Physics:Carbon-14

From HandWiki
Carbon-14, 14C
General
Symbol14C
Namescarbon-14, C-14, radiocarbon
Protons6
Neutrons8
Nuclide data
Natural abundance1.2 ppt (1.2×10−12)
Half-life5700±30 years[1]
Isotope mass14.0032420[2] u
Spin0+
Decay modes
Decay modeDecay energy (MeV)
Beta0.1565[3]
Isotopes of Chemistry:carbon
Complete table of nuclides

Carbon-14, C-14, 14C 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 Kurie in 1934.[4]

There are three naturally occurring isotopes of carbon on Earth: carbon-12 (12C), which makes up 99% of all carbon on Earth; carbon-13 (13C), which makes up 1%; and carbon-14 (14C), which occurs in trace amounts, making up about 1.2[5] atoms per 1012 atoms of carbon in the atmosphere. 12C and 13C are both stable; 14C is unstable, with half-life 5700±30 years, decaying into nitrogen-14 (14N) through beta decay.[6] Pure carbon-14 would have a specific activity of 62.4 mCi/mmol (2.31 GBq/mmol), or 164.9 GBq/g.[7] 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:

146C → 147N + e−
 + νe + 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 5700±30 years[1]) 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.[8] 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[9] 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.[10]

Radiocarbon dating

Main article: Physics:Radiocarbon dating

Radiocarbon dating is a radiometric dating method that uses 14C 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[11] during his tenure as a professor at the University of Chicago. Libby estimated that the radioactivity of exchangeable 14C 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.[12][13] In 1960, Libby was awarded the Nobel Prize in chemistry for this work.[14]

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 14C in plants and animals when they die, roughly equals the level of 14C in the atmosphere at that time. However, it thereafter decreases exponentially, so the date of death or fixation can be estimated. The initial 14C 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 14C 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.[15][16]

Cosmogenic nuclides are also used as proxy data to characterize cosmic particle and solar activity of the distant past.[17][18]

Origin

Natural production in the atmosphere

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):

147N + n → 146C + p + 0.626 MeV

The highest rate of carbon-14 production takes place at altitudes of 9 to 15 kilometres (30,000 to 49,000 ft) and at high geomagnetic latitudes.

The rate of 14C production can be modeled, yielding values of 16,400[19] or 18,800[20] atoms of 14C per second per square meter of Earth's surface, which agrees with the global carbon budget that can be used to backtrack,[21] 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. [21][22] Occasional spikes may occur, known as Miyake events; for example, there is evidence for an unusually high production rate in AD 774–775,[23] caused by an extreme solar energetic particle event, the strongest such event to have occurred within the last ten millennia.[24][25] Another "extraordinarily large" 14C increase (2%) has been associated with a 5480 BC event, which is unlikely to be a solar energetic particle event.[26] The largest known Miyake event took place ca. 12350 BCE.[27]

Carbon-14 may also be produced by lightning[28][29] 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 13C(n,γ)14C and 17O(n,α)14C with thermal neutrons, and 15N(n,d)14C and 16O(n,3He)14C with fast neutrons.[30] The most notable routes for 14C 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 10−8 relative to alpha decay, so radiogenic carbon-14 is extremely rare.

14C production routes[31]
Parent isotope Natural abundance, % Cross section for thermal neutron capture, b Reaction
14N 99.634 1.81 14N(n,p)14C
13C 1.103 0.0009 13C(n,γ)14C
17O 0.0383 0.235 17O(n,α)14C

Formation during nuclear tests

Atmospheric 14C, New Zealand[32] and Austria.[33] 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 14C concentration of the Northern Hemisphere.[34] 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 14C in the atmosphere and subsequently the biosphere; after the tests ended, the atmospheric concentration of the isotope began to decrease, as radioactive CO2 was fixed into plant and animal tissue, and dissolved in the oceans.

