View source for Carbon-14

{{Short description|Radiosotope of carbon}}
{{Redirect|Radiocarbon|the scientific journal|Radiocarbon (journal)|the dating technique|Radiocarbon dating}}
{{Infobox isotope
|num_neutrons = 8
|num_protons = 6
|image=碳-14原子核+電子軌道.png
|
|alternate_names = radiocarbon
|mass_number=14
|abundance = 1 part per trillion = <math>1/10^{12}</math>
|symbol=C
|decay_product = Nitrogen-14 ({{chem|14|N}})
|halflife = {{val|5,700|30|u=years}}{{NUBASE2020|ref}}
|mass = 14.0032420<ref name="Waptstra-2003"/>
|binding_energy =
|spin = 0+
|decay_mode1 = Beta
|decay_energy1 = 0.156476<ref name="Waptstra-2003">{{cite web |title=AME atomic mass evaluation 2003 |url=https://www-nds.iaea.org/amdc/ame2003/mass.mas03 | vauthors = Waptstra AH, Audi G, Thibault C |access-date= |url-status=live |archive-url=https://web.archive.org/web/20230505001859/https://www-nds.iaea.org/amdc/ame2003/mass.mas03 |archive-date=5 May 2023 |publisher = IAEA.org}}</ref>
}}

'''Carbon-14''', '''C-14''', '''{{sup|14}}C''' or '''radiocarbon''', is a [[radioactive isotope]] of [[carbon]] with an [[atomic nucleus]] containing 6 [[protons]] and 8 [[neutrons]]. Its presence in organic matter is the basis of the [[radiocarbon dating]] method pioneered by [[Willard Libby]] and colleagues (1949) to date archaeological, geological and hydrogeological samples. Carbon-14 was discovered on February 27, 1940, by [[Martin Kamen]] and [[Sam Ruben]] at the [[University of California Radiation Laboratory]] in [[Berkeley, California]]. Its existence had been suggested by [[Franz N. D. Kurie|Franz Kurie]] in 1934.<ref>{{cite journal | vauthors = Kamen MD | title = Early History of Carbon-14: Discovery of this supremely important tracer was expected in the physical sense but not in the chemical sense | journal = Science | volume = 140 | issue = 3567 | pages = 584–590 | date = May 1963 | pmid = 17737092 | doi = 10.1126/science.140.3567.584 | bibcode = 1963Sci...140..584K }}</ref>

There are three naturally occurring [[isotope]]s of carbon on Earth: [[carbon-12]] ({{sup|12}}C), which makes up 99% of all carbon on Earth; [[carbon-13]] ({{sup|13}}C), which makes up 1%; and carbon-14 ({{sup|14}}C), which occurs in trace amounts, making up about 1-1.5 atoms per 10{{sup|12}} atoms of carbon in the atmosphere. {{sup|12}}C and {{sup|13}}C are both stable; {{sup|14}}C is unstable, with [[half-life]] {{val|5700|30}} years.<ref>{{Cite journal |last=Kondev |first=F.G. |last2=Wang |first2=M. |last3=Huang |first3=W.J. |last4=Naimi |first4=S. |last5=Audi |first5=G. |date=2021-03-01 |title=The NUBASE2020 evaluation of nuclear physics properties * |url=https://iopscience.iop.org/article/10.1088/1674-1137/abddae |journal=Chinese Physics C |volume=45 |issue=3 |pages=030001 |doi=10.1088/1674-1137/abddae |issn=1674-1137|doi-access=free }}</ref> Carbon-14 has a specific activity of 62.4&nbsp;mCi/mmol (2.31&nbsp;GBq/mmol), or 164.9&nbsp;GBq/g.<ref>{{cite journal | vauthors = Babin V, Taran F, Audisio D | title = Late-Stage Carbon-14 Labeling and Isotope Exchange: Emerging Opportunities and Future Challenges | journal = JACS Au | volume = 2 | issue = 6 | pages = 1234–1251 | date = June 2022 | pmid = 35783167 | pmc = 9241029 | doi = 10.1021/jacsau.2c00030 }}</ref> Carbon-14 decays into [[nitrogen-14]] ({{chem|14|N}}) through [[beta decay]].<ref>{{cite web|url=http://www.nosams.whoi.edu/about/carbon_dating.html |title=What is carbon dating? |publisher=National Ocean Sciences Accelerator Mass Spectrometry Facility |access-date=2007-06-11 |url-status=dead |archive-url=https://web.archive.org/web/20070705182336/http://www.nosams.whoi.edu/about/carbon_dating.html |archive-date=July 5, 2007 }}</ref> A gram of carbon containing 1 atom of carbon-14 per 10{{sup|12}} atoms, emits ~0.2<ref>(1 per 10{{sup|12}}) × (1 gram / (12 grams per mole)) × ([[Avogadro constant]]) / ((5,730 years) × (31,557,600 seconds per [[Julian year (astronomy)|Julian year]]) / [[ln(2)]])</ref> beta (β) particles per second. The primary natural source of carbon-14 on Earth is [[cosmic ray]] action on nitrogen in the atmosphere, and it is therefore a [[cosmogenic nuclide]]. However, open-air [[nuclear test]]ing between 1955 and 1980 contributed to this pool.

