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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 (1949) 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 or 1.5 atoms per 1012 atoms 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.〔(【引用サイトリンク】title=What is carbon dating? )〕 A gram of carbon containing 1 atom of carbon-14 per 1012 atoms will emit 0.192 beta rays 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 1955–1980 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 (half-life of 5730 years) decays into the stable (non-radioactive) isotope nitrogen-14. The emitted beta particles have a 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 (G-M) detectors; it is estimated that G-M detectors will not normally detect contamination of less than about 100 000 disintegration per minute (0.05 µCi). Liquid scintillation counting is the preferred method.〔("Radiation Safety Manual for Laboratory Users, Appendix B: The Characteristics of Common Radioisotopes" ), Princeton University.〕 The G-M counting efficiency is estimated to be 3%. The half-distance layer in water is 0.05 mm.〔("Material Safety Data Sheet. Carbon-14" ), University of Michigan.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Carbon-14」の詳細全文を読む スポンサード リンク
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