Carbon-14
Template:Short description Template:Redirect Template:Infobox isotope
Carbon-14, C-14, Template:SupC 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.<ref>Template:Cite journal</ref>
There are three naturally occurring isotopes of carbon on Earth: carbon-12 (Template:SupC), which makes up 99% of all carbon on Earth; carbon-13 (Template:SupC), which makes up 1%; and carbon-14 (Template:SupC), which occurs in trace amounts, making up about 1-1.5 atoms per 10Template:Sup atoms of carbon in the atmosphere. Template:SupC and Template:SupC are both stable; Template:SupC is unstable, with half-life Template:Val years.<ref>Template:Cite journal</ref> Carbon-14 has a specific activity of 62.4 mCi/mmol (2.31 GBq/mmol), or 164.9 GBq/g.<ref>Template:Cite journal</ref> Carbon-14 decays into nitrogen-14 (Template:Chem) through beta decay.<ref>Template:Cite web</ref> A gram of carbon containing 1 atom of carbon-14 per 10Template:Sup atoms, emits ~0.2<ref>(1 per 10Template:Sup) × (1 gram / (12 grams per mole)) × (Avogadro constant) / ((5,730 years) × (31,557,600 seconds per 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 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 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
[edit]Carbon-14 undergoes beta decay:
- Template:Chem → Template:Chem + Template:Chem2 + Template:Subatomic particle + 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 Template:Val yearsTemplate:NUBASE2020) 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">Template:Cite web</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>Template:Cite book</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 mm.<ref>Template:Cite web</ref>
Radiocarbon dating
[edit]Template:Main Radiocarbon dating is a radiometric dating method that uses Template:SupC 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>Template:Cite journal</ref> during his tenure as a professor at the University of Chicago. Libby estimated that the radioactivity of exchangeable Template:SupC 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>Template:Cite web</ref><ref>Template:Cite web</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 fix atmospheric carbon during photosynthesis, so the level of Template:SupC in plants and animals when they die, roughly equals the level of Template:SupC in the atmosphere at that time. However, it thereafter decreases exponentially, so the date of death or fixation can be estimated. The initial Template:SupC 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 Template:SupC 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>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Cosmogenic nuclides are also used as proxy data to characterize cosmic particle and solar activity of the distant past.<ref name="Reimer-2020">Template:Cite journal</ref><ref name="Brehm-2021">Template:Cite journal</ref>
Origin
[edit]Natural production in the atmosphere
[edit]
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):
- Template:Chem + n → Template:Chem + p + 0.626 MeV
The highest rate of carbon-14 production takes place at altitudes of Template:Convert and at high geomagnetic latitudes.
The rate of Template:SupC production can be modeled, yielding values of 16,400<ref name="Kovaltsov-2012">Template:Cite journal</ref> or 18,800<ref name="Poluianov-2016">Template:Cite journal</ref> atoms of Template:Chem 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">Template:Cite journal</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">Template:Cite journal</ref> Occasional spikes may occur; for example, there is evidence for an unusually high production rate in AD 774–775,<ref>Template:Cite journal</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">Template:Cite journal</ref><ref name="Mekhaldi-2015">Template:Cite journal</ref> Another "extraordinarily large" Template:SupC increase (2%) has been associated with a 5480 BC event, which is unlikely to be a solar energetic particle event.<ref>Template:Cite journal</ref>
Carbon-14 may also be produced by lightning<ref>Template:Cite journal</ref><ref>Template:Cite journal</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
[edit]Carbon-14 can also be produced by other neutron reactions, including in particular [[Carbon-13|Template:SupC]](n,γ)Template:SupC and [[Oxygen-17|Template:SupO]](n,α)Template:SupC with thermal neutrons, and [[Nitrogen-15|Template:SupN]](n,d)Template:SupC and [[Oxygen-16|Template:SupO]](n,Template:SupHe)Template:SupC with fast neutrons.<ref>Template:Cite report</ref> The most notable routes for Template:SupC 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 Template:Val relative to alpha decay, so radiogenic carbon-14 is extremely rare.
| Parent isotope | Natural abundance, % | Cross section for thermal neutron capture, b | Reaction |
|---|---|---|---|
| Template:SupN | 99.634 | 1.81 | Template:SupN(n,p)Template:SupC |
| Template:SupC | 1.103 | 0.0009 | Template:SupC(n,γ)Template:SupC |
| Template:SupO | 0.0383 | 0.235 | Template:SupO(n,α)Template:SupC |
Formation during nuclear tests
[edit]
The above-ground nuclear tests that occurred in several countries in 1955-1980 (see List of nuclear tests) dramatically increased the amount of Template:SupC in the atmosphere and subsequently the biosphere; after the tests ended, the atmospheric concentration of the isotope began to decrease, as radioactive COTemplate:Sub was fixed into plant and animal tissue, and dissolved in the oceans.
