View source for Neutron emission

{{Short description|Type of radioactive decay}}
{{One source|date=March 2016}}
{{Nuclear physics}}'''Neutron emission''' is a mode of [[radioactive decay]] in which one or more [[neutron]]s are ejected from a [[Atomic nucleus|nucleus]].<ref>{{Citation |last=Guinn |first=Vincent P. |title=Radioactivity |date=2003 |encyclopedia=Encyclopedia of Physical Science and Technology |pages=661–674 |url=https://www.sciencedirect.com/topics/physics-and-astronomy/neutron-emission |access-date=2025-04-26 |publisher=Elsevier |language=en |doi=10.1016/B0-12-227410-5/00643-8 |isbn=978-0-12-227410-7|url-access=subscription }}</ref> It occurs in the most neutron-rich/proton-deficient [[nuclides]], and also from excited states of other nuclides as in [[photodisintegration|photoneutron emission]] and beta-delayed neutron emission. As only a neutron is lost by this process the number of [[proton]]s remains unchanged, and an atom does not become an atom of a different element, but a different [[isotope]] of the same element.<ref>{{Cite web |title=What is radiation? |url=https://www.arpansa.gov.au/understanding-radiation/what-is-radiation/ionising-radiation/radiation-decay |website=www.arpansa.gov.au/}}</ref>

Neutrons are also produced in the [[spontaneous fission|spontaneous]] and [[nuclear fission|induced fission]] of certain heavy nuclides.

==Spontaneous neutron emission==
As a consequence of the [[Pauli exclusion principle]], nuclei with an excess of protons or neutrons have a higher average energy per nucleon. Nuclei with a sufficient excess of neutrons have a greater energy than the combination of a free neutron and a nucleus with one less neutron, and therefore can decay by neutron emission. Nuclei which can decay by this process are described as lying beyond the [[neutron drip line]].

Two examples of isotopes that emit neutrons are [[beryllium-13]] (decaying to [[beryllium-12]] with a mean life {{val|2.7|e=-21|ul=s}}) and [[helium-5]] ([[helium-4]], {{val|7|e=-22|u=s}}).<ref>{{cite web |url=http://education.jlab.org/glossary/neutron_emission.html |title=Neutron Emission |format=webpage |access-date=2014-10-30 |archive-date=2016-11-21 |archive-url=https://web.archive.org/web/20161121022030/http://education.jlab.org/glossary/neutron_emission.html }}</ref>

In tables of nuclear decay modes, neutron emission is commonly denoted by the abbreviation ''n''.

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=== Double neutron emission ===
Some neutron-rich isotopes decay by the emission of two or more neutrons. For example, hydrogen-5 and helium-10 decay by the emission of two neutrons, hydrogen-6 by the emission of 3 or 4 neutrons, and hydrogen-7 by emission of 4 neutrons.

==Photoneutron emission==
{{Main|Photodisintegration}}
Some nuclides can be induced to eject a neutron by [[gamma radiation]]. One such nuclide is [[beryllium-9|<sup>9</sup>Be]]; its photodisintegration is significant in nuclear astrophysics, pertaining to the abundance of beryllium and the consequences of the instability of [[beryllium-8|<sup>8</sup>Be]]. This also makes this isotope useful as a neutron source in nuclear reactors.<ref name="9Ber">{{cite journal |last1=Odsuren |first1=M. |last2=Katō |first2=K. |last3=Kikuchi |first3=Y. |last4=Aikawa |first4=M. |last5=Myo |first5=T. |title=A resonance problem on the low-lying resonant state in the 9Be system |journal=Journal of Physics: Conference Series |date=2014 |volume=569 |issue=1 |article-number=012072 |doi=10.1088/1742-6596/569/1/012072 |bibcode=2014JPhCS.569a2072O |url=http://inspirehep.net/record/1333594/files/1742-6596_569_1_012072.pdf|doi-access=free }}</ref> Another nuclide, [[tantalum-181|<sup>181</sup>Ta]], is also known to be readily capable of photodisintegration; this process is thought to be responsible for the creation of [[tantalum-180m|<sup>180m</sup>Ta]], the only primordial [[nuclear isomer]] and the rarest [[primordial nuclide]].<ref>{{cite journal |last1=Utsonomiya |first1=H. |last2=Akimune |first2=H. |last3=Goko |first3=S. |last4=Yamagata |first4=T. |last5=Ohta |first5=M. |last6=Ohgaki |first6=H. |last7=Toyokawa |first7=H. |last8=Sumiyoshi |first8=K. |last9=Lui |first9=Y.-W. |title=Photoneutron Cross Sections for Nuclear Astrophysics |journal=Journal of Nuclear Science and Technology |volume=Supplement 2 |pages=542–545 |date=2002 |doi=10.1080/00223131.2002.10875158 |bibcode=2002JNST...39S.542U |s2cid=124167982 |doi-access=free }}</ref>