One side-effect of the change in atmospheric 14C is that this has enabled some options (e.g. bomb-pulse dating[35]) for determining the birth year of an individual, in particular, the amount of 14C in tooth enamel,[36][37] or the carbon-14 concentration in the lens of the eye.[38]

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.[39]

The concentration of 14C in atmospheric CO2, reported as the 14C/12C 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.[40][41] Though the extra 14C generated by those nuclear tests has not disappeared from the atmosphere, oceans and biosphere,[42] 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.[43] Best practice for nuclear power plant operator management of carbon-14 includes releasing it at night, when plants are not photosynthesizing.[44] 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 14C is released, for example as CO2 during PUREX.[45][46]

Occurrence

Dispersion in the environment

After production in the upper atmosphere, the carbon-14 reacts rapidly to form mostly (about 93%) 14CO (carbon monoxide), which subsequently oxidizes at a slower rate to form 14CO2, 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.[22] The atmospheric half-life for removal of 14CO2 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.[31] In 2009 the activity of 14C was 238 Bq per kg carbon of fresh terrestrial biomatter, close to the values before atmospheric nuclear testing (226 Bq/kg C; 1950).[47]

Total inventory

The inventory of carbon-14 in Earth's biosphere is about 300 megacuries (11 EBq), of which most is in the oceans.[48] The following inventory of carbon-14 has been given:[49]

  • 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

Main article: Chemistry:Suess effect

Many human-made chemicals are derived from fossil fuels (such as petroleum or coal) in which 14C is greatly depleted because the age of fossils far exceeds the half-life of 14C. The relative absence of 14CO2 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.[50]

Dating a specific sample of fossilized carbonaceous material is more complicated. Such deposits often contain trace amounts of 14C. 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).[51] This may indicate contamination by small amounts of bacteria, underground sources of radiation causing a 14N(n,p)14C reaction, direct uranium decay (though reported measured ratios of 14C/U in uranium-bearing ores[52] would imply roughly 1 uranium atom for every two carbon atoms in order to cause the 14C/12C ratio, measured to be on the order of 10−15), or other unknown secondary sources of 14C production. The presence of 14C 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 14C content. In the Borexino Counting Test Facility, a 14C/12C ratio of 1.94×10−18 was determined;[53] probable reactions responsible for varied levels of 14C in different petroleum reservoirs, and the lower 14C levels in methane, have been discussed by Bonvicini et al.[54]

In the human body

Since many sources of human food are ultimately derived from terrestrial plants, the relative concentration of 14C in human bodies is nearly identical to the relative concentration in the atmosphere. The rates of disintegration of potassium-40 (40K) and 14C in the normal adult body are comparable (a few thousand decays per second).[55] Because of the different decay energies, though, K-40 gives about 0.17 mSv/year and C-14 0.012 mSv/year internal dose.[56]

14C 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 37 kBq (1.0 μCi) 14C 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.[57]

