles C Thomas 3rd edition</ref>

Beams of low-energy neutrons are used in [[neutron capture therapy of cancer|boron neutron capture therapy]] to treat cancer. In boron neutron capture therapy, the patient is given a drug that contains boron and that preferentially accumulates in the tumor to be targeted. The tumor is then bombarded with very low-energy neutrons (although often higher than thermal energy) which are captured by the [[boron-10]] isotope in the boron, which produces an excited state of boron-11 that then decays to produce [[lithium-7]] and an [[alpha particle]] that have sufficient energy to kill the malignant cell, but insufficient range to damage nearby cells. For such a therapy to be applied to the treatment of cancer, a neutron source having an intensity of the order of a thousand million (10<sup>9</sup>) neutrons per second per cm<sup>2</sup> is preferred. Such fluxes require a research nuclear reactor.

== Protection ==
Exposure to free neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to [[molecule]]s and [[atom]]s, and can also cause reactions that give rise to other forms of [[radiation]] (such as protons).<ref name="ENW"/> The normal precautions of radiation protection apply: Avoid exposure, stay as far from the source as possible, and keep exposure time to a minimum. But particular thought must be given to how to protect from neutron exposure. For other types of radiation, e.g., [[alpha particle]]s, [[beta particle]]s, or [[gamma ray]]s, material of a high atomic number and with high density makes for good shielding; frequently, [[lead]] is used. However, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number, as it does with alpha, beta, and gamma radiation. Instead, one needs to look at the particular interactions neutrons have with matter (see the section on detection above). For example, [[hydrogen]]-rich materials are often used to shield against neutrons, since ordinary hydrogen both scatters and slows neutrons. This often means that simple concrete blocks or even paraffin-loaded plastic blocks afford better protection from neutrons than do far more dense materials. After slowing, neutrons may then be absorbed with an isotope that has high affinity for slow neutrons without causing secondary capture radiation, such as lithium-6.

Hydrogen-rich [[water|ordinary water]] effects neutron absorption in [[nuclear fission]] reactors: Usually, neutrons are so strongly absorbed by normal water that fuel enrichment with a fissionable isotope is required. (The number of neutrons produced per fission depends primarily on the fission products. The average is roughly 2.5 to 3.0 and at least one, on average, must evade capture in order to sustain the [[nuclear chain reaction]].) The [[deuterium]] in [[heavy water]] has a very much lower absorption affinity for neutrons than does protium (normal light hydrogen). Deuterium is, therefore, used in [[CANDU]]-type reactors, in order to slow ([[neutron moderator|moderate]]) neutron velocity, to increase the probability of [[nuclear fission]] compared to [[neutron capture]].

== Neutron temperature ==
{{Main|Neutron temperature}}

=== Thermal neutrons ===
''Thermal neutrons'' are [[free neutron]]s whose energies have a [[Maxwell–Boltzmann distribution]] with kT&nbsp;=&nbsp;{{val|0.0253|ul=eV}} ({{val|4.0|e=-21|ul=J}}) at room temperature. This gives characteristic (not average, or median) speed of 2.2&nbsp;km/s. The name 'thermal' comes from their energy being that of the room temperature gas or material they are permeating. (see ''[[kinetic theory of gases|kinetic theory]]'' for energies and speeds of molecules). After a number of collisions (often in the range of 10–20) with nuclei, neutrons arrive at this energy level, provided that they are not absorbed.

In many substances, thermal neutron reactions show a much larger effective cross-section than reactions involving faster neutrons, and thermal neutrons can therefore be absorbed more readily (i.e., with higher probability) by any [[atomic nucleus|atomic nuclei]] that they collide with, creating a heavier – and often [[unstable isotope|unstable]] – [[isotope]] of the [[chemical element]] as a result.

Most [[nuclear reactor|fission reactors]] use a [[neutron moderator]] to slow down, or ''thermalize'', the neutrons that are emitted by [[nuclear fission]] so that they are more easily captured, causing further fission. Others, called [[fast breeder]] reactors, use fission energy neutrons directly.

