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{{Use dmy dates|date=September 2023}}
{{short description|Induction of radioactivity by neutron radiation}}
{{Science with neutrons}}
'''Neutron activation''' is the process in which [[neutron radiation]] [[induced radioactivity|induces]] [[radioactivity]] in materials, and occurs when [[atomic nucleus|atomic nuclei]] capture [[Free neutron decay|free neutrons]], becoming heavier and entering [[excited state]]s. The excited nucleus decays immediately by emitting [[gamma rays]], or particles such as [[beta decay|beta particles]], [[alpha particle]]s, [[fission products]], and neutrons (in [[nuclear fission]]). Thus, the process of [[neutron capture]], even after any intermediate decay, often results in the formation of an unstable [[activation product]]. Such radioactive nuclei can exhibit [[half-life|half-lives]] ranging from small fractions of a second to many years.

Neutron activation is the only common way that a stable material can be induced into becoming intrinsically radioactive. All naturally occurring materials, including air, water, and soil, can be induced (activated) by neutron capture into some amount of radioactivity in varying degrees, as a result of the production of neutron-rich [[radioisotope]]s.{{fact|date=April 2021}} Some atoms require more than one neutron to become unstable, which makes them harder to activate because the probability of a double or triple capture by a nucleus is below that of single capture. Water, for example, is made up of hydrogen and oxygen. [[Hydrogen]] requires a double capture to attain instability as [[tritium]] ([[isotopes of hydrogen|hydrogen-3]]), while natural [[oxygen]] ([[oxygen-16]]) requires three captures to become unstable [[isotopes of oxygen|oxygen-19]]. Thus water is relatively difficult to activate, as compared to [[sodium chloride]] ([[sodium|Na]][[chlorine|Cl]]), in which both the sodium and chlorine atoms become unstable with a single capture each. These facts were experienced at the ''[[Operation Crossroads]]'' atomic test series in 1946.

==Examples==
{{Main|Neutron capture}}
An example of this kind of a nuclear reaction occurs in the production of [[cobalt-60]] within a [[nuclear reactor]]:
The cobalt-60 then decays by the emission of a [[beta particle]] plus [[gamma ray]]s into [[nickel]]-60. This reaction has a half-life of about 5.27 years, and due to the availability of [[cobalt-59]] (100% of its [[natural abundance]]),  this neutron bombarded isotope of [[cobalt]] is a valuable source of [[nuclear radiation]] (namely gamma radiation) for [[radiotherapy]].<ref>[http://www-pub.iaea.org/MTCD/publications/PDF/te_1340_web.pdf Manual for reactor produced radioisotopes] from the [[International Atomic Energy Agency]]</ref>
:{{nuclide|cobalt|59|link=yes}} + {{Subatomic particle|10neutron|link=yes}} → {{nuclide|cobalt|60|link=yes}}

In other cases, and depending on the [[kinetic energy]] of the neutron, the capture of a neutron can cause [[nuclear fission]]—the splitting of the atomic nucleus into two smaller nuclei. If the fission requires an input of energy, that comes from the kinetic energy of the neutron. An example of this kind of fission in a light element can occur when the stable isotope of [[lithium]], [[lithium-7]], is bombarded with fast neutrons and undergoes the following nuclear reaction:
:{{nuclide|lithium|7|link=yes}} + {{Subatomic particle|10neutron|link=yes}} → {{nuclide|helium|4|link=yes}} + {{nuclide|hydrogen|3|link=yes}} + {{Subatomic particle|10neutron}} + [[gamma ray]]s + [[kinetic energy]]

In other words, the capture of a neutron by lithium-7 causes it to split into an energetic [[helium]] nucleus ([[alpha particle]]), a [[hydrogen-3]] ([[tritium]]) nucleus and a free neutron. The [[Castle Bravo]] accident, in which the thermonuclear bomb test at [[Bikini Atoll]] in 1954 exploded with 2.5 times the expected yield, was caused by the unexpectedly high probability of this reaction.

