View source for Neutron flux

{{Short description|Total distance traveled by neutrons within a volume over a time period}}
{{Use dmy dates|date=January 2020}}
{{More citations needed|date=July 2008}}
{{Science with neutrons}}

The '''neutron flux''' is a [[scalar (physics)|scalar]] quantity used in [[nuclear physics]] and [[nuclear reactor physics]]. It is the total distance travelled by all free [[neutron]]s per unit time and volume.<ref name="Stamm'ler_1983" /> Equivalently, it can be defined as the number of neutrons travelling through a small sphere of radius <math>R</math> in a time interval, divided by a maximal cross section of the sphere (the [[great disk]] area, <math>\pi R^2</math>) and by the duration of the time interval.<ref name="Beckurts_1964" />{{rp|pages=[https://archive.org/details/neutronphysics0000beck/page/82/mode/2up 82{{hyphen}}83]}} The [[dimension (physics)|dimension]] of neutron flux is <math>\mathsf{L}^{-2}\mathsf{T}^{-1}</math> and the usual [[Units of measurement|unit]] is cm<sup>−2</sup>s<sup>−1</sup> (reciprocal [[square centimetre]] times [[reciprocal second]]).

The '''neutron fluence''' is defined as the neutron flux [[integral|integrated]] over a certain time period. So its dimension is <math>\mathsf{L}^{-2}</math> and its usual unit is cm<sup>−2</sup> (reciprocal square centimetre). An older term used instead of cm<sup>−2</sup> was "n.v.t." (neutrons, velocity, time).<ref>{{cite book | author = M. F. Kaplan | title = Nuclear Radiation and the Properties of Concrete | date = August 1983 | url = https://inis.iaea.org/collection/NCLCollectionStore/_Public/15/052/15052206.pdf | accessdate = 14 September 2022 | publisher = University of Cape Town | page = 2}}</ref>

==Natural neutron flux==
Neutron flux in [[asymptotic giant branch]] [[star]]s and in [[supernovae]] is responsible for most of the natural [[nucleosynthesis]] producing [[Chemical element|element]]s heavier than [[iron]]. In stars there is a relatively low neutron flux on the order of 10<sup>5</sup> to 10<sup>11</sup> cm<sup>−2</sup> s<sup>−1</sup>, resulting in nucleosynthesis by the [[s-process]] (slow neutron-capture process). By contrast, after a core-collapse supernova, there is an extremely high neutron flux, on the order of 10<sup>32</sup> cm<sup>−2</sup> s<sup>−1</sup>,<ref>{{cite journal |last1=Burbidge |first1=E. Margaret |last2=Burbidge |first2=G. R. |last3=Fowler|first3=William A.|last4=Hoyle|first4=F. |date=October 1957 |title=Synthesis of the Elements in Stars |journal=Reviews of Modern Physics |volume=29 |issue=4 |pages=548–650 |doi=10.1103/RevModPhys.29.547 |doi-access=free |bibcode=1957RvMP...29..547B }}</ref> resulting in nucleosynthesis by the [[r-process]] (rapid neutron-capture process).

Earth atmospheric neutron flux, apparently from thunderstorms, can reach levels of 3·10<sup>−2</sup> to 9·10<sup>+1</sup> cm<sup>−2</sup> s<sup>−1</sup>.<ref>{{cite journal |last1=Gurevich |first1=A. V. |last2=Antonova |first2=V. P. |year=2012 |title=Strong Flux of Low-Energy Neutrons Produced by Thunderstorms |journal=Physical Review Letters |volume=108 |issue=12 |page= 125001|publisher=American Physical Society |doi=10.1103/PhysRevLett.108.125001 |bibcode = 2012PhRvL.108l5001G |pmid=22540588}} 
</ref><ref>{{cite journal |last1=Gurevich |first1=A. V. |last2=Almenova |first2=A. M. |year=2016 |title=Observations of high-energy radiation during thunderstorms at Tien-Shan |journal=Physical Review D |volume=94 |issue=2 |page= 023003|publisher=American Physical Society |doi=10.1103/PhysRevD.94.023003|bibcode=2016PhRvD..94b3003G }} 
</ref> However, recent results<ref>{{cite journal |last1=Alekseenko |first1=V. |last2=Arneodo |first2=F. |last3=Bruno |first3=G. |last4=Di Giovanni |first4=A. |last5=Fulgion |first5=W. |last6=Gromushkin |first6=D. |last7=Shchegolev |first7=O. |last8=Stenkin |first8=Yu. |last9=Stepanov |first9=V. |last10=Sulakov |first10=V. |last11=Yashin |first11=I. |year=2015 |title=Decrease of Atmospheric Neutron Counts Observed during Thunderstorms |journal=Physical Review Letters |volume=114 |issue=12 |page=125003 |publisher=American Physical Society |doi=10.1103/PhysRevLett.114.125003 |pmid=25860750 |bibcode = 2015PhRvL.114l5003A|url=https://www.openaccessrepository.it/record/140269 }}</ref> (considered invalid by the original investigators<ref>{{cite journal |last1=Gurevich |first1=A. V. |last2=Ptitsyn |first2=M. O. |year=2015 |title=Comment on "Decrease of Atmospheric Neutron Counts Observed during Thunderstorms" |journal=Physical Review Letters |volume=115 |issue=12 |page= 179501|publisher=American Physical Society |doi=10.1103/PhysRevLett.115.179501|bibcode=2015PhRvL.115q9501G |pmid=26551144 }}</ref>) obtained with unshielded scintillation neutron detectors show a decrease in the neutron flux during thunderstorms. Recent research appears to support lightning generating 10<sup>13</sup>–10<sup>15</sup> neutrons per discharge via [[Photodisintegration|photonuclear processes]].<ref>{{cite journal |last1=Köhn |first1=Christoph |last2=Diniz |first2=Gabriel |last3=Harakeh |first3=GMushin |year=2017 |title=Production mechanisms of leptons, photons, and hadrons and their possible feedback close to lightning leaders|journal=Journal of Geophysical Research: Atmospheres |volume=122 |issue=2 |pages= 1366|publisher=American Geophysical Union |doi=10.1002/2016JD025445 |bibcode =2017JGRD..122.1365K   |pmid= 28357174|pmc=5349290 }} 
</ref>

