Neutron flux

The neutron flux is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total distance travelled by all free neutrons 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" /> The dimension of neutron flux is <math>\mathsf{L}^{-2}\mathsf{T}^{-1}</math> and the usual unit is cm⁻²s⁻¹ (reciprocal square centimetre times reciprocal second).

The neutron fluence is defined as the neutron flux integrated over a certain time period. So its dimension is <math>\mathsf{L}^{-2}</math> and its usual unit is cm⁻² (reciprocal square centimetre). An older term used instead of cm⁻² was "n.v.t." (neutrons, velocity, time).<ref>M. F. Kaplan (August 1983). Nuclear Radiation and the Properties of Concrete (PDF). University of Cape Town. p. 2. Retrieved 14 September 2022.</ref>

Natural neutron flux

Neutron flux in asymptotic giant branch stars and in supernovae is responsible for most of the natural nucleosynthesis producing elements heavier than iron. In stars there is a relatively low neutron flux on the order of 10⁵ to 10¹¹ cm⁻² s⁻¹, 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 10³² cm⁻² s⁻¹,<ref>Burbidge, E. Margaret; Burbidge, G. R.; Fowler, William A.; Hoyle, F. (October 1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 548–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547.</ref> resulting in nucleosynthesis by the r-process (rapid neutron-capture process).

Earth atmospheric neutron flux, apparently from thunderstorms, can reach levels of 3·10⁻² to 9·10⁺¹ cm⁻² s⁻¹.<ref>Gurevich, A. V.; Antonova, V. P. (2012). "Strong Flux of Low-Energy Neutrons Produced by Thunderstorms". Physical Review Letters. American Physical Society. 108 (12): 125001. Bibcode:2012PhRvL.108l5001G. doi:10.1103/PhysRevLett.108.125001. PMID 22540588. </ref><ref>Gurevich, A. V.; Almenova, A. M. (2016). "Observations of high-energy radiation during thunderstorms at Tien-Shan". Physical Review D. American Physical Society. 94 (2): 023003. Bibcode:2016PhRvD..94b3003G. doi:10.1103/PhysRevD.94.023003. </ref> However, recent results<ref>Alekseenko, V.; Arneodo, F.; Bruno, G.; Di Giovanni, A.; Fulgion, W.; Gromushkin, D.; Shchegolev, O.; Stenkin, Yu.; Stepanov, V.; Sulakov, V.; Yashin, I. (2015). "Decrease of Atmospheric Neutron Counts Observed during Thunderstorms". Physical Review Letters. American Physical Society. 114 (12): 125003. Bibcode:2015PhRvL.114l5003A. doi:10.1103/PhysRevLett.114.125003. PMID 25860750.</ref> (considered invalid by the original investigators<ref>Gurevich, A. V.; Ptitsyn, M. O. (2015). "Comment on "Decrease of Atmospheric Neutron Counts Observed during Thunderstorms"". Physical Review Letters. American Physical Society. 115 (12): 179501. Bibcode:2015PhRvL.115q9501G. doi:10.1103/PhysRevLett.115.179501. 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 10¹³–10¹⁵ neutrons per discharge via photonuclear processes.<ref>Köhn, Christoph; Diniz, Gabriel; Harakeh, GMushin (2017). "Production mechanisms of leptons, photons, and hadrons and their possible feedback close to lightning leaders". Journal of Geophysical Research: Atmospheres. American Geophysical Union. 122 (2): 1366. Bibcode:2017JGRD..122.1365K. doi:10.1002/2016JD025445. PMC 5349290. PMID 28357174. </ref>

Artificial neutron flux

Further information: 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 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 reactors and nuclear weapons.

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 (PWR) endures in 40 years (32 full reactor years) of operation approximately 6.5×10¹⁹ cm⁻² (E > 1 MeV) of neutron fluence.<ref>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