In the area around a [[pressurized water reactor]] or [[boiling water reactor]] during normal operation, a significant amount of radiation is produced due to the [[fast neutron]] activation of coolant water oxygen via a [[Np reaction|(n,p) reaction]]. The activated oxygen-16 nucleus emits a proton (hydrogen nucleus), and transmutes to nitrogen-16, which has a very short life (7.13 seconds) before decaying back to oxygen-16 (emitting 10.4 MeV beta particles and 6.13 MeV gamma radiations).<ref name="Neeb">{{cite book|last=Neeb|first= Karl Heinz |title=The Radiochemistry of Nuclear Power Plants with Light Water Reactors|publisher=Walter de Gruyter|location=Berlin-New York|date=1997|isbn=3-11-013242-7| url= https://books.google.com/books?id=SJOE00whg44C&pg=PA227| page=227}}</ref>
:{{nuclide|oxygen|16|link=yes}} + {{Subatomic particle|10neutron|link=yes}} → {{Subatomic particle|11proton|link=yes}} + {{nuclide|nitrogen|16|link=yes}} (Decays rapidly)
:{{nuclide|nitrogen|16}} → {{Subatomic particle|gamma|link=yes}} + {{Physics particle|e|TL=0|BL=-1|TR=-|link=electron}} + {{nuclide|oxygen|16}}

This activation of the coolant water requires extra [[biological shield]]ing around the nuclear reactor plant. It is the high energy gamma ray in the second reaction that causes the major concern. This is why water that has recently been inside a nuclear reactor core must be shielded until this radiation subsides. One to two minutes is generally sufficient.

In facilities that housed a cyclotron, the [[reinforced concrete]] foundation can become radioactive due to neutron activation. Six important long-lived radioactive isotopes ([https://hps.org/publicinformation/radardecaydata.cfm <sup>54</sup>Mn], [[Iron-55|<sup>55</sup>Fe]], [[Cobalt-60|<sup>60</sup>Co]], [https://ehs.princeton.edu/laboratory-research/radiation-safety/radioactive-materials/radioisotope-fact-sheets/zinc-65 <sup>65</sup>Zn], [https://uwm.edu/safety-health/wp-content/uploads/sites/405/2016/11/Ba-133.pdf <sup>133</sup>Ba], and [http://hpschapters.org/northcarolina/NSDS/europium.pdf <sup>152</sup>Eu]) can be found within concrete nuclei affected by neutrons.<ref>{{cite journal |last1=Vichi |first1=Sara |title=Efficiency calibration of a portable CZT detector for |journal=Radiation Effects and Defects in Solids |date=2016 |volume=171 |pages=705–713 |doi=10.1080/10420150.2016.1244675 |s2cid=99556734 }}</ref> The residual radioactivity is predominantly due to trace elements present, and thus the amount of radioactivity derived from cyclotron activation is minuscule, i.e., pCi/g or [[Becquerel|Bq/g]]. The release limit for facilities with residual radioactivity is 25 mrem/year.<ref name="NRC">{{cite web |last1=Nuclear Regulatory Commission 10 CFR 20.1402 |title=Standards for Protection Against Radiation |url=https://www.nrc.gov/reading-rm/doc-collections/cfr/part020/full-text.html |ref=10 CFR 20.1402}}</ref> An example of <sup>55</sup>Fe production from the activation of iron in [[Rebar|reinforcement bars]] found in concrete is shown below:

:{{nuclide|Iron|54|link=yes}} + {{Subatomic particle|10neutron|link=yes}} → {{nuclide|Iron|55|link=yes}}

==Occurrence==
Neutron activation is the only common way that a stable material can be induced into becoming intrinsically radioactive. Activation is inherently different than contamination. Neutrons are only free in quantity in the microseconds of a nuclear weapon's explosion, in an active nuclear reactor, or in a [[spallation]] neutron source.

