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Neutron radiation
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{{Short description|Ionizing radiation that presents as free neutrons}} {{Science with neutrons}} '''Neutron radiation''' is a form of [[ionizing radiation]] that presents as [[free neutron]]s. Typical phenomena are [[nuclear fission]] or [[nuclear fusion]] causing the release of free neutrons, which then [[Neutron capture|react]] with [[Atomic nucleus|nuclei]] of other [[atom]]s to form new [[nuclide]]s—which, in turn, may trigger further neutron radiation. Free neutrons are unstable, [[Free neutron decay|decaying]] into a [[proton]], an [[electron]], plus an [[Electron neutrino#Electron antineutrino|electron antineutrino]]. Free neutrons have a mean lifetime of 887 seconds (14 minutes, 47 seconds).<ref>{{cite journal|title=Improved Determination of the Neutron Lifetime|first1=A. T.|last1=Yue|first2=M. S.|last2=Dewey|first3=D. M.|last3=Gilliam|first4=G. L.|last4=Greene|first5=A. B.|last5=Laptev|first6=J. S.|last6=Nico|first7=W. M.|last7=Snow|first8=F. E.|last8=Wietfeldt|date=27 November 2013|journal=Physical Review Letters|volume=111|issue=22|pages=222501|doi=10.1103/PhysRevLett.111.222501|pmid = 24329445|arxiv=1309.2623|bibcode=2013PhRvL.111v2501Y|s2cid=17006418}}</ref> Neutron radiation is distinct from [[alpha radiation|alpha]], [[beta radiation|beta]] and [[gamma radiation|gamma]] radiation.<ref>{{Cite web |title=What Are The Different Types of Radiation? |url=https://www.nrc.gov/reading-rm/basic-ref/students/science-101/what-are-different-types-of-radiation.html}}</ref> == Sources == {{Main|Neutron source}} {{See also|Category:Neutron sources}} [[Neutron]]s may be emitted from [[nuclear fusion]] or [[nuclear fission]], or from other [[nuclear reaction]]s such as [[radioactive decay]] or particle interactions with [[cosmic ray]]s or within [[particle accelerator]]s. Large neutron sources are rare, and usually limited to large-sized devices such as [[nuclear reactor]]s or [[particle accelerator]]s, including the [[Spallation Neutron Source]]. Neutron radiation was discovered from observing an [[alpha particle]] colliding with a [[beryllium]] [[atomic nucleus|nucleus]], which was transformed into a [[carbon]] [[atomic nucleus|nucleus]] while emitting a [[neutron]], [[Beryllium|Be]]([[alpha particle|α]], [[neutron|n]])[[Carbon|C]]. The combination of an alpha particle emitter and an isotope with a large ([[alpha particle|α]], [[neutron|n]]) [[cross section (physics)|nuclear reaction probability]] is still a common neutron source. === Neutron radiation from fission === The neutrons in nuclear reactors are generally categorized as [[slow neutron|slow (thermal) neutrons]] or [[fast neutron]]s depending on their energy. Thermal neutrons are similar in energy distribution (the [[Maxwell–Boltzmann distribution]]) to a gas in [[thermodynamic equilibrium]]; but are easily captured by atomic nuclei and are the primary means by which elements undergo [[nuclear transmutation]]. To achieve an effective fission chain reaction, neutrons produced during fission must be captured by fissionable nuclei, which then split, releasing more neutrons. In most fission reactor designs, the [[nuclear fuel]] is not sufficiently refined to absorb enough fast neutrons to carry on the chain reaction, due to the lower [[Cross section (physics)|cross section]] for higher-energy neutrons, so a [[neutron moderator]] must be introduced to slow the fast neutrons down to thermal velocities to permit sufficient absorption. Common neutron moderators include [[graphite]], ordinary (light) [[water]] and [[heavy water]]. A few reactors ([[fast neutron reactor]]s) and all [[nuclear weapon]]s rely on fast neutrons. === Cosmogenic neutrons === {{uncited section|date=June 2022}} {{Main|Cosmogenic neutron}} Cosmogenic neutrons are produced from cosmic radiation in the Earth's atmosphere or surface, as well as in particle accelerators. They often possess higher energy levels compared to neutrons found in reactors. Many of these neutrons activate atomic nuclei before reaching the Earth's surface, while a smaller fraction interact with nuclei in the atmospheric air.<ref>{{Cite web |date=2022-10-26 |title=Cosmogenic Nucleide Principle - CEREGE |url=https://www.cerege.fr/en/equipment/laboratoire-national-des-nucleides-cosmogeniques/principle-of-cosmogenic-nucleides/ |access-date=2024-07-16 |website=www.cerege.fr |language=en-GB}}</ref> When these neutrons interact with nitrogen-14 atoms, they can transform them into [[carbon-14]] (14C), which is extensively utilized in [[radiocarbon dating]].