 | |
| General | |
|---|---|
| Symbol | 14C |
| Names | carbon-14, 14C, C-14, radiocarbon |
| Protons(Z) | 6 |
| Neutrons(N) | 8 |
| Nuclide data | |
| Natural abundance | 1 part per trillion = |
| Half-life(t1/2) | 5700±30 years[1] |
| Isotope mass | 14.0032420[2]Da |
| Spin | 0+ |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Beta | 0.156476[2] |
| Isotopes of carbon Complete table of nuclides | |
Carbon-14,C-14,14C orradiocarbon, is aradioactive isotope ofcarbon with anatomic nucleus containing 6protons and 8neutrons. Its presence in organic matter is the basis of theradiocarbon dating method pioneered byWillard Libby and colleagues (1949) to date archaeological, geological and hydrogeological samples. Carbon-14 was discovered on February 27, 1940, byMartin Kamen andSam Ruben at theUniversity of California Radiation Laboratory inBerkeley, California. Its existence had been suggested byFranz Kurie in 1934.[3]
There are three naturally occurringisotopes 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 1-1.5 atoms per 1012 atoms of carbon in the atmosphere.12C and13C are both stable;14C is unstable, withhalf-life5700±30 years.[4] Carbon-14 has a specific activity of 62.4 mCi/mmol (2.31 GBq/mmol), or 164.9 GBq/g.[5] Carbon-14 decays intonitrogen-14 (14
N) throughbeta decay.[6] A gram of carbon containing 1 atom of carbon-14 per 1012 atoms, emits ~0.2[7] beta (β) particles per second. The primary natural source of carbon-14 on Earth iscosmic ray action on nitrogen in the atmosphere, and it is therefore acosmogenic nuclide. However, open-airnuclear testing 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 calledcarbon 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.
Carbon-14 undergoesbeta decay:
By emitting anelectron and anelectron antineutrino, one of the neutrons in carbon-14 decays to a proton and the carbon-14 (half-life of5700±30 years[1]) decays into the stable (non-radioactive) isotopenitrogen-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 thedead skin layer is estimated to be 0.11. Small amounts of carbon-14 are not easily detected by typicalGeiger–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-distance layer in water is 0.05 mm.[10]
Radiocarbon dating is aradiometric dating method that uses14C to determine the age ofcarbonaceous materials up to about 60,000 years old. The technique was developed byWillard Libby and his colleagues in 1949[11] during his tenure as a professor at theUniversity of Chicago. Libby estimated that the radioactivity of exchangeable14C would be about 14 decays per minute (dpm) per gram of carbon, and this is still used as the activity of themodern radiocarbon standard.[12][13] In 1960, Libby was awarded theNobel Prize in chemistry for this work.
One of the frequent uses of the technique is to date organic remains from archaeological sites. Plantsfix atmospheric carbon during photosynthesis; so the level of14C in plants and animals when they die, roughly equals the level of14C in the atmosphere at that time. However, it thereafter decreases exponentially; so the date of death or fixation can be estimated. The initial14C 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-deposit14C 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, inpeatland landscapes, radiocarbon can indicate that carbon which was previously stored in organic soils is being released due to land clearance or climate change.[14][15]
Cosmogenic nuclides are also used asproxy data to characterize cosmic particle and solar activity of the distant past.[16][17]
Carbon-14 is produced in the uppertroposphere and thestratosphere bythermal neutrons absorbed bynitrogen atoms. Whencosmic rays enter the atmosphere, they undergo various transformations, including the production ofneutrons. The resulting neutrons (n) participate in the followingn-p reaction (p isproton):
The highest rate of carbon-14 production takes place at altitudes of 9 to 15 kilometres (30,000 to 49,000 ft) and at highgeomagnetic latitudes.
The rate of14C production can be modeled, yielding values of 16,400[18] or 18,800[19] atoms of14
C per second per square meter of Earth's surface, which agrees with the globalcarbon budget that can be used to backtrack,[20] but attempts to measure the production time directlyin 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 theEarth's magnetic field. Changes in thecarbon cycle however can make such effects difficult to isolate and quantify.[20][21]Occasional spikes may occur; for example, there is evidence foran unusually high production rate in AD 774–775,[22] caused by an extremesolar energetic particle event, the strongest such event to have occurred within the last ten millennia.[23][24] Another "extraordinarily large"14C increase (2%) has been associated with a 5480 BC event, which is unlikely to be a solar energetic particle event.[25]
Carbon-14 may also be produced by lightning[26][27] but in amounts negligible, globally, compared to cosmic ray production. Local effects of cloud-ground discharge through sample residues are unclear, but possibly significant.
