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'Carbon-14', '¹⁴C', or 'radiocarbon', is a radioactive isotope of carbon discovered on February 27, 1940, by Martin Kamen and Sam Ruben. Its nucleus contains 6 protons and 8 neutrons. Its presence in organic materials is used extensively as basis of the radiocarbon dating method to date archaeological, geological, and hydrogeological samples.
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, making up as much as 1 part per trillion (0.0000000001%) of the carbon on the Earth. The half-life of carbon-14 is 5730±40 years. It decays into nitrogen-14 through beta-decay.^[2] The activity of the ''modern radiocarbon standard''^[3]is about 14 disintegrations per minute (dpm) per gram carbon ^[4].
The atomic mass of carbon-14 is about 14.003241 amu. The different isotopes of carbon do not differ appreciably in their chemical properties. This is used in chemical research in a technique called carbon labeling: some ¹²C atoms of a given compound are replaced with ¹⁴C atoms (or some ¹³C atoms) in order to trace them along chemical reactions involving the given compound.

Contents

Origin and radioactive decay of C-14

Radiocarbon dating

C-14 and fossil fuels

C-14 and nuclear tests

C-14 in the human body

See also

References

Origin and radioactive decay of C-14

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 (¹n) participate in the following reaction:
:¹n + ¹⁴N → ¹⁴C + ¹H
The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km (30,000 to 50,000 feet) and at high geomagnetic latitudes, but the carbon-14 readily mixes and becomes evenly distributed throughout the atmosphere and reacts with oxygen to form radioactive carbon dioxide. Carbon dioxide also dissolves in water and thus permeates the oceans.
Carbon-14 can also be produced in ice by fast neutrons causing spallation reactions in oxygen.
Carbon-14 then goes through radioactive beta decay.
:$mathrm\{~^\{14\}\_\{6\}C\}$
ightarrowmathrm{~^{14}_{7}N}+ e^- + ar{
u}_e
By emitting an electron and an anti-neutrino, carbon-14 decays into stable (non-radioactive) nitrogen-14 with a half life of 5730 years.

Radiocarbon dating

C-14 and fossil fuels

Most man-made chemicals are made of fossil fuels, such as petroleum or coal, in which the carbon-14 has long since decayed. However, oil deposits often contain trace amounts of carbon-14 (varying significantly, but ranging from 1% the ratio found in living organisms to undetectable amounts, comparable to an apparent age of 40,000 years for oils with the highest levels of carbon-14). This may indicate possible contamination by small amounts of bacteria or an unknown second source of carbon-14 production.
Presence of carbon-14 in the isotopic signature of a sample of carbonaceous material therefore indicates its possible biogenic origin and relatively recent geologic age.

C-14 and nuclear tests

The above-ground nuclear tests that occurred in several countries between 1955 and 1963 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. This enabled to determine the birth year of a deceased individual: the amount of ¹⁴C in tooth enamel is measured with accelerator mass spectrometry and compared to records of past atmospheric ¹⁴C concentrations. Since teeth are formed at a specific age and do not exchange carbon thereafter, this method allows age to be determined to within 1.6 years. This method only works for individuals born after 1943,^[6][7]
and it must be known whether the individual was born in the Northern or the Southern Hemisphere.

C-14 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 beta-decays from this internal radiocarbon contribute approx 1 mrem/year (.01 mSv /year) to each person's dose of ionizing radiation.^[8] This is small compared to the doses from potassium-40 (0.39 mSv/year) and radon (which vary).
Carbon-14 can be used as a radioactive tracer in medicine. In the urea breath test, a diagnostic test for ''Helicobacter pylori'', urea labeled with approx 1 μCi (37kBq) 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.^[9]

See also

★ Isotopic tracer

★ Radiocarbon dating

References

1. AME atomic mass evaluation 2003 A.H Waptstra, G. Audi, and C. Thibault
2. What is carbon dating?
3. Carbon 14:age calculation
4. Class notes for Isotope Hydrology EESC W 4886: Radiocarbon ¹⁴C
5. Atmospheric δ¹⁴C record from Wellington
6. Radiation in Teeth Can Help Date, ID Bodies, Experts Say, ''National Geographic News'', 22 September 2005
7. Spalding KL, Buchholz BA, Bergman LE, Druid H, Frisen J. Forensics: age written in teeth by nuclear tests. ''Nature''. 2005 Sep 15;437(7057):333-4. PMID 16163340
8. Ionizing Radiation Exposure of the Population of the United States, NCRP Report No. 93, , , National Council on Radiation Protection and Measurements, , (excerpt)
9. Society of Nuclear Medicine Procedure Guideline for C-14 Urea Breath Test


★ Kamen, Martin D. (1985). ''Radiant Science, Dark Politics: A Memoir of the Nuclear Age'' (foreword by Edwin M. McMillan), University of California Press, USA.

★ What is Carbon Dating?, Woods Hole Oceanographic Institute