In an atomic weapon, neutrons are generated for only between 1 and 50 microseconds, but in huge numbers. Most are absorbed by the metallic bomb casing, which is only just starting to be affected by the explosion within it. The neutron activation of the soon-to-be vaporized metal is responsible for a significant portion of the [[nuclear fallout]] in nuclear bursts high in the atmosphere. In other types of activation,  neutrons may irradiate soil that is dispersed in a mushroom cloud at or near the Earth's surface, resulting in fallout from activation of soil chemical elements.

==Effects on materials over time==
In any location with high [[neutron flux]]es, such as within the cores of nuclear reactors, neutron activation contributes to material erosion and periodically the lining materials themselves must be disposed of, as low-level [[radioactive waste]]. Some materials are more subject to neutron activation than others, so a suitably chosen low-activation material can significantly reduce this problem (see [[International Fusion Materials Irradiation Facility]]). For example, [[isotopes of chromium|Chromium-51]] will form by neutron activation in [[chrome steel]] (which contains Cr-50) that is exposed to a typical reactor neutron flux.<ref>{{Cite web |url=http://ie.lbl.gov/toi/nuclide.asp?iZA=240051 |title=Table of Isotopes decay data |access-date=2014-03-05 |archive-url=https://web.archive.org/web/20140305004349/http://ie.lbl.gov/toi/nuclide.asp?iZA=240051 |archive-date=2014-03-05 |url-status=dead }}</ref>

[[Carbon-14]], most frequently but not solely, generated by the neutron activation of atmospheric nitrogen-14 with a [[thermal neutron]], is (together with its dominant natural production pathway from cosmic ray-air interactions and historical production from [[nuclear testing|atmospheric nuclear testing]]) also generated in comparatively minute amounts inside many designs of nuclear reactors which contain nitrogen gas impurities in their [[fuel cladding]], [[coolant water]] and by neutron activation of the oxygen contained in the water itself. [[Fast breeder reactor]]s (FBR) produce about an order of magnitude less C-14 than the most common reactor type, the [[pressurized water reactor]], as FBRs do not use water as a primary coolant.<ref>{{Cite web|url=http://www-pub.iaea.org/MTCD/publications/PDF/TRS421_web.pdf|title=IAEA Technical report series no.421, Management of Waste Containing Tritium and Carbon-14}}</ref>

==Uses==

===Radiation safety===
For physicians and radiation safety officers, activation of sodium in the human body to sodium-24, and phosphorus to phosphorus-32, can give a good immediate estimate of acute accidental neutron exposure.<ref>[http://web.ornl.gov/info/reports/1993/3445603755103.pdf ORNL Report] {{Webarchive|url=https://web.archive.org/web/20131001184002/http://web.ornl.gov/info/reports/1993/3445603755103.pdf |date=2013-10-01 }} on determination of dose from criticality accidents</ref>

====Neutron detection====
One way to demonstrate that [[nuclear fusion]] has occurred inside a [[fusor|fusor device]] is to use a [[Geiger counter]] to measure the gamma ray radioactivity that is produced from a sheet of [[aluminium foil]].

In the [[inertial confinement fusion|ICF]] fusion approach, the fusion yield of the experiment (directly proportional to neutron production) is usually determined by measuring the gamma-ray emissions of aluminium or copper neutron activation targets.<ref>{{cite web |url=http://www.geneseo.edu/nuclear/aluminum-activation-results |title=DT neutron yield measurements using neutron activation of aluminum|author1=Stephen Padalino |author2=Heather Oliver |author3=Joel Nyquist |others= LLE Collaborators: Vladimir Smalyukand, Nancy Rogers |name-list-style=amp}}</ref> Aluminium can capture a neutron and generate radioactive [[sodium-24]], which has a half life of 15 hours<ref>{{cite web|url=http://www.aanda.org/articles/aa/full/2001/10/aah2362/node4.html |title= 4 Identified radioactive isotopes |publisher=Aanda.org |date=1998-03-02 |access-date=2019-11-14}}</ref><ref>{{Cite web|url=http://kubchemistry.weebly.com/uploads/6/9/8/7/6987088/chapter_22_nuclear_reactions.ppt|title=Nuclear Reactions|format=PPT|date=November 29, 2014|archive-url=https://web.archive.org/web/20141129070410/http://kubchemistry.weebly.com/uploads/6/9/8/7/6987088/chapter_22_nuclear_reactions.ppt|archive-date=2014-11-29}}</ref> and a beta decay energy of 5.514 MeV.<ref>[http://www.site.uottawa.ca:4321/astronomy/index.html#sodium24] {{webarchive|url=https://web.archive.org/web/20060705214728/http://www.site.uottawa.ca:4321/astronomy/index.html|date=2006-07-05}}</ref>