Carbon-14 can also be produced by other neutron reactions, including in particular13C(n,γ)14C and17O(n,α)14C withthermal neutrons, and15N(n,d)14C and16O(n,3He)14C withfast neutrons.[28] The most notable routes for14C production by thermal neutron irradiation of targets (e.g., in a nuclear reactor) are summarized in the table.
Another source of carbon-14 iscluster decay branches from traces of naturally occurringisotopes of radium, though this decay mode has abranching ratio on the order of10−8 relative toalpha decay, soradiogenic carbon-14 is extremely rare.
| 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 |
The above-groundnuclear tests that occurred in several countries in 1955-1980 (seeList of nuclear tests) dramatically increased the amount of14C 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 atmospheric14C is that this has enabled some options (e.g.bomb-pulse dating[33]) for determining the birth year of an individual, in particular, the amount of14C intooth enamel,[34][35] or the carbon-14 concentration in the lens of the eye.[36]
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, theMariana Trench in the Pacific Ocean.[37]
The concentration of14C in atmospheric CO2, reported as the14C/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.[38][39] Though the extra14C generated by those nuclear tests has not disappeared from the atmosphere, oceans and biosphere,[40] it is diluted due to theSuess effect.
Carbon-14 is produced in coolant atboiling water reactors (BWRs) andpressurized water reactors (PWRs). It is typically released into the air in the form ofcarbon dioxide at BWRs, andmethane at PWRs.[41] Best practice for nuclear power plant operator management of carbon-14 includes releasing it at night, when plants are notphotosynthesizing.[42] Carbon-14 is also generated inside nuclear fuels (some due to transmutation of oxygen in theuranium oxide, but most significantly from transmutation of nitrogen-14 impurities), and if the spent fuel is sent tonuclear reprocessing then the14C is released, for example as CO2 duringPUREX.[43][44]
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 form14
CO
2, radioactivecarbon 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 theoceans, but at a slower rate.[21] The atmospheric half-life for removal of14
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 ofbicarbonates in the ocean depths occurs at a limited rate.[29]In 2009 the activity of14
C was 238 Bq per kg carbon of fresh terrestrial biomatter, close to the values before atmospheric nuclear testing (226 Bq/kg C; 1950).[45]
The inventory of carbon-14 in Earth's biosphere is about 300megacuries (11 EBq), of which most is in the oceans.[46]The following inventory of carbon-14 has been given:[47]
Many human-made chemicals are derived fromfossil fuels (such aspetroleum orcoal) in which14C is greatly depleted because the age of fossils far exceeds the half-life of14C. The relative absence of14
CO
2 is therefore used to determine the relative contribution (ormixing ratio) of fossil fuel oxidation to the totalcarbon dioxide in a given region of Earth'satmosphere.[48]
Dating a specific sample of fossilized carbonaceous material is more complicated. Such deposits often contain trace amounts of14C. 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).[49] This may indicate contamination by small amounts of bacteria, underground sources of radiation causing a14N(n,p)14C reaction, directuranium decay (though reported measured ratios of14C/U in uranium-bearing ores[50] would imply roughly 1 uranium atom for every two carbon atoms in order to cause the14C/12C ratio, measured to be on the order of 10−15), or other unknown secondary sources of14C production. The presence of14C in theisotopic 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 theBorexino solar neutrino observatory, petroleum feedstock (for synthesizing the primary scintillant) was obtained with low14C content. In the Borexino Counting Test Facility, a14C/12C ratio of 1.94×10−18 was determined;[51] probable reactions responsible for varied levels of14C in differentpetroleum reservoirs, and the lower14C levels in methane, have been discussed by Bonvicini et al.[52]
Since many sources of human food are ultimately derived from terrestrial plants, the relative concentration of14C in human bodies is nearly identical to the relative concentration in the atmosphere. The rates of disintegration ofpotassium-40 (40K) and14C in the normal adult body are comparable (a few thousand decays per second).[53] The beta decays from external (environmental) radiocarbon contribute about 0.01 mSv/year (1 mrem/year) to each person'sdose ofionizing radiation.[54] This is small compared to the doses from40K (0.39 mSv/year) andradon (variable).
14C can be used as aradioactive tracer in medicine. In the initial variant of theurea breath test, a diagnostic test forHelicobacter 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 aH. pylori infection, the bacterialurease enzyme breaks down theurea intoammonia and radioactively-labeledcarbon dioxide, which can be detected by low-level counting of the patient's breath.[55]
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.
| Lighter: carbon-13 | Carbon-14 is an isotope ofcarbon | Heavier: carbon-15 |
| Decay product of: boron-14,nitrogen-18 | Decay chain of carbon-14 | Decays to: nitrogen-14 |
