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 = 1.2 ppt ({{val|1.2|e=-12}}) |symbol=C |decay_product = Nitrogen-14 ({{chem|14|N}}) |halflife = {{val|5,700|30|u=years}}{{NUBASE2020|ref}} |mass = 14.0032420<ref>{{AME2020 II}}</ref> |binding_energy = |spin = 0+ |decay_mode1 = Beta |decay_energy1 = 0.1565<ref>{{NNDC}}</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. 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 Kurie in 1934. 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.<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 isotopes 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 1.2<ref name="14calc">Activity at 1 ppt: (10{{sup|-12}} * Avogadro number/ 12.011) / ((5,700 years) × (31,557,600 seconds per Julian year) / ln(2)) = 0.1932 Bq/g; concentration calculated from that and measured activities.</ref> 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, decaying into nitrogen-14 ({{chem|14|N}}) through beta decay.<ref>{{Cite journal |last1=Kondev |first1=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 * |journal=Chinese Physics C |volume=45 |issue=3 |page=030001 |doi=10.1088/1674-1137/abddae |bibcode=2021ChPhC..45c0001K |issn=1674-1137|doi-access=free }}</ref> Pure carbon-14 would have a molar activity of 62.4 mCi/mmol (2.31 GBq/mmol), or specific activity of 164.9 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> 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. Open-air nuclear testing between 1955 and 1980 contributed to this pool, however.

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 labeling: 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}} + 0.1565 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 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=dead|archive-url=https://web.archive.org/web/20110815171408/http://www.nucleide.org/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 cm in air and 0.27 mm in body tissue. The fraction of the radiation transmitted through the dead skin layer is estimated to be 0.11. Small amounts of carbon-14 are not easily detected by typical 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 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-value layer in water is 0.05 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 carbonaceous 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=https://www.ldeo.columbia.edu/~martins/isohydro/c_14.html|title=Class notes for Isotope Hydrology EESC W 4886: Radiocarbon 14C|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.<ref>{{Cite web|url=https://www.bbc.com/future/article/20250926-the-natural-clocks-that-can-pinpoint-the-time-of-death|title=The natural clocks that can pinpoint someone's time of death|date=Oct 2, 2025|website=www.bbc.com}}</ref>

One of the frequent uses of the technique is to date organic remains from archaeological sites. Plants 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 (speleothems), 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 = https://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 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=== right|thumb| 1: Formation of carbon-14 2: Decay of carbon-14 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 neutrons absorbed by nitrogen atoms. When cosmic rays enter the atmosphere, they undergo various transformations, including the production of neutrons. The resulting neutrons (n) participate in the following 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 latitudes.

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 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 | bibcode=2008Archa..50..249B }}</ref> Occasional spikes may occur, known as Miyake events; for example, there is evidence for 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 | 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 | page = 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 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> The largest known Miyake event took place ca. 12350 BCE.<ref>{{cite journal | vauthors = Bard E, Miramont C, Capano M, Guibal F, Marschal C, Rostek F, Tuna T, Fagault Y, Heaton TJ | display-authors = 6 | title = A radiocarbon spike at 14 300 cal yr BP in subfossil trees provides the impulse response function of the global carbon cycle during the Late Glacial | journal = Philosophical Transactions of the Royal Society A | volume = 281| issue = 2261 | pages = id.20220206 | date = 2023 | doi = 10.1098/rsta.2022.0206 | bibcode = 2023RSPTA.38120206B }}</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 {{sup|13}}C(n,γ){{sup|14}}C and {{sup|17}}O(n,α){{sup|14}}C with thermal neutrons, and {{sup|15}}N(n,d){{sup|14}}C and {{sup|16}}O(n,{{sup|3}}He){{sup|14}}C with fast neutrons.<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 |osti=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, %||Cross section for thermal neutron capture, 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 |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 | 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 tests 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 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 |url-access=subscription}}</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 | 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 | article-number = 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|url-access=subscription }}</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. |author3-link=Phil Duffy |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 reactors (BWRs) and pressurized water reactors (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|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|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 oceans, 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 bicarbonates 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 |issue=1 |pages=2–36 |bibcode=2006PNuE...48....2Y }}</ref> In 2009 the activity of {{chem|14|C}} was 238 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 megacuries (11 E 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 |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, 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 PBq (about 50 t) ** Atmosphere: 140 PBq (840 kg) ** Terrestrial materials: the balance * From nuclear testing (until 1990): 220 PBq (1.3 t)

===In fossil fuels===

Many human-made chemicals are derived from fossil fuels (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 = https://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 14C 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 reservoirs, 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 14C 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> Because of the different decay energies, though, K-40 gives about 0.17 mSv/year and C-14 0.012 mSv/year internal dose.<ref>{{cite book |author=United Nations Scientific Committee on the Effects of Atomic Radiation|title=Sources and effects of ionizing radiation |date=2008 |publication-date=2010 |publisher=United Nations |location=New York |isbn=978-92-1-142274-0 |url=https://www.unscear.org/unscear/en/publications/2008_1.html |access-date=9 November 2012|page=4}}</ref>

{{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 |archive-url=https://web.archive.org/web/20070926152956/http://interactive.snm.org/docs/pg_ch07_0403.pdf |archive-date=2007-09-26 }}</ref>

== 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}}

{{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