One side-effect of the change in atmospheric Template:SupC is that this has enabled some options (e.g. bomb-pulse dating<ref>Template:Cite journal</ref>) for determining the birth year of an individual, in particular, the amount of Template:SupC in tooth enamel,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> or the carbon-14 concentration in the lens of the eye.<ref>Template:Cite journal</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>Template:Cite web</ref>
The concentration of Template:SupC in atmospheric COTemplate:Sub, reported as the Template:SupC/Template:SupC 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>Template:Cite news</ref><ref>Template:Cite journal</ref> Though the extra Template:SupC generated by those nuclear tests has not disappeared from the atmosphere, oceans and biosphere,<ref>Template:Cite journal</ref> it is diluted due to the Suess effect.
Emissions from nuclear power plants
[edit]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>Template:Cite web</ref> Best practice for nuclear power plant operator management of carbon-14 includes releasing it at night, when plants are not photosynthesizing.<ref>Template:Cite web</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 Template:SupC is released, for example as COTemplate:Sub during PUREX.<ref>Template:Cite book</ref><ref>Template:Cite web</ref>
Occurrence
[edit]Dispersion in the environment
[edit]After production in the upper atmosphere, the carbon-14 reacts rapidly to form mostly (about 93%) Template:SupCO (carbon monoxide), which subsequently oxidizes at a slower rate to form Template:Chem, 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 Template:Chem 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">Template:Cite journal</ref> In 2009 the activity of Template:Chem was 238 Bq per kg carbon of fresh terrestrial biomatter, close to the values before atmospheric nuclear testing (226 Bq/kg C; 1950).<ref>Template:Cite web</ref>
Total inventory
[edit]The inventory of carbon-14 in Earth's biosphere is about 300 megacuries (11 EBq), of which most is in the oceans.<ref>Template:Cite web</ref> The following inventory of carbon-14 has been given:<ref name="Choppin-2002">Template:Cite book</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
[edit]Many human-made chemicals are derived from fossil fuels (such as petroleum or coal) in which Template:SupC is greatly depleted because the age of fossils far exceeds the half-life of Template:SupC. The relative absence of Template:Chem 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">Template:Cite web</ref>
Dating a specific sample of fossilized carbonaceous material is more complicated. Such deposits often contain trace amounts of Template:SupC. 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>Template:Cite journal</ref> This may indicate contamination by small amounts of bacteria, underground sources of radiation causing a Template:SupN(n,p)Template:SupC reaction, direct uranium decay (though reported measured ratios of Template:SupC/U in uranium-bearing ores<ref>Template:Cite journal</ref> would imply roughly 1 uranium atom for every two carbon atoms in order to cause the Template:SupC/Template:SupC ratio, measured to be on the order of 10Template:Sup), or other unknown secondary sources of Template:SupC production. The presence of Template:SupC 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 Template:SupC content. In the Borexino Counting Test Facility, a Template:SupC/Template:SupC ratio of 1.94×10Template:Sup was determined;<ref>Template:Cite journal</ref> probable reactions responsible for varied levels of Template:SupC in different petroleum reservoirs, and the lower Template:SupC levels in methane, have been discussed by Bonvicini et al.<ref>Template:Cite arXiv</ref>
In the human body
[edit]Since many sources of human food are ultimately derived from terrestrial plants, the relative concentration of Template:SupC in human bodies is nearly identical to the relative concentration in the atmosphere. The rates of disintegration of potassium-40 (Template:SupK) and Template:SupC in the normal adult body are comparable (a few thousand decays per second).<ref>Template:Cite web</ref> The beta decays from external (environmental) radiocarbon contribute about 0.01 mSv/year (1 mrem/year) to each person's dose of ionizing radiation.<ref>Template:Cite book)</ref> This is small compared to the doses from potassium-40, Template:SupK (0.39 mSv/year) and radon (variable depending on where you live).
Template:SupC 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 Template:Convert Template:SupC 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>Template:Cite web</ref>