==Artificial neutron flux==
{{Further |Neutron radiation}}

Artificial neutron flux refers to neutron flux which is man-made, either as byproducts from weapons or nuclear energy production or for a specific application such as from a [[research reactor]] or by [[spallation]]. A flow of neutrons is often used to initiate the [[nuclear fission|fission]] of unstable large nuclei. The additional neutron(s) may cause the nucleus to become unstable, causing it to decay (split) to form more stable products. This effect is essential in [[fission reactor]]s and [[nuclear weapon]]s.

Within a nuclear fission reactor, the neutron flux is the primary quantity measured to control the reaction inside. The flux shape is the term applied to the density or relative strength of the flux as it moves around the reactor. Typically the strongest neutron flux occurs in the middle of the reactor core, becoming lower toward the edges. The higher the neutron flux the greater the chance of a nuclear reaction occurring as there are more neutrons going through an area per unit time.

=== Reactor vessel wall neutron fluence ===
A [[reactor vessel]] of a typical nuclear power plant ([[Pressurized water reactor|PWR]]) endures in 40 years (32 full reactor years) of operation approximately 6.5×10<sup>19</sup> cm<sup>−2</sup> ([[Energy|''E'']] > 1 [[Electronvolt|MeV]]) of neutron fluence.<ref>[http://www.government.nl/documents-and-publications/reports/2012/07/12/kcb-rpv-safety-assessment.html Nuclear Power Plant Borssele Reactor Pressure Vessel Safety Assessment], p. 29, 5.6 Neutron Fluence Calculation.</ref> Neutron flux causes reactor vessels to suffer from [[neutron embrittlement]].

==See also==
*[[Neutron radiation]]
*[[Neutron transport]]

==References==
{{Reflist|30em|refs=

<ref name="Stamm'ler_1983">{{cite book | last1 = Stamm'ler | first1 = Rudi J. J. | last2 = Abbate | first2 = Máximo Julio | date = 1983-07-01 | chapter =  | chapter-url =  | chapter-url-access =  | title = Methods of Steady-State Reactor Physics in Nuclear Design | title-link =  | url = | url-status =  | url-access =  | language = en | edition = 1st | publisher = [[Academic Press]] | pages =  | isbn = 978-0126633207 | lccn = 82072342 | oclc = 9915614 | ol = OL3512075M | df = dmy-all}}</ref>

<ref name="Beckurts_1964">{{cite book | last1 = Beckurts | first1 = Karl-Heinrich | last2 = Wirtz | first2 = Karl | translator-last1 = Dresner | translator-first1 = L. | date = 1964 | chapter = 5.1.1 Neutron Flux, Neutron Density, and Neutron Current | chapter-url = https://archive.org/details/neutronphysics0000beck/page/82/mode/2up | chapter-url-access = registration  | title = Neutron Physics | title-link =  | url = https://archive.org/details/neutronphysics0000beck | url-access = registration  | language = en | edition = 1st | publisher = [[Springer Science+Business Media|Springer-Verlang]] | pages =  | isbn = 978-3540030966 | lccn = 64025646 | oclc = 569910840 | ol = OL27986790M | via = [[Internet Archive]] | df = dmy-all}}</ref>

}}

{{Fusion power}}

{{DEFAULTSORT:Neutron Flux}}
[[Category:Neutron|Flux]]
[[Category:Physical quantities]]