The activation of a number of test target elements such as [[sulfur]], copper, [[tantalum]], and [[gold]] have been used to determine the yield of both [[pure fission weapon|pure fission]]<ref name=Kerr2005>{{cite book |chapter-url=http://www.rerf.or.jp/shared/ds02/pdf/chapter01/cha01-p42-61.pdf |chapter=Bomb Parameters |last1=Kerr |first1=George D. |last2=Young |first2=Robert W. |last3=Cullings |first3=Harry M. |last4=Christy |first4=Robert F. |pages=42–43 |title=Reassessment of the Atomic Bomb Radiation Dosimetry for Hiroshima and Nagasaki – Dosimetry System 2002 |year=2005 |publisher=The Radiation Effects Research Foundation |editor=Robert W. Young, George D. Kerr |access-date=2014-03-13 |archive-url=https://web.archive.org/web/20150810210007/http://www.rerf.or.jp/shared/ds02/pdf/chapter01/cha01-p42-61.pdf |archive-date=2015-08-10 |url-status=dead }}</ref><ref name=Malik1985>{{cite web |url=http://www.osti.gov/manhattan-project-history/publications/LANLHiroshimaNagasakiYields.pdf |title=The Yields of the Hiroshima and Nagasaki Explosions |last=Malik |first=John |date=September 1985 |publisher=Los Alamos National Laboratory |access-date=March 9, 2014}}</ref> and [[thermonuclear weapon]]s.<ref>{{cite book|title=Operation Ivy Final Report Joint Task Force 132|year=1952|url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a995443.pdf|archive-url=https://web.archive.org/web/20140311064141/http://www.dtic.mil/dtic/tr/fulltext/u2/a995443.pdf|url-status=live|archive-date=March 11, 2014|author=US Army}}</ref>

====Materials analysis====
{{Main|Neutron activation analysis}}
[[Neutron activation analysis]] is one of the most sensitive and precise methods of trace element analysis. It requires no sample preparation or solubilization and can therefore be applied to objects that need to be kept intact such as a valuable piece of art. Although the activation induces radioactivity in the object, its level is typically low and its lifetime may be short, so that its effects soon disappear. In this sense, neutron activation is a non-destructive analysis method.

Neutron activation analysis can be done in situ. For example, aluminium (Al-27) can be activated by capturing relatively low-energy neutrons to produce the [[isotopes of aluminium|isotope Al-28]], which decays with a half-life of 2.3 minutes with a decay energy of 4.642 MeV.<ref>[http://www.site.uottawa.ca:4321/astronomy/index.html#aluminium28] {{webarchive|url=https://web.archive.org/web/20060705214728/http://www.site.uottawa.ca:4321/astronomy/index.html|date=2006-07-05}}</ref> This activated isotope is used in oil drilling to determine the [[phyllosilicate#Phyllosilicates|clay]] content (clay is generally an [[alumino-silicate]]) of the underground area under exploration.<ref>{{cite web|url=https://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term+name&filter=aluminum+activation+log|title=Search Results - Schlumberger Oilfield Glossary|website=www.glossary.oilfield.slb.com}}</ref>