=== Cold neutrons ===
''Cold neutrons'' are thermal neutrons that have been equilibrated in a very cold substance such as liquid [[deuterium]]. Such a ''cold source'' is placed in the moderator of a research reactor or spallation source. Cold neutrons are particularly valuable for [[neutron scattering]] experiments.<ref>{{Cite journal|last=brian.maranville@nist.gov|date=2017-04-17|title=How neutrons are useful|url=https://www.nist.gov/ncnr/how-neutrons-are-useful|access-date=2021-01-21|journal=NIST|language=en|archive-date=2021-01-25|archive-url=https://web.archive.org/web/20210125082444/https://www.nist.gov/ncnr/how-neutrons-are-useful|url-status=live}}</ref>

The use of cold and very cold neutrons (VCN) has been a bit limited compared to the use of thermal neutrons due to the relatively lower flux and lack in optical components. However, Innovative solutions have been proposed to offer more options to the scientific community to promote the use of VCN.<ref>{{Cite journal |last1=E Hadden |last2=Y Iso |last3=A Kume |last4=K Umemoto |last5=T Jenke |last6=M Fally |last7=J Klepp |last8=Y Tomita |date=2022 |title=Highly efficient holographic optical elements for cold neutron experiments |url=https://rgdoi.net/10.13140/RG.2.2.26033.04963 |language=en |doi=10.13140/RG.2.2.26033.04963 |website=ResearchGate |access-date=2022-09-14 |archive-date=2024-05-12 |archive-url=https://web.archive.org/web/20240512232159/https://www.researchgate.net/publication/363272925_HIGHLY_EFFICIENT_HOLOGRAPHIC_OPTICAL_ELEMENTS_FOR_COLD_NEUTRON_EXPERIMENTS?channel=doi&linkId=6315e5c75eed5e4bd14b8c9d&showFulltext=true |url-status=live }}</ref><ref>{{Cite book |last1=Hadden |first1=Elhoucine |last2=Iso |first2=Yuko |last3=Kume |first3=Atsushi |last4=Umemoto |first4=Koichi |last5=Jenke |first5=Tobias |last6=Fally |first6=Martin |last7=Klepp |first7=Jürgen |last8=Tomita |first8=Yasuo |title=Photosensitive Materials and their Applications II |chapter=Nanodiamond-based nanoparticle-polymer composite gratings with extremely large neutron refractive index modulation |editor-first1=Robert R |editor-first2=Yasuo |editor-first3=John T |editor-first4=Inmaculada |editor-last1=McLeod |editor-last2=Tomita |editor-last3=Sheridan |editor-last4=Pascual Villalobos |date=2022-05-24 |chapter-url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/12151/1215109/Nanodiamond-based-nanoparticle-polymer-composite-gratings-with-extremely-large-neutron/10.1117/12.2623661.full |publisher=SPIE |volume=12151 |pages=70–76 |doi=10.1117/12.2623661|bibcode=2022SPIE12151E..09H |isbn=9781510651784 |s2cid=249056691 }}</ref>

[[File:Cold Neutron Source.jpg|right|thumb|350px|Cold neutron source providing neutrons at about the temperature of liquid hydrogen]]

=== Ultracold neutrons ===
[[Ultracold neutrons]] are produced by inelastic scattering of cold neutrons in substances with a low neutron absorption cross section at a temperature of a few kelvins, such as solid [[deuterium]]<ref name="psiucnsource">{{cite journal|author1=B. Lauss|title=Startup of the high-intensity ultracold neutron source at the Paul Scherrer Institute|journal=Hyperfine Interact.|volume=211|issue=1|date=May 2012|pages = 21–25|doi=10.1007/s10751-012-0578-7|arxiv = 1202.6003|bibcode = 2012HyInt.211...21L|s2cid=119164071}}</ref> or superfluid [[helium]].<ref name="ucnheliumsource">{{cite journal|author1=R. Golub|author2=J. M. Pendlebury|name-list-style=amp|title=The interaction of Ultra-Cold Neutrons (UCN) with liquid helium and a superthermal UCN source|journal=Phys. Lett. A|volume=62|issue=5|date=1977|pages=337–339|doi=10.1016/0375-9601(77)90434-0|bibcode=1977PhLA...62..337G}}</ref> An alternative production method is the mechanical deceleration of cold neutrons exploiting the Doppler shift.<ref name="turbineconstruction">{{cite journal|author1=A. Steyerl|author2=H. Nagel|author3=F.-X. Schreiber|author4=K.-A. Steinhauser|author5=R. Gähler|author6=W. Gläser|author7=P. Ageron|author8=J. M. Astruc|author9=W. Drexel|author10=G. Gervais|author11=W. Mampe|name-list-style=amp|title=A new source of cold and ultracold neutrons|journal=Phys. Lett. A|volume=116|issue=7|date=1986|pages=347–352|doi=10.1016/0375-9601(86)90587-6|bibcode=1986PhLA..116..347S}}</ref><ref name="turbinespectrum">{{cite journal|url=http://www.sciencedirect.com/science/article/pii/S0168900219314445|author1=Stefan Döge|author2=Jürgen Hingerl|author3=Christoph Morkel|name-list-style=amp|title=Measured velocity spectra and neutron densities of the PF2 ultracold-neutron beam ports at the Institut Laue–Langevin|journal=Nucl. Instrum. Methods A|volume=953|pages=163112|date=Feb 2020|doi=10.1016/j.nima.2019.163112|arxiv=2001.04538|bibcode=2020NIMPA.95363112D|s2cid=209942845|access-date=2020-04-24|archive-date=2021-02-24|archive-url=https://web.archive.org/web/20210224162316/https://www.sciencedirect.com/science/article/pii/S0168900219314445|url-status=live}}</ref>