The different isotopes of carbon do not differ appreciably in their chemical properties. This resemblance is used in chemical and biological research, in a technique called [[carbon label]]ing: carbon-14 atoms can be used to replace nonradioactive carbon, in order to trace chemical and biochemical reactions involving carbon atoms from any given organic compound.

==Radioactive decay and detection==
Carbon-14 undergoes [[beta decay]]:

:{{chem|14|6|C}} → {{chem|14|7|N}} + {{chem2|e-}} + {{subatomic particle|electron antineutrino}} + 0.156.5 MeV

By emitting an [[electron]] and an [[electron antineutrino]], one of the neutrons in carbon-14 decays to a proton and the carbon-14 ([[half-life]] of {{val|5,700|30}} years{{NUBASE2020|ref}}) decays into the stable (non-radioactive) isotope [[nitrogen-14]].

As usual with beta decay, almost all the decay energy is carried away by the beta particle and the neutrino. The emitted beta particles have a maximum energy of about 156 keV, while their weighted mean energy is 49&nbsp;keV.<ref name="Nicols-2011">{{cite web| vauthors = Nicols AL |title=14C Comments on evaluation of decay data|url=http://www.nucleide.org/DDEP_WG/Nuclides/Tl-208_com.pdf|website=www.nucleide.org|publisher=LNHB|access-date=30 October 2021|url-status=live|archive-url=https://web.archive.org/web/20110815171408/http://www.nucleide.org:80/DDEP_WG/Nuclides/Tl-208_com.pdf |archive-date=2011-08-15 }}</ref> These are relatively low energies; the maximum distance traveled is estimated to be 22&nbsp;cm in air and 0.27&nbsp;mm in body tissue. The fraction of the radiation transmitted through the [[Stratum lucidum|dead skin layer]] is estimated to be 0.11. Small amounts of carbon-14 are not easily detected by typical [[Geiger–Müller tube|Geiger–Müller (G-M) detectors]]; it is estimated that G-M detectors will not normally detect contamination of less than about 100,000 decays per minute (0.05&nbsp;μCi). [[Liquid scintillation counting]] is the preferred method<ref>{{cite book | chapter-url = http://web.princeton.edu/sites/ehs/radmanual/radman_app_b.htm#c14 | title = Radiation Safety Manual for Laboratory Users | chapter = Appendix B: The Characteristics of Common Radioisotopes | archive-url = https://web.archive.org/web/20131002005809/http://web.princeton.edu/sites/ehs/radmanual/radman_app_b.htm | archive-date=2013-10-02 | publisher = Princeton University }}</ref> although more recently, accelerator mass spectrometry has become the method of choice; it counts all the carbon-14 atoms in the sample and not just the few that happen to decay during the measurements; it can therefore be used with much smaller samples (as small as individual plant seeds), and gives results much more quickly. The G-M counting efficiency is estimated to be 3%. The half-distance layer in water is 0.05&nbsp;mm.<ref>{{cite web | url = http://www.oseh.umich.edu/radiation/c14.shtml | work = Material Safety Data Sheet. | title = Carbon-14 | archive-url = https://web.archive.org/web/20130312103041/http://www.oseh.umich.edu/radiation/c14.shtml | archive-date=2013-03-12 | publisher = University of Michigan }}</ref>

