Editing Neutron (section)

=== Free neutron ===
{{main|Free neutron decay}}

Neutrons are tightly bound in atomic nuclei, requiring MeV sized energies to bust out. Once free, neutrons decay in a quarter of an hour on average. Thus free neutrons are rare compared to other components of atoms: electrons are freed by heating a light bulb filament and protons are freed in rapid hydrogen gas combustion. 
Moreover, once freed in say a nuclear reactor, the charge-free neutrons are difficult to direct, confine, or detect.<ref>{{Cite journal |last1=Dubbers |first1=Dirk |last2=Schmidt |first2=Michael G. |date=2011-10-24 |title=The neutron and its role in cosmology and particle physics |url=https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.83.1111 |journal=Reviews of Modern Physics |volume=83 |issue=4 |pages=1111–1171 |doi=10.1103/RevModPhys.83.1111|arxiv=1105.3694 |bibcode=2011RvMP...83.1111D }}</ref>

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|spin-{{sfrac|1|2}}]] [[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>

Different experimental methods for measuring the neutron's lifetime, the "bottle" and "beam" methods, produce slightly different values.<ref>{{Cite journal |last1=Czarnecki |first1=Andrzej |last2=Marciano |first2=William J. |last3=Sirlin |first3=Alberto |date=2018-05-16 |title=Neutron Lifetime and Axial Coupling Connection |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.202002 |journal=Physical Review Letters |volume=120 |issue=20 |pages=202002 |doi=10.1103/PhysRevLett.120.202002|pmid=29864332 |arxiv=1802.01804 |bibcode=2018PhRvL.120t2002C }}</ref><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 proportion of free neutrons (about four per million) 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 energy that is necessary for it to escape the proton ({{val|13.6|ul=eV}}, the [[ionization energy]] of [[hydrogen]]), and therefore 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}}.)