<ref>{{Cite web |title=What is Carbon Dating? {{!}} University of Chicago News |url=https://news.uchicago.edu/explainer/what-is-carbon-14-dating#how |access-date=2024-09-19 |website=news.uchicago.edu |language=en}}</ref> == Uses == [[Neutron temperature|''Cold'', ''thermal'' and ''hot'']] neutron radiation is most commonly used in [[neutron scattering|scattering]] and [[neutron diffraction|diffraction]] experiments, to assess the properties and the structure of materials in [[crystallography]], [[condensed matter physics]], [[biology]], [[solid state chemistry]], [[materials science]], [[geology]], [[mineralogy]], and related sciences. Neutron radiation is also used in [[Boron Neutron Capture Therapy]] to treat cancerous tumors due to its highly penetrating and damaging nature to cellular structure. Neutrons can also be used for imaging of industrial parts termed [[neutron radiography]] when using film, neutron radioscopy when taking a digital image, such as through image plates, and [[neutron tomography]] for three-dimensional images. [[Neutron imaging]] is commonly used in the nuclear industry,<ref>{{Cite journal |last1=Craft |first1=Aaron E. |last2=Barton |first2=John P. |date=2017-01-01 |title=Applications of Neutron Radiography for the Nuclear Power Industry |url=https://www.sciencedirect.com/science/article/pii/S1875389217300597 |journal=Physics Procedia |series=Neutron Imaging for Applications in Industry and Science Proceedings of the 8th International Topical Meeting on Neutron Radiography (ITMNR-8) Beijing, China, September 4-8, 2016 |volume=88 |pages=73–80 |doi=10.1016/j.phpro.2017.06.009 |bibcode=2017PhPro..88...73C |issn=1875-3892}}</ref> the space and aerospace industry,<ref>{{Cite web |title= |url=https://www.ndt.net/article/wcndt2004/pdf/aerospace/264_bastuerk.pdf |archive-url=http://web.archive.org/web/20220603210640/https://www.ndt.net/article/wcndt2004/pdf/aerospace/264_bastuerk.pdf |archive-date=2022-06-03 |access-date=2025-03-17 |website=www.ndt.net}}</ref> as well as the high reliability explosives industry. == Ionization mechanisms and properties == Neutron radiation is often called ''indirectly [[ionizing radiation]]''. It does not ionize atoms in the same way that charged particles such as [[proton]]s and [[electron]]s do (exciting an electron), because neutrons have no charge. However, neutron interactions are largely ionizing, for example when neutron absorption results in gamma emission and the [[gamma ray]] (photon) subsequently removes an electron from an atom, or a nucleus recoiling from a neutron interaction is ionized and causes more traditional subsequent ionization in other atoms. Because neutrons are uncharged, they are more penetrating than [[alpha radiation]] or [[beta radiation]]. In some cases they are more penetrating than gamma radiation, which is impeded in materials of high [[atomic number]]. In materials of low atomic number such as [[hydrogen]], a low energy gamma ray may be more penetrating than a high energy neutron. == Health hazards and protection == In [[health physics]], neutron radiation is a type of radiation hazard. Another, more severe hazard of neutron radiation, is [[neutron activation]], the ability of neutron radiation to induce [[radioactivity]] in most substances it encounters, including bodily tissues.<ref>{{Cite web|url=https://web.pa.msu.edu/courses/2000fall/PHY232/lectures/radioactive/damage.html|title=How Radiation Damages Tissue|website=Michigan State University|access-date=2017-12-21}}</ref> This occurs through the capture of neutrons by atomic nuclei, which are transformed to another [[nuclide]], frequently a [[radionuclide]]. This process accounts for much of the radioactive material released by the detonation of a [[nuclear weapon]]. It is also a problem in nuclear fission and nuclear fusion installations as it gradually renders the equipment radioactive such that eventually it must be replaced and disposed of as low-level [[radioactive waste]]. Neutron [[radiation protection]] relies on [[radiation shielding]]. Due to the high kinetic energy of neutrons, this radiation is considered the most severe and dangerous radiation to the whole body when it is exposed to external radiation sources. In comparison to conventional ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and slowed (absorbed) by light nuclei so hydrogen-rich material is more effective at shielding