==Radiocarbon dating==
{{main|Radiocarbon dating}}
Radiocarbon dating is a [[radiometric dating]] method that uses {{sup|14}}C to determine the age of [[carbon]]aceous materials up to about 60,000 years old. The technique was developed by [[Willard Libby]] and his colleagues in 1949<ref>{{cite journal | vauthors = Arnold JR, Libby WF | title = Age determinations by radiocarbon content; checks with samples of known age | journal = Science | volume = 110 | issue = 2869 | pages = 678–680 | date = December 1949 | pmid = 15407879 | doi = 10.1126/science.110.2869.678 | bibcode = 1949Sci...110..678A }}</ref> during his tenure as a professor at the [[University of Chicago]]. Libby estimated that the radioactivity of exchangeable {{sup|14}}C would be about 14 decays per minute (dpm) per gram of carbon, and this is still used as the activity of the ''modern radiocarbon standard''.<ref>{{cite web|url=http://www.c14dating.com/agecalc.html|title=Carbon 14:age calculation|publisher=C14dating.com|access-date=2007-06-11|url-status=live|archive-url=https://web.archive.org/web/20070610195000/http://www.c14dating.com/agecalc.html|archive-date=2007-06-10}}</ref><ref>{{cite web|url=http://www.ldeo.columbia.edu/~martins/isohydro/c_14.html|title=Class notes for Isotope Hydrology EESC W 4886: Radiocarbon <sup>14</sup>C|access-date=2007-06-11|publisher=Martin Stute's homepage at Columbia|url-status=live|archive-url=https://web.archive.org/web/20060924135028/http://www.ldeo.columbia.edu/%7Emartins/isohydro/c_14.html|archive-date=2006-09-24}}</ref> In 1960, Libby was awarded the [[Nobel Prize in chemistry]] for this work.

One of the frequent uses of the technique is to date organic remains from archaeological sites. Plants [[Carbon fixation|fix]] atmospheric carbon during photosynthesis; so the level of {{sup|14}}C in plants and animals when they die, roughly equals the level of {{sup|14}}C in the atmosphere at that time. However, it thereafter decreases exponentially; so the date of death or fixation can be estimated. The initial {{sup|14}}C level for the calculation can either be estimated, or else directly compared with known year-by-year data from tree-ring data ([[dendrochronology]]) up to 10,000 years ago (using overlapping data from live and dead trees in a given area), or else from cave deposits ([[speleothem]]s), back to about 45,000 years before present. A calculation or (more accurately) a direct comparison of carbon-14 levels in a sample, with tree ring or cave-deposit {{sup|14}}C levels of a known age, then gives the wood or animal sample age-since-formation. Radiocarbon is also used to detect disturbance in natural ecosystems; for example, in [[peatland]] landscapes, radiocarbon can indicate that carbon which was previously stored in organic soils is being released due to land clearance or climate change.<ref>{{cite journal | vauthors = Moore S, Evans CD, Page SE, Garnett MH, Jones TG, Freeman C, Hooijer A, Wiltshire AJ, Limin SH, Gauci V | display-authors = 6 | title = Deep instability of deforested tropical peatlands revealed by fluvial organic carbon fluxes | journal = Nature | volume = 493 | issue = 7434 | pages = 660–663 | date = January 2013 | pmid = 23364745 | doi = 10.1038/nature11818 | s2cid = 205232299 | bibcode = 2013Natur.493..660M | url = http://nora.nerc.ac.uk/id/eprint/21405/1/N021405PP.pdf }}</ref><ref>{{Cite journal| vauthors = Dean JF, Garnett MH, Spyrakos E, Billett MF |date=2019|title=The Potential Hidden Age of Dissolved Organic Carbon Exported by Peatland Streams|journal=Journal of Geophysical Research: Biogeosciences|language=en|volume=124|issue=2|pages=328–341|doi=10.1029/2018JG004650|bibcode=2019JGRG..124..328D|issn=2169-8953|doi-access=free|hdl=1893/28684|hdl-access=free}}</ref>

