Editing Carbon-14 (section)

==Origin==

===Natural production in the atmosphere===
[[File:Carbon 14 formation and decay.svg|right|thumb| 1: Formation of carbon-14 <br>2: Decay of carbon-14 <br>3: The "equal" equation is for living organisms, and the unequal one is for dead organisms, in which the C-14 then decays (See 2).]]
Carbon-14 is produced in the upper [[troposphere]] and the [[stratosphere]] by [[thermal neutron]]s absorbed by [[nitrogen]] atoms. When [[cosmic ray]]s enter the atmosphere, they undergo various transformations, including the production of [[neutron]]s. The resulting neutrons (n) participate in the following [[(n-p) reaction|n-p]] reaction (p is [[proton]]):

:{{chem|14|7|N}} + n → {{chem|14|6|C}} + p + 0.626 MeV

The highest rate of carbon-14 production takes place at altitudes of {{convert|9|to|15|km|ft}} and at high [[geomagnetic latitude]]s.

The rate of {{sup|14}}C production can be modeled, yielding values of 16,400<ref name="Kovaltsov-2012">{{cite journal | vauthors = Kovaltsov GA, Mishev A, Usoskin IG |title=A new model of cosmogenic production of radiocarbon 14C in the atmosphere |journal=Earth and Planetary Science Letters |volume=337–338 |year=2012 |pages=114–20 |issn=0012-821X |doi=10.1016/j.epsl.2012.05.036 |arxiv=1206.6974 |bibcode=2012E&PSL.337..114K |s2cid=118602346}}</ref> or 18,800<ref name="Poluianov-2016">{{cite journal | vauthors = Poluianov SV, Kovaltsov GA, Mishev AL, Usoskin IG |title=Production of cosmogenic isotopes 7Be, 10Be, 14C, 22Na, and 36Cl in the atmosphere: Altitudinal profiles of yield functions |journal=Journal of Geophysical Research: Atmospheres |volume=121 |issue=13 |year=2016 |pages=8125–36 |doi=10.1002/2016JD025034 |arxiv=1606.05899 |bibcode=2016JGRD..121.8125P|s2cid=119301845 }}</ref> atoms of {{chem|14|C}} per second per square meter of Earth's surface, which agrees with the global [[Emissions budget|carbon budget]] that can be used to backtrack,<ref name="Hain-2014">{{cite journal | vauthors = Hain MP, Sigman DM, Haug GH |title=Distinct roles of the Southern Ocean and North Atlantic in the deglacial atmospheric radiocarbon decline |journal=Earth and Planetary Science Letters |volume=394 |year=2014 |pages=198–208 |issn=0012-821X |doi=10.1016/j.epsl.2014.03.020 |url=https://earth-system-biogeochemistry.net/wp-content/uploads/2021/05/Hain_et_al_2014_EPSL.pdf |bibcode=2014E&PSL.394..198H |url-status=live |archive-url=https://web.archive.org/web/20151222120109/http://www.mathis-hain.net/resources/Hain_et_al_2014_EPSL.pdf |archive-date=2015-12-22}}</ref> but attempts to measure the production time directly ''in situ'' were not very successful. Production rates vary because of changes to the cosmic ray flux caused by the heliospheric modulation (solar wind and solar magnetic field), and, of great significance, due to variations in the [[Earth's magnetic field]]. Changes in the [[carbon cycle]] however can make such effects difficult to isolate and quantify.
<ref name="Hain-2014"/><ref name="Ramsey-2008">{{cite journal | year=2008 | author=Ramsey, C. Bronk | journal =Archaeometry | volume=50 | pages=249–75 | doi=10.1111/j.1475-4754.2008.00394.x | issue=2 | title=Radiocarbon Dating: Revolutions in Understanding}}</ref>
Occasional spikes may occur; for example, there is evidence for [[774–775 carbon-14 spike|an unusually high production rate in AD 774–775]],<ref>{{cite journal | vauthors = Miyake F, Nagaya K, Masuda K, Nakamura T | title = A signature of cosmic-ray increase in AD 774-775 from tree rings in Japan | journal = Nature | volume = 486 | issue = 7402 | pages = 240–242 | date = June 2012 | pmid = 22699615 | doi = 10.1038/nature11123 | url = http://sciences.blogs.liberation.fr/files/c14-774-apr%C3%A8s-jc.pdf | url-status = dead | s2cid = 4368820 | bibcode = 2012Natur.486..240M | archive-url = https://web.archive.org/web/20150706121714/http://sciences.blogs.liberation.fr/files/c14-774-apr%C3%A8s-jc.pdf | archive-date = 2015-07-06 }}</ref> caused by an extreme
solar energetic particle event, the strongest such event to have occurred within the last ten millennia.<ref name="Usoskin-2013">{{cite journal | year=2013 | vauthors = Usoskin IG, Kromer B, Ludlow F, Beer J, Friedrich M, Kovaltsov GA, Solanki SK, Wacker L | display-authors = 6 | journal =Astron. Astrophys.| volume=552 | pages=L3 | doi=10.1051/0004-6361/201321080 | title=The AD775 cosmic event revisited: the Sun is to blame | bibcode=2013A&A...552L...3U|arxiv=1302.6897|s2cid=55137950 }}</ref><ref name="Mekhaldi-2015">{{cite journal | vauthors = Mekhaldi F, Muscheler R, Adolphi F, Aldahan A, Beer J, McConnell JR, Possnert G, Sigl M, Svensson A, Synal HA, Welten KC, Woodruff TE | display-authors = 6 | title = Multiradionuclide evidence for the solar origin of the cosmic-ray events of ᴀᴅ 774/5 and 993/4 | journal = Nature Communications | volume = 6 | pages = 8611 | date = October 2015 | pmid = 26497389 | pmc = 4639793 | doi = 10.1038/ncomms9611 | bibcode = 2015NatCo...6.8611M }}</ref> Another "extraordinarily large" {{sup|14}}C increase (2%) has been associated with a 5480&nbsp;BC event, which is unlikely to be a solar energetic particle event.<ref>{{cite journal | vauthors = Miyake F, Jull AJ, Panyushkina IP, Wacker L, Salzer M, Baisan CH, Lange T, Cruz R, Masuda K, Nakamura T | display-authors = 6 | title = Large 14C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 114 | issue = 5 | pages = 881–884 | date = January 2017 | pmid = 28100493 | pmc = 5293056 | doi = 10.1073/pnas.1613144114 | doi-access = free | bibcode = 2017PNAS..114..881M }}</ref>