=== Fission energy neutrons ===
{{Main|Nuclear fission}}
A ''fast neutron'' is a free neutron with a kinetic energy level close to {{val|1|ul=MeV}} ({{val|1.6|e=-13|ul=J}}), hence a speed of ~{{val|14000|u=km/s}} (~{{hsp}}5% of the speed of light). They are named ''fission energy'' or ''fast'' neutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes such as [[nuclear fission]]. Neutrons produced in fission, as noted above, have a [[Maxwell–Boltzmann distribution]] of kinetic energies from 0 to ~14 MeV, a mean energy of 2 MeV (for <sup>235</sup>U fission neutrons), and a [[mode (statistics)|mode]] of only 0.75 MeV, which means that more than half of them do not qualify as fast (and thus have almost no chance of initiating fission in [[fertile material]]s, such as <sup>238</sup>U and <sup>232</sup>Th).

Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a [[neutron moderator]]. In reactors, typically [[heavy water]], [[light water reactor|light water]], or [[graphite]] are used to moderate neutrons.

=== Fusion neutrons ===
[[File:fusion rxnrate.svg|right|300px|thumb|The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The D–T rate peaks at a lower temperature (about 70&nbsp;keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.]]

{{Further|Nuclear fusion#Criteria and candidates for terrestrial reactions}}

D–T ([[deuterium]]–[[tritium]]) fusion is the [[deuterium–tritium fusion|fusion reaction]] that produces the most energetic neutrons, with 14.1&nbsp;[[MeV]] of [[kinetic energy]] and traveling at 17% of the [[speed of light]]. D–T fusion is also the easiest fusion reaction to ignite, reaching near-peak rates even when the deuterium and tritium nuclei have only a thousandth as much kinetic energy as the 14.1&nbsp;MeV that will be produced.

14.1&nbsp;MeV neutrons have about 10 times as much energy as fission neutrons, and they are very effective at fissioning even non-[[fissile]] [[actinides|heavy nuclei]]. These high-energy fissions also produce more neutrons on average than fissions by lower-energy neutrons.  D–T fusion neutron sources, such as proposed [[tokamak]] power reactors, are therefore useful for [[nuclear transmutation|transmutation]] of transuranic waste. 14.1&nbsp;MeV neutrons can also produce neutrons by [[spallation#Nuclear spallation|knocking them loose from nuclei]].

On the other hand, these very high-energy neutrons are less likely to simply [[neutron capture|be captured without causing fission or spallation]]. For these reasons, [[nuclear weapon design]] extensively uses D–T fusion 14.1&nbsp;MeV neutrons to [[fusion boosting|cause more fission]]. Fusion neutrons are able to cause fission in ordinarily non-fissile materials, such as [[depleted uranium]] (uranium-238), and these materials have been used in the jackets of [[thermonuclear weapon]]s. Fusion neutrons also can cause fission in substances that are unsuitable or difficult to make into primary fission bombs, such as [[reactor grade plutonium]]. This physical fact thus causes ordinary non-weapons grade materials to become of concern in certain [[nuclear proliferation]] discussions and treaties.

Other fusion reactions produce much less energetic neutrons. D–D fusion produces a 2.45&nbsp;MeV neutron and [[helium-3]] half of the time and produces [[tritium]] and a proton but no neutron the rest of the time. D–<sup>3</sup>He fusion produces no neutron.

=== Intermediate-energy neutrons ===
[[File:Sasahara.svg|thumb|right|300px|Transmutation flow in [[light water reactor]], which is a thermal-spectrum reactor]]

A fission energy neutron that has slowed down but not yet reached thermal energies is called an epithermal neutron.