Cosmogenic nuclides are also used as [[Proxy (statistics)|proxy]] data to characterize cosmic particle and solar activity of the distant past.<ref name="Reimer-2020">{{cite journal |journal=Radiocarbon |date=August 2020 | vauthors = Reimer PJ, Austin WE, Bard E, Bayliss A, Blackwell PG, Ramsey CB, Butzin M, Cheng H, Edwards RL, Friedrich M, Grootes PM | display-authors = 6 |title = The INTCAL20 Northern Hemisphere Radiocarbon Age Calibration Curve (0–55 CAL kBP) |volume=62 |issue=4 |pages=725–757 |doi=10.1017/RDC.2020.41 |doi-access=free |bibcode=2020Radcb..62..725R |hdl=11585/770531 |hdl-access=free }}</ref><ref name="Brehm-2021">{{cite journal | vauthors = Brehm N, Bayliss A, Christl M, Synal HA, Adolphi F, Beer J, Kromer B, Muscheler R, Solanki SK, Usoskin I, Bleicher N | display-authors = 6 |title = Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings |journal = Nature Geoscience |volume = 14 |pages = 10–15 |date = 2021 |issue = 1 |doi = 10.1038/s41561-020-00674-0 |bibcode = 2021NatGe..14...10B |s2cid = 230508539 |url = https://www.dora.lib4ri.ch/eawag/islandora/object/eawag%3A21905 }}</ref>

==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>

==Occurrence==

=== Dispersion in the environment ===
After production in the upper atmosphere, the carbon-14 reacts rapidly to form mostly (about 93%) {{sup|14}}CO ([[carbon monoxide]]), which subsequently oxidizes at a slower rate to form {{chem|14|CO|2}}, radioactive [[carbon dioxide]]. The gas mixes rapidly and becomes evenly distributed throughout the atmosphere (the mixing timescale on the order of weeks). Carbon dioxide also dissolves in water and thus permeates the [[ocean]]s, but at a slower rate.<ref name="Ramsey-2008"/> The atmospheric half-life for removal of {{chem|14|CO|2}} has been estimated at roughly 12 to 16 years in the Northern Hemisphere. The transfer between the ocean shallow layer and the large reservoir of [[bicarbonate]]s in the ocean depths occurs at a limited rate.<ref name="Yim-2006">{{cite journal|doi=10.1016/j.pnucene.2005.04.002 |title=Life cycle and management of carbon-14 from nuclear power generation |year=2006 | vauthors = Yim MS, Caron F |journal=Progress in Nuclear Energy |volume=48 |pages=2–36 }}</ref>
In 2009 the activity of {{chem|14|C}} was 238&nbsp;Bq per kg carbon of fresh terrestrial biomatter, close to the values before atmospheric nuclear testing (226 Bq/kg C; 1950).<ref>{{cite web | url= http://www.irsn.fr/EN/Research/publications-documentation/radionuclides-sheets/environment/Pages/carbon14-environment.aspx#3 | publisher= Institute for Radiological Protection and Nuclear Safety | title= Carbon-14 and the environment | url-status= live | archive-url= https://web.archive.org/web/20150418012710/http://www.irsn.fr/EN/Research/publications-documentation/radionuclides-sheets/environment/Pages/carbon14-environment.aspx#3 | archive-date= 2015-04-18 }}</ref>

===Total inventory===
The inventory of carbon-14 in Earth's biosphere is about 300 [[Curie (unit)|megacuries]] (11&nbsp;[[Exa-|E]][[Becquerel|Bq]]), of which most is in the oceans.<ref>{{cite web|title=Human Health Fact Sheet – Carbon 14 |publisher=Argonne National Laboratory, EVS |date=August 2005 |url=http://www.ead.anl.gov/pub/doc/carbon14.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110716164724/http://www.ead.anl.gov/pub/doc/carbon14.pdf |archive-date=2011-07-16 }}</ref>
The following inventory of carbon-14 has been given:<ref name="Choppin-2002">{{cite book | vauthors = Choppin GR, [[Jan-Olov Liljenzin|Liljenzin JO]], Rydberg J | date = 2002 | title = Radiochemistry and Nuclear Chemistry | edition = 3rd | publisher = Butterworth-Heinemann | isbn = 978-0-7506-7463-8}}</ref>
* Global inventory: ~8500&nbsp;PBq (about 50&nbsp;[[tonne|t]])
** Atmosphere: 140&nbsp;PBq (840&nbsp;kg)
** Terrestrial materials: the balance
* From nuclear testing (until 1990): 220&nbsp;PBq (1.3&nbsp;t)

