Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing
6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon
dating method pioneered by Willard Libby and colleagues to date archaeological, geological
and hydrogeological samples. Carbon-14 was discovered on 27 February 1940, by Martin
Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley. Its existence
had been suggested by Franz Kurie in 1934. There are three naturally occurring isotopes
of carbon on Earth: 99% of the carbon is carbon-12, 1% is carbon-13, and carbon-14 occurs in trace
amounts, i.e., making up about 1 part per trillion of the carbon in the atmosphere.
The half-life of carbon-14 is 5,730±40 years. Carbon-14 decays into nitrogen-14 through
beta decay. The primary natural source of carbon-14 on Earth is cosmic ray action upon
nitrogen in the atmosphere, and it is therefore a cosmogenic nuclide. However, open-air nuclear
testing between 1955–1980 contributed to this pool.
The different isotopes of carbon do not differ appreciably in their chemical properties.
This 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 goes through radioactive beta decay:
By emitting an electron and an electron antineutrino, one of the neutrons in the carbon-14 atom
decays to a proton and the carbon-14 decays into the stable isotope nitrogen-14.
The emitted beta particles have maximum energy of 156 keV, while their average energy is
49 keV. 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 detectors; it is estimated that G-M detectors
will not normally detect contamination of less than about 100 000 disintegration per
minute. Liquid scintillation counting is the preferred method. The G-M counting efficiency
is estimated to be 3%. The half-distance layer in water is 0.05 mm.
Radiocarbon dating
Radiocarbon dating is a radiometric dating method that uses 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 during his tenure as a professor at the University of Chicago.
Libby estimated that the radioactivity of exchangeable carbon-14 would be about 14 disintegrations
per minute per gram of pure carbon, and this is still used as the activity of the modern
radiocarbon standard. 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 14C in plants
and animals when they die approximately equals the level of 14C in the atmosphere at that
time. However, it decreases thereafter from radioactive decay, allowing the date of death
or fixation to 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 up to 10,000 years
ago, or else from cave deposits, back to about 45,000 years before the present. A calculation
or a direct comparison of carbon-14 levels in a sample, with tree ring or cave-deposit
carbon-14 levels of a known age, then gives the wood or animal sample age-since-formation.
Origin Natural production in the atmosphere
Carbon-14 is produced in the upper layers of the 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 participate in the following reaction: 1n + 14N → 14C + 1p
The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km and at high
geomagnetic latitudes. As of 2008, the rate of carbon-14 production
was poorly known – while the reaction can be modelled or the current concentrations
and the global carbon budget can be used to backtrack, attempts to directly measure the
production rate had not agreed with these models very well. Production rates vary because
of changes to the cosmic ray flux incident, such as supernovae, and due to variations
in the Earth's magnetic field. The latter can create significant variations in carbon-14
production rates, although the changes of the carbon cycle can make these effects difficult
to tease out. The natural atmospheric yield of carbon-14
has been estimated to be about 22 000 atoms 14C per meter square of the surface of the
earth per second, resulting in the global production rate of about 1 PBq/a. Another
estimate of the average production rate gives a value of 20 500 atoms m−2s−1. Occasional
spikes are possible; for example, there is evidence for an unusual 10-fold increase of
the production rate in AD 774–775. Other carbon-14 sources
Carbon-14 can also be produced by other neutron reactions, including in particular 13C(n,gamma)14C
and 17O(n,alpha)14C with thermal neutrons, and 15N(n,d)14C and 16O(n,3He)14C with fast
neutrons. The most notable routes for 14C production by thermal neutron irradiation
of targets are summarized in the table. Carbon-14 may also be radiogenic. However,
this origin is extremely rare. Formation during nuclear tests
The above-ground nuclear tests that occurred in several countries between 1955 and 1980
dramatically increased the amount of carbon-14 in the atmosphere and subsequently in the
biosphere; after the tests ended, the atmospheric concentration of the isotope began to decrease.
One side-effect of the change in atmospheric carbon-14 is that this has enabled some options
for determining the birth year of an individual, in particular, the amount of carbon-14 in
tooth enamel, or the carbon-14 concentration in the lens of the eye.
Occurrence Dispersion in the environment
After production in the upper atmosphere, the carbon-14 atoms react rapidly to form
mostly 14CO, which subsequently oxidizes at a slower rate to form 14CO2, radioactive carbon
dioxide. The gas mixes rapidly and becomes evenly distributed throughout the atmosphere.
Carbon dioxide also dissolves in water and thus permeates the oceans, but at a slower
rate. The atmospheric half-life for removal of 14CO2 has been estimated to be 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.
Total inventory The inventory of carbon-14 in Earth's biosphere
is about 300 megacuries, of which most is in the oceans. The following inventory of
carbon-14 has been given: Global inventory: ~8500 PBq
Atmosphere: 140 PBq Terrestrial materials: the balance
From nuclear testing: 220 PBq In fossil fuels
Most man-made chemicals are made of fossil fuels, such as petroleum or coal, in which
the carbon-14 should have long since decayed. However, such deposits often contain trace
amounts of carbon-14. This may indicate possible contamination by small amounts of bacteria,
underground sources of radiation causing the 14N(n,p) 14C reaction, direct uranium decay,
or other unknown secondary sources of carbon-14 production. Presence of carbon-14 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 was obtained
with low 14C content. In the Borexino Counting Test Facility, a 14C/12C ratio of 1.94×10−18
was determined; 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.
In the human body Since essentially all sources of human food
are derived from plants, the carbon that comprises our bodies contains carbon-14 at the same
concentration as the atmosphere. The rates of disintegration of potassium-40 and carbon-14
in the normal adult body are comparable. The beta-decays from external radiocarbon contribute
approximately 0.01 mSv/year to each person's dose of ionizing radiation. This is small
compared to the doses from potassium-40 and radon.
Carbon-14 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 approximately 37 kBq
carbon-14 is fed to a patient. 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. The 14-C urea breath test
has been largely replaced by the 13-C urea breath test which has no radiation issues.
See also Isotopic labeling
Radiocarbon dating References
Further reading Kamen, Martin D.. Radiant Science, Dark Politics:
A Memoir of the Nuclear Age. Berkeley: University of California Press. ISBN 0-520-04929-2.
External links What is Carbon Dating?, Woods Hole Oceanographic
Institute