than [[iron]] nuclei. The light atoms serve to slow down the neutrons by [[elastic scattering]] so they can then be absorbed by [[nuclear reaction]]s. However, gamma radiation is often produced in such reactions, so additional shielding must be provided to absorb it. Care must be taken to avoid using materials whose nuclei undergo fission or [[neutron capture]] that causes [[radioactive decay]] of nuclei, producing gamma rays. Neutrons readily pass through most material, and hence the absorbed dose (measured in [[Gray (unit)|grays]]) from a given amount of radiation is low, but interact enough to cause biological damage. The most effective shielding materials are [[water]], or [[hydrocarbon]]s like [[polyethylene]] or [[paraffin wax]]. Water-extended polyester (WEP) is effective as a shielding wall in harsh environments due to its high hydrogen content and resistance to fire, allowing it to be used in a range of nuclear, health physics, and defense industries.<ref>{{Cite web|url=http://www.frontier-cf252.com/custom-shielding-walls.html|title=Neutron Radiation Shielding|website=www.frontier-cf252.com|publisher=Frontier Technology Corporation|access-date=2017-12-21}}</ref> Hydrogen-based materials are suitable for shielding as they are proper barriers against radiation.<ref>{{Cite news|url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/39/075/39075326.pdf|title=Neutron Shielding Performance of Water-Extended Polyester|last=Carrillo|first=Héctor René Vega|date=2006-05-15|work=TA-3 Dosimetry and Instrumentation|access-date=2017-12-21}}</ref> [[Concrete]] (where a considerable number of water molecules chemically bind to the cement) and [[gravel]] provide a cheap solution due to their combined shielding of both gamma rays and neutrons. [[Boron]] is also an excellent neutron absorber (and also undergoes some neutron scattering). Boron decays into carbon or helium and produces virtually no gamma radiation with [[boron carbide]], a shield commonly used where concrete would be cost prohibitive. Commercially, tanks of water or fuel oil, concrete, gravel, and B<sub>4</sub>C are common shields that surround areas of large amounts of [[neutron flux]], e.g., nuclear reactors. Boron-impregnated silica glass, standard [[borosilicate glass]], high-[[boron steel]], paraffin, and [[Plexiglas]] have niche uses. Because neutrons that strike the hydrogen nucleus ([[proton]], or [[deuteron]]) impart energy to that nucleus, they in turn break from their chemical bonds and travel a short distance before stopping. Such hydrogen nuclei are high [[linear energy transfer]] particles, and are in turn stopped by ionization of the material they travel through. Consequently, in living tissue, neutrons have a relatively high [[relative biological effectiveness]], and are roughly ten times more effective at causing biological damage compared to gamma or beta radiation of equivalent energy exposure. These neutrons can either cause cells to change in their functionality or to completely stop replicating, causing damage to the body over time.<ref>{{Cite web|url=https://ehss.energy.gov/ohre/roadmap/achre/intro_9_5.html|title=Advisory Committee On Human Radiation Experiments Final Report|last=Specialist|first=WPI, Environmental Information Services -- Shawn Denny, Information Architect; Mike Pizzuti, Graphic Designer; Chelene Neal, Web Information Specialist; Kate Bessiere, Web Information|website=ehss.energy.gov|access-date=2017-12-21}}</ref> Neutrons are particularly damaging to soft tissues like the [[cornea]] of the eye. == Effects on materials == High-energy neutrons damage and degrade materials over time; bombardment of materials with neutrons creates [[collision cascade]]s that can produce [[point defect]]s and [[dislocation]]s in the material, the creation of which is the primary driver behind microstructural changes occurring over time in materials exposed to radiation. At high neutron [[fluence]]s this can lead to [[embrittlement]] of metals and other materials, and to [[neutron-induced swelling]] in some of them. This poses a problem for nuclear reactor vessels and significantly limits their lifetime (which can be somewhat prolonged by controlled [[Annealing (metallurgy)|annealing]] of the vessel, reducing the number of the built-up dislocations). Graphite [[neutron moderator]] blocks are especially susceptible to this effect, known as [[Wigner effect]], and must be annealed periodically. The [[Windscale fire]] was caused by a mishap during such an annealing operation. Radiation