How Does Radioisotope Dating Work?
Radiocarbon dating is performed by measuring the ratio of carbon 14 to carbon 13 in organic matter. Carbon 14 is unstable and decays into carbon 13 with the release of radioactivity (a beta particle). Radiocarbon dating cannot reliably determine the age of an object older than 60,000 years. The age of objects older than 1,000 years can generally be determined within plus or minus forty years.
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Isotopes
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Protons and neutrons, together called nucleons, compose the nucleus of an atom. The number of protons in the nucleus of an element determines an element's identity. Carbon has 6 protons. Forms of an element that vary in the number of neutrons in the nucleus are called isotopes. The number of nucleons is given after the elemental symbol or name to specify a particular isotope. Carbon 12, the most abundant isotope, has six protons and neutrons. Carbon 13 has seven neutrons and carbon 14 has eight neutrons. The approximate relative abundance of these three isotopes is carbon 12, 98.9%; carbon 13, 1.1%; and carbon 14, 0.0000000001%.
Radioactive Isotopes
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Some isotopes, including carbon 14, are unstable and decay into other isotopes or other elements with the release of radioactivity. These elements can either be measured indirectly by measuring the radiation given off in a particle counter, or measured directly in an accelerator mass spectrometer.
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Carbon Exchange
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A living organism exchanges carbon with its environment and has a fixed ratio of carbon 13 to carbon 14. When it dies, it quits taking in carbon (as food or carbon dioxide). The carbon 14 decays at a constant rate, changing the ratio of carbon 13 to carbon 14. The ratio of carbon 13 to carbon 14 can be used to determine the age of the object.
Half-life
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Carbon 14 has an exponential decay. After 5,730 years, half of the carbon 14 decays. In another 5,730 years, another half will decay and so forth. If there was 1 gram of carbon 14 in a sample, after 5,730 years, there would be half, or 0.5 grams; after another 5,730 years, another half of the 0.5 grams would decay, and there would be 0.25 grams left; after another 5,730 years, there would be 0.125 grams left and so on. The number of years it takes for half a radioactive isotope to decay is called a half-life.
Other Radioisotopes
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Radioactive isotopes other than carbon can be used to date samples. Igneous rocks can be dated using a uranium-to-lead decay, while other rocks can be dated using a potassium-to-argon decay. The oldest reliable dating limit is typically ten half-lives, so an isotope with a half-life greater than 5,730 years should be selected for dating something older than 60,000 years old.
Instruments
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Measurement destroys the sample. When a sample is plentiful (a gram or more available), a particle counter can be used to determine its age. An accelerator mass spectrometer, requiring about 1 milligram, is used when a sample is scarce. Fresh material (e.g., fingerprints and dust) must be cleaned off the sample leaving only the old organic material for measurement.
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References
- Photo Credit radioactive image by Soja Andrzej from Fotolia.com