===In fossil fuels===

{{main|Suess effect}}

Many human-made chemicals are derived from [[fossil fuel]]s (such as [[petroleum]] or [[coal]]) in which {{sup|14}}C is greatly depleted because the age of fossils far exceeds the half-life of {{sup|14}}C. The relative absence of {{chem|14|CO|2}} is therefore used to determine the relative contribution (or [[mixing ratio]]) of fossil fuel oxidation to the total [[carbon dioxide]] in a given region of Earth's [[atmosphere]].<ref name="NOAA-2015">{{cite web
 | url = http://www.esrl.noaa.gov/gmd/outreach/isotopes/c14tracer.html
 | title = The Basics: 14C and Fossil Fuels
 | website = NOAA ESRL GMD Education and Outreach
 | archive-url = https://web.archive.org/web/20150925082306/http://esrl.noaa.gov/gmd/outreach/isotopes/c14tracer.html
 | archive-date = 25 September 2015
 | access-date = 9 Dec 2015
 | quote = All other atmospheric carbon dioxide comes from young sources–namely land-use changes (for example, cutting down a forest in order to create a farm) and exchange with the ocean and terrestrial biosphere. This makes 14C an ideal tracer of carbon dioxide coming from the combustion of fossil fuels. Scientists can use 14C measurements to determine the age of carbon dioxide collected in air samples, and from this can calculate what proportion of the carbon dioxide in the sample comes from fossil fuels.
}}</ref>

Dating a specific sample of fossilized carbonaceous material is more complicated. Such deposits often contain trace amounts of {{sup|14}}C. These amounts can vary significantly between samples, ranging up to 1% of the ratio found in living organisms (an apparent age of about 40,000 years).<ref>{{cite journal|title=Problems associated with the use of coal as a source of C14-free background material| vauthors = Lowe D |journal=Radiocarbon|year=1989|volume=31|issue=2|pages=117–120|url=https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/1127/1132|url-status=live|archive-url=https://web.archive.org/web/20130724153305/https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/1127/1132|archive-date=2013-07-24|doi=10.1017/S0033822200044775|doi-access=free| bibcode = 1989Radcb..31..117L }}</ref> This may indicate contamination by small amounts of bacteria, underground sources of radiation causing a {{sup|14}}N(n,p){{sup|14}}C reaction, direct [[uranium]] decay (though reported measured ratios of {{sup|14}}C/U in uranium-bearing ores<ref>{{cite journal |title=Carbon-14 Abundances in Uranium Ores and Possible Spontaneous Exotic Emission from U-Series Nuclides | vauthors = Jull AJ, Barker D, Donahue DJ |journal=Meteoritics |volume=20 |year=1985 |page=676 |bibcode=1985Metic..20..676J}}</ref> would imply roughly 1 uranium atom for every two carbon atoms in order to cause the {{sup|14}}C/{{sup|12}}C ratio, measured to be on the order of 10{{sup|−15}}), or other unknown secondary sources of {{sup|14}}C production. The presence of {{sup|14}}C in the [[isotopic signature]] of a sample of carbonaceous material possibly indicates its contamination by biogenic sources or the decay of radioactive material in surrounding geologic strata. In connection with building the [[Borexino]] solar neutrino observatory, petroleum feedstock (for synthesizing the primary scintillant) was obtained with low {{sup|14}}C content. In the Borexino Counting Test Facility, a {{sup|14}}C/{{sup|12}}C ratio of 1.94×10{{sup|−18}} was determined;<ref>{{cite journal | vauthors = Alimonti G, Angloher G, Arpesella C, Balata M, Bellini G, Benziger J, Bonetti S, Cadonati L, Calaprice FP, Cecchet G, Chen M | display-authors = 6 |doi = 10.1016/S0370-2693(97)01565-7|title = Measurement of the <sup>14</sup>C abundance in a low-background liquid scintillator |journal = Physics Letters B|volume = 422 |issue=1–4 |year=1998|pages=349–358|bibcode = 1998PhLB..422..349B }}</ref> probable reactions responsible for varied levels of {{sup|14}}C in different [[petroleum reservoir]]s, and the lower {{sup|14}}C levels in methane, have been discussed by Bonvicini et al.<ref>{{cite arXiv|eprint=hep-ex/0308025| vauthors = Bonvicini G, Harris N, Paolone V |title=The chemical history of <sup>14</sup>C in deep oilfields|year=2003}}</ref>