damage to materials occurs as a result of the interaction of an energetic incident particle (a neutron, or otherwise) with a lattice atom in the material. The collision causes a massive transfer of kinetic energy to the lattice atom, which is displaced from its lattice site, becoming what is known as the [[primary knock-on atom]] (PKA). Because the PKA is surrounded by other lattice atoms, its displacement and passage through the lattice results in many subsequent collisions and the creations of additional knock-on atoms, producing what is known as the collision cascade or displacement cascade. The knock-on atoms lose energy with each collision, and terminate as [[Interstitial defect|interstitials]], effectively creating a series of [[Frenkel defect]]s in the lattice. Heat is also created as a result of the collisions (from electronic energy loss), as are possibly [[nuclear transmutation|transmuted atoms]]. The magnitude of the damage is such that a single 1 [[MeV]] neutron creating a PKA in an iron lattice produces approximately 1,100 Frenkel pairs.<ref name="Lecture">Dunand, David. "Materials in Nuclear Power Generation." Materials Science & Engineering 381: Materials for Energy Efficient Technology. Northwestern University, Evanston. 3 Feb. 2015. Lecture</ref> The entire cascade event occurs over a timescale of 1 × 10<sup>−13</sup> seconds, and therefore, can only be "observed" in computer simulations of the event.<ref name="Thermal Spike Lifetime">A. Struchbery, E. Bezakova "Thermal-Spike Lifetime from Picosecond-Duration Preequilibrium Effects in Hyperfine Magnetic Fields Following Ion Implantation". 3 May. 1999.</ref> The knock-on atoms terminate in non-equilibrium interstitial lattice positions, many of which annihilate themselves by diffusing back into neighboring vacant lattice sites and restore the ordered lattice. Those that do not or cannot leave vacancies, which causes a local rise in the vacancy concentration far above that of the equilibrium concentration. These vacancies tend to migrate as a result of [[Thermal transpiration|thermal diffusion]] towards vacancy sinks (i.e., [[grain boundaries]], [[dislocations]]) but exist for significant amounts of time, during which additional high-energy particles bombard the lattice, creating collision cascades and additional vacancies, which migrate towards sinks. The main effect of irradiation in a lattice is the significant and persistent flux of defects to sinks in what is known as the [[defect wind]]. Vacancies can also annihilate by combining with one another to form [[pinning point|dislocation loops]] and later, [[crystallographic defect|lattice voids]].<ref name="Lecture" /> The collision cascade creates many more vacancies and interstitials in the material than equilibrium for a given temperature, and [[diffusivity]] in the material is dramatically increased as a result. This leads to an effect called [[radiation-enhanced diffusion]], which leads to microstructural evolution of the material over time. The mechanisms leading to the evolution of the microstructure are many, may vary with temperature, flux, and fluence, and are a subject of extensive study.<ref name ="Radiation Effects in Nuclear Ceramics">{{cite journal|title=Radiation Effects in Nuclear Ceramics|first1=L.|last1=Thomé|first2=S.|last2=Moll|first3=A.|last3=Debelle|first4=F.|last4=Garrido|first5=G.|last5=Sattonnay|first6=J.|last6=Jagielski|date=1 June 2018|journal=Advances in Materials Science and Engineering|volume=2012|pages=1–13|doi=10.1155/2012/905474|doi-access=free}}</ref> * [[Radiation-induced segregation]] results from the aforementioned flux of vacancies to sinks, implying a flux of lattice atoms away from sinks; but not necessarily in the same proportion to alloy composition in the case of an alloyed material. These fluxes may therefore lead to depletion of alloying elements in the vicinity of sinks. For the flux of interstitials introduced by the cascade, the effect is reversed: the interstitials diffuse toward sinks resulting in alloy enrichment near the sink.<ref name="Lecture" /> * [[pinning point|Dislocation loops]] are formed if vacancies form clusters on a lattice plane. If these vacancy concentration expand in three dimensions, a [[Vacuum|void]] forms. By definition, voids are under vacuum, but may became gas-filled in the case of [[alpha particle|alpha-particle radiation]] (helium) or if the gas is produced as a result of [[nuclear transmutation|transmutation reactions]]. The void is then called a bubble, and leads to dimensional instability (neutron-induced swelling) of parts subject to radiation. Swelling presents a major long-term design problem, especially in reactor components made out of stainless steel.