See also

References

  1. ↑ 1.0 1.1 Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties". Chinese Physics C 45 (3). doi:10.1088/1674-1137/abddae. https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf. 
  2. ↑ Template:AME2020 II
  3. ↑ National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/. 
  4. ↑ "Early History of Carbon-14: Discovery of this supremely important tracer was expected in the physical sense but not in the chemical sense". Science 140 (3567): 584–590. May 1963. doi:10.1126/science.140.3567.584. PMID 17737092. Bibcode: 1963Sci...140..584K. 
  5. ↑ Activity at 1 ppt: (10-12 * 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.
  6. ↑ Kondev, F.G.; Wang, M.; Huang, W.J.; Naimi, S.; Audi, G. (2021-03-01). "The NUBASE2020 evaluation of nuclear physics properties *". Chinese Physics C 45 (3): 030001. doi:10.1088/1674-1137/abddae. ISSN 1674-1137. Bibcode: 2021ChPhC..45c0001K. 
  7. ↑ "Late-Stage Carbon-14 Labeling and Isotope Exchange: Emerging Opportunities and Future Challenges". JACS Au 2 (6): 1234–1251. June 2022. doi:10.1021/jacsau.2c00030. PMID 35783167. 
  8. ↑ "14C Comments on evaluation of decay data". LNHB. http://www.nucleide.org/DDEP_WG/Nuclides/Tl-208_com.pdf. 
  9. ↑ "Appendix B: The Characteristics of Common Radioisotopes". Radiation Safety Manual for Laboratory Users. Princeton University. http://web.princeton.edu/sites/ehs/radmanual/radman_app_b.htm#c14. 
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  11. ↑ "Age determinations by radiocarbon content; checks with samples of known age". Science 110 (2869): 678–680. December 1949. doi:10.1126/science.110.2869.678. PMID 15407879. Bibcode: 1949Sci...110..678A. 
  12. ↑ "Carbon 14:age calculation". C14dating.com. http://www.c14dating.com/agecalc.html. 
  13. ↑ "Class notes for Isotope Hydrology EESC W 4886: Radiocarbon 14C". Martin Stute's homepage at Columbia. http://www.ldeo.columbia.edu/~martins/isohydro/c_14.html. 
  14. ↑ "The natural clocks that can pinpoint someone's time of death". Oct 2, 2025. https://www.bbc.com/future/article/20250926-the-natural-clocks-that-can-pinpoint-the-time-of-death. 
  15. ↑ "Deep instability of deforested tropical peatlands revealed by fluvial organic carbon fluxes". Nature 493 (7434): 660–663. January 2013. doi:10.1038/nature11818. PMID 23364745. Bibcode: 2013Natur.493..660M. http://nora.nerc.ac.uk/id/eprint/21405/1/N021405PP.pdf. 
  16. ↑ "The Potential Hidden Age of Dissolved Organic Carbon Exported by Peatland Streams" (in en). Journal of Geophysical Research: Biogeosciences 124 (2): 328–341. 2019. doi:10.1029/2018JG004650. ISSN 2169-8953. Bibcode: 2019JGRG..124..328D. 
  17. ↑ "The INTCAL20 Northern Hemisphere Radiocarbon Age Calibration Curve (0–55 CAL kBP)". Radiocarbon 62 (4): 725–757. August 2020. doi:10.1017/RDC.2020.41. Bibcode: 2020Radcb..62..725R. 
  18. ↑ "Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings". Nature Geoscience 14 (1): 10–15. 2021. doi:10.1038/s41561-020-00674-0. Bibcode: 2021NatGe..14...10B. https://www.dora.lib4ri.ch/eawag/islandora/object/eawag%3A21905. 
  19. ↑ "A new model of cosmogenic production of radiocarbon 14C in the atmosphere". Earth and Planetary Science Letters 337–338: 114–20. 2012. doi:10.1016/j.epsl.2012.05.036. ISSN 0012-821X. Bibcode: 2012E&PSL.337..114K. 
  20. ↑ "Production of cosmogenic isotopes 7Be, 10Be, 14C, 22Na, and 36Cl in the atmosphere: Altitudinal profiles of yield functions". Journal of Geophysical Research: Atmospheres 121 (13): 8125–36. 2016. doi:10.1002/2016JD025034. Bibcode: 2016JGRD..121.8125P. 