Carbon-14 may also be produced by lightning<ref>{{Cite journal | doi=10.1029/JB078i026p05902| bibcode=1973JGR....78.5902L| title=Production of radiocarbon in tree rings by lightning bolts| year=1973| vauthors = Libby LM, Lukens HR | journal=Journal of Geophysical Research| volume=78| issue=26| pages=5902–5903}}</ref><ref>{{cite journal | vauthors = Enoto T, Wada Y, Furuta Y, Nakazawa K, Yuasa T, Okuda K, Makishima K, Sato M, Sato Y, Nakano T, Umemoto D, Tsuchiya H | display-authors = 6 | title = Photonuclear reactions triggered by lightning discharge | journal = Nature | volume = 551 | issue = 7681 | pages = 481–484 | date = November 2017 | pmid = 29168803 | doi = 10.1038/nature24630 | bibcode = 2017Natur.551..481E | arxiv = 1711.08044 | s2cid = 4388159 }}</ref> but in amounts negligible, globally, compared to cosmic ray production. Local effects of cloud-ground discharge through sample residues are unclear, but possibly significant.

===Other carbon-14 sources===
Carbon-14 can also be produced by other neutron reactions, including in particular [[Carbon-13|{{sup|13}}C]](n,γ){{sup|14}}C and [[Oxygen-17|{{sup|17}}O]](n,α){{sup|14}}C with [[thermal neutron]]s, and [[Nitrogen-15|{{sup|15}}N]](n,d){{sup|14}}C and [[Oxygen-16|{{sup|16}}O]](n,{{sup|3}}He){{sup|14}}C with [[fast neutron]]s.<ref>{{cite report | vauthors = Davis Jr W | title = Carbon-14 production in nuclear reactors. | work = U.S. Nuclear Regulatory Commission | publisher = Oak Ridge National Lab. | location = TN (USA) | date = January 1977 | url = https://www.osti.gov/scitech/servlets/purl/7114972 | doi = 10.2172/7114972 }}</ref> The most notable routes for {{sup|14}}C production by thermal neutron irradiation of targets (e.g., in a nuclear reactor) are summarized in the table. 

Another source of carbon-14 is [[cluster decay]] branches from traces of naturally occurring [[isotopes of radium]], though this decay mode has a [[branching ratio]] on the order of {{val|e=-8}} relative to [[alpha decay]], so [[radiogenic]] carbon-14 is extremely rare.