<ref name="Voids in Irradiated Stainless Steel">{{cite journal|title=Voids in Irradiated Stainless Steel|first1=C.|last1=CAWTHORNE|first2=E. J.|last2=FULTON|date=1 November 1967|journal=Nature|volume=216|issue=5115|pages=575–576|doi=10.1038/216575a0|bibcode=1967Natur.216..575C|s2cid=4238714}}</ref> Alloys with crystallographic [[isotropy]], such as [[Zircaloy]]s are subject to the creation of dislocation loops, but do not exhibit void formation. Instead, the loops form on particular lattice planes, and can lead to [[irradiation-induced growth]], a phenomenon distinct from swelling, but that can also produce significant dimensional changes in an alloy.<ref name="Effects of Neutron Radiation on Microstructure and Properties of Zircaloy">Adamson, R. "Effects of Neutron Radiation on Microstructure and the Properties of Zircaloy" 1977. 08 Feb. 2015.</ref> *Irradiation of materials can also induce [[phase transformation]]s in the material: in the case of a [[solid solution]], the solute enrichment or depletion at sinks radiation-induced segregation can lead to the precipitation of new phases in the material.<ref name="Neutron irradiation performance of Zircaloy-4 under research reactor operating conditions">Hyun Ju Jin, Tae Kyu Kim. "Neutron irradiation performance of Zircaloy-4 under research reactor operating conditions." Annals of Nuclear Energy. 13 Sept. 2014 Web. 08 Feb. 2015.</ref> The mechanical effects of these mechanisms include [[irradiation hardening]], [[embrittlement]], [[creep (deformation)|creep]], and [[stress corrosion cracking|environmentally-assisted cracking]]. The defect clusters, dislocation loops, voids, bubbles, and precipitates produced as a result of radiation in a material all contribute to the strengthening and [[embrittlement]] (loss of [[ductility]]) in the material.<ref name="Effect of Irradiation">{{cite book|chapter-url=http://www.astm.org/DIGITAL_LIBRARY/STP/PAGES/STP33683S.htm|title=Effects of Radiation on Structural Materials|first=CJ|last=Baroch|publisher=ASTM International|website=astm.org|pages=129–129–14|doi=10.1520/STP33683S|chapter=Effect of Irradiation at 130, 650, and 775°F on Tensile Properties of Zircaloy-4 at 70, 650, and 775°F|year=1975|doi-broken-date=25 March 2025 |isbn=978-0-8031-0539-3}}</ref> Embrittlement is of particular concern for the material comprising the reactor pressure vessel, where as a result the energy required to fracture the vessel decreases significantly. It is possible to restore ductility by annealing the defects out, and much of the life-extension of nuclear reactors depends on the ability to safely do so. [[Creep (deformation)|Creep]] is also greatly accelerated in irradiated materials, though not as a result of the enhanced diffusivities, but rather as a result of the interaction between lattice stress and the developing microstructure. Environmentally-assisted cracking or, more specifically, [[irradiation assisted stress corrosion cracking|irradiation-assisted stress corrosion cracking]] (IASCC) is observed especially in alloys subject to neutron radiation and in contact with water, caused by [[hydrogen embrittlement|hydrogen absorption]] at crack tips resulting from [[radiolysis]] of the water, leading to a reduction in the required energy to propagate the crack.<ref name="Lecture" /> == See also == * [[Neutron emission]] * [[Neutron flux]] * [[Neutron radiography]] == References == {{Reflist}} https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.111.222501 == External links == *[http://www.epa.gov/radiation/terms/termnop.htm#neutronradiation EPA definitions of various terms] *[http://www.nray.ca/nray/res_XvsN.php Comparison of Neutron Radiographic and X-Radiographic Images] {{Webarchive|url=https://web.archive.org/web/20140314120105/http://www.nray.ca/nray/res_XvsN.php |date=2014-03-14 }} *[http://www.ill.eu/industry/solutions/applications Neutron techniques A unique tool for research and development] {{Webarchive|url=https://web.archive.org/web/20111031080251/http://www.ill.eu/industry/solutions/applications |date=2011-10-31 }} {{Nuclear technology}} {{Authority control}} [[Category:IARC Group 1 carcinogens]] [[Category:Ionizing radiation]] [[Category:Neutron|Radiation]] [[Category:Neutron-related techniques]]
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