===In the human body===
Since many sources of human food are ultimately derived from terrestrial plants, the relative concentration of {{sup|14}}C in human bodies is nearly identical to the relative concentration in the atmosphere. The rates of disintegration of [[potassium-40]] ({{sup|40}}K) and {{sup|14}}C in the normal adult body are comparable (a few thousand decays per second).<ref>{{cite web | vauthors = Rowland RE | url = http://www.rerowland.com/BodyActivity.htm | title = The Radioactivity of the Normal Adult Body | archive-url = https://web.archive.org/web/20110205025628/http://www.rerowland.com/BodyActivity.htm | archive-date=2011-02-05 | work = rerowland.com }}</ref> The beta decays from external (environmental) radiocarbon contribute about 0.01&nbsp;[[mSv]]/year (1&nbsp;mrem/year) to each person's [[Equivalent dose|dose]] of [[ionizing radiation]].<ref>{{cite book| title=Ionizing Radiation Exposure of the Population of the United States {{!}} NCRP Report No. 93 | publisher=National Council on Radiation Protection and Measurements | year=1987 | url = http://lbl.gov/abc/wallchart/chapters/15/3.html | archive-url = https://web.archive.org/web/20070711052408/http://www.lbl.gov/abc/wallchart/chapters/15/3.html | archive-date=2007-07-11 }})</ref> This is small compared to the doses from {{sup|40}}K (0.39&nbsp;mSv/year) and [[radon]] (variable).

{{sup|14}}C can be used as a [[radioactive tracer]] in medicine. In the initial variant of the [[urea breath test]], a diagnostic test for ''[[Helicobacter pylori]]'', urea labeled with about {{convert|37|kBq|uCi|abbr=on|lk=on}} {{sup|14}}C is fed to a patient (i.e. 37,000 decays per second). In the event of a ''H. pylori'' infection, the bacterial [[urease]] enzyme breaks down the [[urea]] into [[ammonia]] and radioactively-labeled [[carbon dioxide]], which can be detected by low-level counting of the patient's breath.<ref>{{cite web|title=Society of Nuclear Medicine Procedure Guideline for C-14 Urea Breath Test |date=2001-06-23 |url=http://interactive.snm.org/docs/pg_ch07_0403.pdf |access-date=2007-07-04 |work=snm.org |url-status=dead |archive-url=https://web.archive.org/web/20070926152956/http://interactive.snm.org/docs/pg_ch07_0403.pdf |archive-date=2007-09-26 }}</ref>

== See also ==
* [[Carbon-to-nitrogen ratio]]
* [[Diamond battery]]
* [[Isotopes of carbon]]
* [[Isotopic labeling]]
* [[Radiocarbon dating]]

== References ==
{{Reflist|30em}}

== Further reading ==
{{refbegin}}
* {{cite book |title=Radiant Science, Dark Politics: A Memoir of the Nuclear Age | vauthors = Kamen MD |year=1985 |publisher=University of California Press |location=Berkeley |isbn=978-0-520-04929-1 |url=https://archive.org/details/radiantscienceda00kame |url-access=registration }}
{{refend}}

== External links ==
* [http://www.whoi.edu/nosams/page.do?pid=40138 What is Carbon Dating?], [[Woods Hole Oceanographic Institute]]

{{Isotope sequence
|element=carbon
|lighter=[[carbon-13]]
|heavier=[[carbon-15]]
|before=[[boron-14]], [[nitrogen-18]]
|after=[[nitrogen-14]]
}}

[[Category:Isotopes of carbon]]
[[Category:Carbon-14| ]]
[[Category:Environmental isotopes]]
[[Category:Radionuclides used in radiometric dating]]