{|class="wikitable"
|+'''{{sup|14}}C production routes<ref name="Yim-2006"/>'''
|-
!Parent isotope!!Natural abundance, %||[[Neutron cross section|Cross section for thermal neutron capture]], [[barn (unit)|b]]!!Reaction
|-
|{{sup|14}}N||99.634||1.81||{{sup|14}}N(n,p){{sup|14}}C
|-
|{{sup|13}}C||1.103||0.0009||{{sup|13}}C(n,γ){{sup|14}}C
|-
|{{sup|17}}O||0.0383||0.235||{{sup|17}}O(n,α){{sup|14}}C
|-
|}

===Formation during nuclear tests===
[[File:Radiocarbon bomb spike.svg|thumb|300px|right|Atmospheric {{sup|14}}C, [[New Zealand]]<ref>{{cite journal | vauthors = Manning MR, Melhuish WH|url=http://cdiac.esd.ornl.gov/trends/co2/welling.html |title=Atmospheric δ{{sup|14}}C record from Wellington |access-date=2007-06-11 |journal=Trends: A Compendium of Data on Global Change. | publisher = Carbon Dioxide Information Analysis Center |year=1994 |url-status=dead |archive-url=https://web.archive.org/web/20140201222225/http://cdiac.esd.ornl.gov/trends/co2/welling.html |archive-date=2014-02-01 }}</ref> and [[Austria]].<ref>{{cite journal| url=http://cdiac.esd.ornl.gov/trends/co2/cent-verm.html| vauthors = Levin I, Kromer B, Schoch-Fischer H, Bruns M, Münnich M, Berdau D, Vogel JW, Münnich KO | title=δ{{sup|14}}C record from Vermunt| journal=Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center| year=1994 | url-status=dead| archive-url=https://web.archive.org/web/20080923105819/http://cdiac.esd.ornl.gov/trends/co2/cent-verm.html| archive-date=2008-09-23| access-date=2009-03-25}}</ref> The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere. Atmospheric nuclear tests almost doubled the {{sup|14}}C concentration of the Northern Hemisphere.<ref>{{cite web | url=http://www1.phys.uu.nl/ams/Radiocarbon.htm | publisher=University of Utrecht | title=Radiocarbon dating | access-date=2008-02-19 | url-status=live | archive-url=https://web.archive.org/web/20071209151357/http://www1.phys.uu.nl/ams/Radiocarbon.htm | archive-date=2007-12-09 }}</ref> PTBT = [[Partial Nuclear Test Ban Treaty]].]]
The above-ground [[nuclear test]]s that occurred in several countries in 1955-1980 (see [[List of nuclear tests]]) dramatically increased the amount of {{sup|14}}C in the atmosphere and subsequently the biosphere; after the tests ended, the atmospheric concentration of the isotope began to decrease, as radioactive CO{{sub|2}} was fixed into plant and animal tissue, and dissolved in the oceans.

One side-effect of the change in atmospheric {{sup|14}}C is that this has enabled some options (e.g. [[Bomb pulse|bomb-pulse dating]]<ref>{{cite journal|url=https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/3713|title=Bomb-Pulse Dating of Human Material: Modeling the Influence of Diet|journal=Radiocarbon|volume=52|issue=2|pages=800–07|url-status=live|archive-url=https://web.archive.org/web/20141020085949/https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/3713|archive-date=2014-10-20|date=August 2010| vauthors = Stenstrom K, Georgiadou E |doi=10.1017/S0033822200045811|doi-access=free|bibcode=2010Radcb..52..800G }}</ref>) for determining the birth year of an individual, in particular, the amount of {{sup|14}}C in [[tooth enamel]],<ref>{{cite journal | url=http://news.nationalgeographic.com/news/2005/09/0922_050922_nuke_body.html | title=Radiation in Teeth Can Help Date, ID Bodies, Experts Say | journal=National Geographic News | date=2005-09-22 | url-status=dead | archive-url=https://web.archive.org/web/20070425080623/http://news.nationalgeographic.com/news/2005/09/0922_050922_nuke_body.html | archive-date=2007-04-25 }}</ref><ref>{{cite journal | vauthors = Spalding KL, Buchholz BA, Bergman LE, Druid H, Frisén J | title = Forensics: age written in teeth by nuclear tests | journal = Nature | volume = 437 | issue = 7057 | pages = 333–334 | date = September 2005 | pmid = 16163340 | doi = 10.1038/437333a | s2cid = 4407447 | bibcode = 2005Natur.437..333S }}</ref> or the carbon-14 concentration in the lens of the eye.<ref>{{cite journal | vauthors = Lynnerup N, Kjeldsen H, Heegaard S, Jacobsen C, Heinemeier J | title = Radiocarbon dating of the human eye lens crystallines reveal proteins without carbon turnover throughout life | journal = PLOS ONE | volume = 3 | issue = 1 | pages = e1529 | date = January 2008 | pmid = 18231610 | pmc = 2211393 | doi = 10.1371/journal.pone.0001529 | veditors = Gazit E | doi-access = free | bibcode = 2008PLoSO...3.1529L }}</ref>

