Editing Neutron (section)

=== 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}}