  21. ↑ 21.0 21.1 "Distinct roles of the Southern Ocean and North Atlantic in the deglacial atmospheric radiocarbon decline". Earth and Planetary Science Letters 394: 198–208. 2014. doi:10.1016/j.epsl.2014.03.020. ISSN 0012-821X. Bibcode: 2014E&PSL.394..198H. https://earth-system-biogeochemistry.net/wp-content/uploads/2021/05/Hain_et_al_2014_EPSL.pdf. 
  22. ↑ 22.0 22.1 Ramsey, C. Bronk (2008). "Radiocarbon Dating: Revolutions in Understanding". Archaeometry 50 (2): 249–75. doi:10.1111/j.1475-4754.2008.00394.x. Bibcode: 2008Archa..50..249B. 
  23. ↑ "A signature of cosmic-ray increase in AD 774-775 from tree rings in Japan". Nature 486 (7402): 240–242. June 2012. doi:10.1038/nature11123. PMID 22699615. Bibcode: 2012Natur.486..240M. http://sciences.blogs.liberation.fr/files/c14-774-apr%C3%A8s-jc.pdf. 
  24. ↑ "The AD775 cosmic event revisited: the Sun is to blame". Astron. Astrophys. 552: L3. 2013. doi:10.1051/0004-6361/201321080. Bibcode: 2013A&A...552L...3U. 
  25. ↑ "Multiradionuclide evidence for the solar origin of the cosmic-ray events of ᴀᴅ 774/5 and 993/4". Nature Communications 6: 8611. October 2015. doi:10.1038/ncomms9611. PMID 26497389. Bibcode: 2015NatCo...6.8611M. 
  26. ↑ "Large 14C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene". Proceedings of the National Academy of Sciences of the United States of America 114 (5): 881–884. January 2017. doi:10.1073/pnas.1613144114. PMID 28100493. Bibcode: 2017PNAS..114..881M. 
  27. ↑ "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". Philosophical Transactions of the Royal Society A 281 (2261): id.20220206. 2023. doi:10.1098/rsta.2022.0206. Bibcode: 2023RSPTA.38120206B. 
  28. ↑ "Production of radiocarbon in tree rings by lightning bolts". Journal of Geophysical Research 78 (26): 5902–5903. 1973. doi:10.1029/JB078i026p05902. Bibcode: 1973JGR....78.5902L. 
  29. ↑ "Photonuclear reactions triggered by lightning discharge". Nature 551 (7681): 481–484. November 2017. doi:10.1038/nature24630. PMID 29168803. Bibcode: 2017Natur.551..481E. 
  30. ↑ "Carbon-14 production in nuclear reactors.". U.S. Nuclear Regulatory Commission (TN (USA): Oak Ridge National Lab.). January 1977. doi:10.2172/7114972. https://www.osti.gov/scitech/servlets/purl/7114972. 
  31. ↑ 31.0 31.1 "Life cycle and management of carbon-14 from nuclear power generation". Progress in Nuclear Energy 48 (1): 2–36. 2006. doi:10.1016/j.pnucene.2005.04.002. Bibcode: 2006PNuE...48....2Y. 
  32. ↑ "Atmospheric δ14C record from Wellington". Trends: A Compendium of Data on Global Change. (Carbon Dioxide Information Analysis Center). 1994. http://cdiac.esd.ornl.gov/trends/co2/welling.html. Retrieved 2007-06-11. 
  33. ↑ "δ14C record from Vermunt". Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center. 1994. http://cdiac.esd.ornl.gov/trends/co2/cent-verm.html. Retrieved 2009-03-25. 
  34. ↑ "Radiocarbon dating". University of Utrecht. http://www1.phys.uu.nl/ams/Radiocarbon.htm. 
  35. ↑ "Bomb-Pulse Dating of Human Material: Modeling the Influence of Diet". Radiocarbon 52 (2): 800–07. August 2010. doi:10.1017/S0033822200045811. Bibcode: 2010Radcb..52..800G. https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/3713. 
  36. ↑ "Radiation in Teeth Can Help Date, ID Bodies, Experts Say". National Geographic News. 2005-09-22. http://news.nationalgeographic.com/news/2005/09/0922_050922_nuke_body.html. 
  37. ↑ "Forensics: age written in teeth by nuclear tests". Nature 437 (7057): 333–334. September 2005. doi:10.1038/437333a. PMID 16163340. Bibcode: 2005Natur.437..333S. 
  38. ↑ "Radiocarbon dating of the human eye lens crystallines reveal proteins without carbon turnover throughout life". PLOS ONE 3 (1). January 2008. doi:10.1371/journal.pone.0001529. PMID 18231610. Bibcode: 2008PLoSO...3.1529L. 
  39. ↑ "'Bomb Carbon' Has Been Found in Deep-Ocean Creatures". Scientific American. 15 May 2019. https://www.scientificamerican.com/article/bomb-carbon-has-been-found-in-deep-ocean-creatures/. 