[[Cross section (physics)|Cross sections]] for both [[neutron capture|capture]] and [[nuclear fission|fission]] reactions often have multiple [[resonance]] peaks at specific energies in the epithermal energy range. These are of less significance in a [[fast-neutron reactor]], where most neutrons are absorbed before slowing down to this range, or in a well-[[neutron moderator|moderated]] [[thermal reactor]], where epithermal neutrons interact mostly with moderator nuclei, not with either [[fissile]] or [[fertile material|fertile]] [[actinide]] nuclides. But in a partially moderated reactor with more interactions of epithermal neutrons with heavy metal nuclei, there are greater possibilities for [[transient state (chemical engineering)|transient]] changes in [[nuclear chain reaction|reactivity]] that might make reactor control more difficult.

Ratios of capture reactions to fission reactions are also worse (more captures without fission) in most [[nuclear fuel]]s such as [[plutonium-239]], making epithermal-spectrum reactors using these fuels less desirable, as captures not only waste the one neutron captured but also usually result in a [[nuclide]] that is not [[fissile]] with thermal or epithermal neutrons, though still [[fissionable]] with fast neutrons. The exception is [[uranium-233]] of the [[thorium cycle]], which has good capture-fission ratios at all neutron energies.

=== High-energy neutrons ===
High-energy neutrons have much more energy than fission energy neutrons and are generated as secondary particles by [[particle accelerator]]s or in the atmosphere from [[cosmic ray]]s. These high-energy neutrons are extremely efficient at [[ionization]] and far more likely to cause [[cell (biology)|cell]] death than [[X-ray]]s or protons.<ref>{{cite web |url= http://medicalphysicsweb.org/cws/article/research/34364 |title= Facing up to secondary neutrons |access-date= 2011-02-08 |author= Freeman, Tami |date= May 23, 2008 |publisher= Medical Physics Web |archive-url= https://web.archive.org/web/20101220151101/http://medicalphysicsweb.org/cws/article/research/34364 |archive-date= 2010-12-20 |url-status=dead }}</ref><ref>{{cite journal |title= Expand+Overview of secondary neutron production relevant to shielding in space |journal= [[Radiation Protection Dosimetry]] |year= 2005 |volume= 116 |issue= 1–4 |pages= 140–143 |doi= 10.1093/rpd/nci033 |pmid= 16604615 |last1= Heilbronn |first1= L. |last2= Nakamura |first2= T |last3= Iwata |first3= Y |last4= Kurosawa |first4= T |last5= Iwase |first5= H |last6= Townsend |first6= LW |url= http://www.escholarship.org/uc/item/1nh2s8n5 |access-date= 2019-01-25 |archive-date= 2019-01-26 |archive-url= https://web.archive.org/web/20190126060915/https://escholarship.org/uc/item/1nh2s8n5 |url-status= live }}</ref>

== See also ==
{{Commons category}}
* [[Ionizing radiation]]
* [[Isotope]]
* [[List of particles]]
* [[Neutron radiation]] and the [[Sievert|Sievert radiation scale]]
* [[Neutronium]]
* [[Nuclear reaction]]
* [[Nucleosynthesis]]
** [[Neutron capture nucleosynthesis]]
** [[R-process]]
** [[S-process]]
* [[Thermal-neutron reactor]]

=== Neutron sources ===
* [[Neutron generator]]
* [[Neutron source]]

=== Processes involving neutrons ===
* [[Neutron bomb]]
* [[Neutron diffraction]]
* [[Neutron flux]]
* [[Neutron imaging]]
* [[Neutron transport]]
* [[Cosmogenic radionuclide dating]]

== References ==
{{reflist|25em}}

== Further reading ==
* James Byrne, ''Neutrons, Nuclei and Matter: An Exploration of the Physics of Slow Neutrons''. Mineola, New York: Dover Publications, 2011. {{ISBN|0486482383}}.
* [[Abraham Pais]], ''Inward Bound'', Oxford: Oxford University Press, 1986. {{ISBN|0198519974}}.
* [[Sin-Itiro Tomonaga]], ''The Story of Spin'', The University of Chicago Press, 1997
* [[Herwig Schopper]], ''Weak interactions and nuclear beta decay'', Publisher, North-Holland Pub. Co., 1966.
* [http://webarchive.loc.gov/all/20150107225156/http%3A//alsos.wlu.edu/qsearch.aspx?browse%3Dscience/Neutrons Annotated bibliography for neutrons from the Alsos Digital Library for Nuclear Issues]
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