Historians can use neutron activation products to authenticate atomic artifacts and materials subjected to neutron fluxes from fission incidents. For example, one of the rare isotopes found in [[trinitite]] is [[isotopes of barium|barium-133]], an activation product formed from the [[Baratol]] used in the [[detonation velocity|slow explosive]] [[explosive lens|lens]] employed in the [[Trinity (nuclear test)|Trinity device]]. This barium isotope can be used to authenticate trinitite samples, with its absence indicating a fraudulent sample.<ref>{{cite journal |title=Radioactivity in Trinitite six decades later |doi=10.1016/j.jenvrad.2005.01.017 |volume=85 |issue=1 |journal=Journal of Environmental Radioactivity |pages=103–120 |pmid=16102878 |citeseerx=10.1.1.494.5179 |year=2006 |last1=Parekh |first1=PP |last2=Semkow |first2=TM |last3=Torres |first3=MA |last4=Haines |first4=DK |last5=Cooper |first5=JM |last6=Rosenberga |first6=PM |last7=Kittoa |first7=ME|bibcode=2006JEnvR..85..103P |url=http://www.ecolo.org/documents/documents_in_english/Trinitite-JER-2006.pdf }}</ref>

=== Semiconductor production ===
Neutron irradiation may be used for [[float-zone silicon]] slices ([[Wafer (electronics)|wafers]]) to trigger fractional transmutation of Si atoms into phosphorus (P) and therefore doping it into n-type  silicon <ref name=":0">{{Cite book |last=Sze |first=S. M. |url=https://www.worldcat.org/oclc/869833419 |title=Semiconductor devices : physics and technology |date=2012 |publisher=Wiley |others=M. K. Lee |isbn=978-0-470-53794-7 |edition=3 |location=New York, NY |oclc=869833419}}</ref>{{Rp|page=366}}

:<math chem>\ce{_14^{30}Si} + \mathrm{neutron} \xrightarrow{} \ce{_14^{31}Si} + \gamma\mathrm{-ray}\xrightarrow{2.62\mathrm{hr}}\ce{_15^{31}P} + \beta\mathrm{-ray}</math>

==See also==
*[[Induced radioactivity]]
*[[Neutron activation analysis]]
*[[Neutron embrittlement]]
*[[Phosphorus-32]] produced when sulfur captures a neutron.
*[[Salted bomb]]
*[[Table of nuclides]]

==References==
{{Reflist}}

==External links==
*[https://web.archive.org/web/20161126143224/http://www.naa-online.net/ Neutron Activation Analysis web]
*[http://www-nds.iaea.org/publications/tecdocs/technical-reports-series-156.pdf Handbook on Nuclear Activation Cross-Sections, IAEA, 1974]
*[https://www.orau.org/PTP/PTP%20Library/library/DOE/bnl/nuclidedata/table.htm Decay Data in MIRD Format from the National Nuclear Data Center at Brookhaven National Laboratory]
*[https://web.archive.org/web/20150924072849/http://www.physics.rutgers.edu/~abragg/110/lecture16.html Neutron capture as it relates to nucleosynthesis]
*[https://web.archive.org/web/20150810210025/http://www.orau.gov/tdd/QualPrgm/learningmaterials/RadiationProtection/rp12.pdf Neutron capture and the Chart of the nuclides]
*[http://www.nndc.bnl.gov/chart/ The chart of the Nuclides]
*[http://www.nscl.msu.edu/~thoennes/2009/chromium-adndt.pdf Discovery of the Chromium isotopes, Chromium-55 by Cr-54 neutron capture]
*[https://orill.readthedocs.io ORILL] : 1D transmutation, fuel depletion, and radiological protection code

==Further reading==
*{{cite book|title=Operation Ivy Final Report Joint Task Force 132|year=1952|url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a995443.pdf|archive-url=https://web.archive.org/web/20140311064141/http://www.dtic.mil/dtic/tr/fulltext/u2/a995443.pdf|url-status=live|archive-date=March 11, 2014|author=US Army}}

[[Category:Neutron|Activation]]
[[Category:Radiation effects]]
[[Category:Radiation]]