== Beta-delayed neutron emission ==
{{Unreferenced section|date=April 2023}}
Neutron emission usually happens from nuclei that are in an excited state, such as the excited [[Oxygen-17|<sup>17</sup>O*]] produced from the beta decay of [[Nitrogen-17|<sup>17</sup>N]]. The neutron emission process itself is controlled by the [[nuclear force]] and therefore is extremely fast, sometimes referred to as "nearly instantaneous". This process allows unstable atoms to become more stable. The ejection of the neutron may be as a product of the movement of many nucleons, but it is ultimately mediated by the repulsive action of the nuclear force that exists at extremely short-range distances between nucleons.

=== Delayed neutrons in reactor control ===
{{Main|Nuclear reactor physics}}

Most neutron emission outside prompt neutron production associated with fission (either induced or spontaneous), is from neutron-heavy isotopes produced as [[fission products]]. These neutrons are sometimes emitted with a delay, giving them the term [[delayed neutron]]s, but the actual delay in their production is a delay waiting for the [[beta decay]] of fission products to produce the excited-state nuclear precursors that immediately undergo prompt neutron emission. Thus, the delay in neutron emission is not from the neutron-production process, but rather its precursor beta decay, which is controlled by the weak force, and thus requires a far longer time. The beta decay half lives for the precursors to delayed neutron-emitter radioisotopes, are typically fractions of a second to tens of seconds.

Nevertheless, the delayed neutrons emitted by neutron-rich [[fission product]]s aid control of [[nuclear reactor]]s by making reactivity change far more slowly than it would if it were controlled by prompt neutrons alone. About 0.65% of neutrons are released in a [[nuclear chain reaction]] in a delayed way due to the mechanism of neutron emission, and it is this fraction of neutrons that allows a nuclear reactor to be controlled on human reaction time-scales, without proceeding to a [[prompt critical]] state, and runaway melt down.

== Neutron emission in fission ==
{{Unreferenced section|date=April 2023}}

===Induced fission===
A synonym for such neutron emission is "[[prompt neutron]]" production, of the type that is best known to occur simultaneously with induced [[nuclear fission]]. Induced fission happens only when a nucleus is bombarded with neutrons, gamma rays, or other carriers of energy. Many heavy isotopes, most notably [[californium-252]], also emit prompt neutrons among the products of a similar spontaneous radioactive decay process, [[spontaneous fission]].

===Spontaneous fission===
Spontaneous fission happens when a nucleus splits into two (occasionally [[ternary fission|three]]) smaller nuclei and generally one or more neutrons.

==See also==
*[[Neutron radiation]]
*[[Neutron source]]
*[[Proton emission]]

==References==
{{Reflist}}

==External links==
*[https://web.archive.org/web/20080408125305/http://www.epa.gov/rpdweb00/understand/radiation.html "Why Are Some Atoms Radioactive?"] EPA. Environmental Protection Agency, n.d. Web. 31 Oct. 2014
*[http://www-nds.iaea.org/livechart The LIVEChart of Nuclides – IAEA ] with filter on delayed neutron emission decay
*[http://www-nds.iaea.org/queryensdf Nuclear Structure and Decay Data – IAEA ] with query on Neutron Separation Energy

{{Nuclear processes}}

[[Category:Neutron|Emission]]
[[Category:Nuclear physics]]
[[Category:Radioactivity]]