  40. ↑ Jones, Nicola (27 July 2022). "Carbon dating hampered by rising fossil-fuel emissions". Nature News. https://www.nature.com/articles/d41586-022-02057-4. 
  41. ↑ Graven, H.; Keeling, R.; Xu, X. (19 July 2022). "Radiocarbon dating: going back in time". Nature 607 (7919): 449. doi:10.1038/d41586-022-01954-y. PMID 35854150. Bibcode: 2022Natur.607R.449G. https://www.nature.com/articles/d41586-022-01954-y. 
  42. ↑ Caldeira, K.; Rau, G.H.; Duffy, PB (1998). "Predicted net efflux of radio- carbon from the ocean and increase in atmospheric radiocarbon content". Geophysical Research Letters 25 (20): 3811–3814. doi:10.1029/1998GL900010. Bibcode: 1998GeoRL..25.3811C. 
  43. ↑ "EPRI | Product Abstract | Impact of Nuclear Power Plant Operations on Carbon-14 Generation, Chemical Forms, and Release". http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001023023. 
  44. ↑ "EPRI | Product Abstract | Carbon-14 Dose Calculation Methods at Nuclear Power Plants". http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001024827. 
  45. ↑ "Environmental Impact of Atmospheric Carbon-14 Emissions Resulting from the Nuclear Energy Cycle.". Radiocarbon After Four Decades.. New York, NY: Springer. 1992. 
  46. ↑ "Carbon-14 and the environment". Institute for Radiological Protection and Nuclear Safety. https://www.irsn.fr/EN/Research/publications-documentation/radionuclides-sheets/environment/Pages/carbon14-environment.aspx. 
  47. ↑ "Carbon-14 and the environment". Institute for Radiological Protection and Nuclear Safety. http://www.irsn.fr/EN/Research/publications-documentation/radionuclides-sheets/environment/Pages/carbon14-environment.aspx#3. 
  48. ↑ "Human Health Fact Sheet – Carbon 14". Argonne National Laboratory, EVS. August 2005. http://www.ead.anl.gov/pub/doc/carbon14.pdf. 
  49. ↑ Radiochemistry and Nuclear Chemistry (3rd ed.). Butterworth-Heinemann. 2002. ISBN 978-0-7506-7463-8. 
  50. ↑ "The Basics: 14C and Fossil Fuels". http://www.esrl.noaa.gov/gmd/outreach/isotopes/c14tracer.html. "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." 
  51. ↑ "Problems associated with the use of coal as a source of C14-free background material". Radiocarbon 31 (2): 117–120. 1989. doi:10.1017/S0033822200044775. Bibcode: 1989Radcb..31..117L. https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/1127/1132. 
  52. ↑ "Carbon-14 Abundances in Uranium Ores and Possible Spontaneous Exotic Emission from U-Series Nuclides". Meteoritics 20: 676. 1985. Bibcode: 1985Metic..20..676J. 
  53. ↑ "Measurement of the 14C abundance in a low-background liquid scintillator". Physics Letters B 422 (1–4): 349–358. 1998. doi:10.1016/S0370-2693(97)01565-7. Bibcode: 1998PhLB..422..349B. 
  54. ↑ Bonvicini G, Harris N, Paolone V (2003). "The chemical history of 14C in deep oilfields". arXiv:hep-ex/0308025.
  55. ↑ "The Radioactivity of the Normal Adult Body". rerowland.com. http://www.rerowland.com/BodyActivity.htm. 
  56. ↑ United Nations Scientific Committee on the Effects of Atomic Radiation (2008). Sources and effects of ionizing radiation. New York: United Nations (published 2010). p. 4. ISBN 978-92-1-142274-0. http://www.unscear.org/unscear/en/publications/2008_1.html. Retrieved 9 November 2012. 
  57. ↑ "Society of Nuclear Medicine Procedure Guideline for C-14 Urea Breath Test". snm.org. 2001-06-23. http://interactive.snm.org/docs/pg_ch07_0403.pdf. 

Further reading


Lighter:
carbon-13 		Carbon-14 is an
isotope of carbon 		Heavier:
carbon-15
Decay product of:
boron-14, nitrogen-18 		Decay chain
of carbon-14 		Decays to:
nitrogen-14



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