In 2019, [[Scientific American]] reported that carbon-14 from nuclear testing has been found in animals from one of the most inaccessible regions on Earth, the [[Mariana Trench]] in the Pacific Ocean.<ref>{{cite web | vauthors = Levy A | url = https://www.scientificamerican.com/article/bomb-carbon-has-been-found-in-deep-ocean-creatures/ | title = 'Bomb Carbon' Has Been Found in Deep-Ocean Creatures | work = Scientific American | date = 15 May 2019 }}</ref>

The concentration of {{sup|14}}C in atmospheric CO{{sub|2}}, reported as the {{sup|14}}C/{{sup|12}}C ratio with respect to a standard, has (since about 2022) declined to levels similar to those prior to the above-ground nuclear tests of the 1950s and 1960s.<ref>{{cite news |last1=Jones |first1=Nicola |title=Carbon dating hampered by rising fossil-fuel emissions |url=https://www.nature.com/articles/d41586-022-02057-4 |access-date=5 November 2023 |publisher=Nature News |date=27 July 2022}}</ref><ref>{{cite journal |last1=Graven |first1=H. |last2=Keeling |first2=R. |last3=Xu |first3=X. |title=Radiocarbon dating: going back in time |journal=Nature |date=19 July 2022 |volume=607 |issue=7919 |page=449 |doi=10.1038/d41586-022-01954-y |pmid=35854150 |bibcode=2022Natur.607R.449G |url=https://www.nature.com/articles/d41586-022-01954-y}}</ref> Though the extra {{sup|14}}C generated by those nuclear tests has not disappeared from the atmosphere, oceans and biosphere,<ref>{{cite journal |last1=Caldeira |first1=K. |last2=Rau |first2=G.H. |last3=Duffy |first3=PB |title=Predicted net efflux of radio- carbon from the ocean and increase in atmospheric radiocarbon content |journal=Geophysical Research Letters |date=1998 |volume=25 |issue=20 |pages=3811–3814 |doi=10.1029/1998GL900010 |doi-access=free |bibcode=1998GeoRL..25.3811C }}</ref> it is diluted due to the [[Suess effect]].

=== Emissions from nuclear power plants ===
Carbon-14 is produced in coolant at [[boiling water reactor]]s (BWRs) and [[pressurized water reactor]]s (PWRs). It is typically released into the air in the form of [[carbon dioxide]] at BWRs, and [[methane]] at PWRs.<ref>{{Cite web|url=http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001023023|archive-url=https://web.archive.org/web/20160818161716/http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001023023|url-status=dead|archive-date=2016-08-18|title=EPRI {{!}} Product Abstract {{!}} Impact of Nuclear Power Plant Operations on Carbon-14 Generation, Chemical Forms, and Release|website=www.epri.com|access-date=2016-07-07}}</ref> Best practice for nuclear power plant operator management of carbon-14 includes releasing it at night, when plants are not [[photosynthesizing]].<ref>{{Cite web|url=http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001024827|archive-url=https://web.archive.org/web/20160818174331/http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001024827|url-status=dead|archive-date=2016-08-18|title=EPRI {{!}} Product Abstract {{!}} Carbon-14 Dose Calculation Methods at Nuclear Power Plants|website=www.epri.com|access-date=2016-07-07}}</ref> Carbon-14 is also generated inside nuclear fuels (some due to transmutation of oxygen in the [[uranium oxide]], but most significantly from transmutation of nitrogen-14 impurities), and if the spent fuel is sent to [[nuclear reprocessing]] then the {{sup|14}}C is released, for example as CO{{sub|2}} during [[PUREX]].<ref>{{cite book | vauthors = Otlet RL, Fulker MJ, Walker AJ | date = 1992 | chapter = Environmental Impact of Atmospheric Carbon-14 Emissions Resulting from the Nuclear Energy Cycle. | veditors = Taylor RE, Long A, Kra RS | title = Radiocarbon After Four Decades. | publisher = Springer | location = New York, NY }}</ref><ref>{{Cite web|url=https://www.irsn.fr/EN/Research/publications-documentation/radionuclides-sheets/environment/Pages/carbon14-environment.aspx|title=Carbon-14 and the environment | publisher = Institute for Radiological Protection and Nuclear Safety }}</ref>