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Kent Archaeological Review extract

Science in Archaeology --
Radio-carbon dating.
by Tony Cox.

An essential part of history and archaeology is the correct dating of events and the associated items of culture, which can vary from potsherds to parchment. Numerous phenomena have been used for estimating the correct age, and while some have counted the annual growth rings in trees, dendrochronology, others have placed their faith in knowing how the earth's magnetic field has changed with time. All these methods have their particular uses, but the one which has general application is known as radio-carbon dating.

Few naturally occurring elements are radio-active, but those that are, spontaneously emit small particles that can be detected by electronic counters and revert in a predictable way to another element. The big advantage, as far as archaeologists are concerned, is that the time keeping of this atomic clock is unaffected by the surroundings, so the sample's previous history does not matter.

Carbon is a commonly occurring element, and is found in plants and animals as carbon-twelve, the twelve indicating the weight of the central part of the atom. This particular form is not radio-active, but the type described as carbon-fourteen is, and this is manufactured in the upper atmosphere when cosmic rays (from outer space), interact with atoms of nitrogen gas. Despite the slight differences in weight, both types of carbon behave the same chemically, and so are absorbed into plants and animals in the same proportion as they exist in the world. Once the organism dies, the absorption of carbon ceases, but whereas the amount of carbon twelve present remains the same, the carbon-fourteen decreases as it changes to nitrogen. The rate of decay is described in terms of the "half-life," which is the time of half the initial amount to disappear, and for carbon-fourteen is 5,568±30 years. Dating is thus a matter of taking the item of interest, be it charcoal, nuts, hair, paper or burnt bone and calculating the age from the radio-activity remaining.

In practice, like most things, the measurement is not quite so straightforward, for archaeological items are only mildly radio-active and precautions have to be taken to prevent the cosmic rays, which are all around us, from penetrating the measuring device and completely swamping the effect. Because of this background radiation, radio-carbon dating is limited to samples less than 50,000 years old, although using specially enriched samples this may be extended to 70,000 years.

The accuracy of dating is often open to discussion, as it depends upon how many measurements are taken, and ages should be given as a probability of lying within a given range, i.e. 99.7% chance of being in the range 9,800-10,200 years.

In the future, radio-carbon dating will become even more difficult, for the assumption that the proportion of the two varieties of carbon in the atmosphere is constant throughout history no longer applies, for the Industrial Revolution has seen the release of large amounts of carbon-twelve into the atmosphere by the combustion of fossil fuels, and in latter years, nuclear explosions have produced large local concentrations of carbon-fourteen.

Books for further information on dating:
The Scientist and Archaeology, by E. Pyddoke. Published by Phoenix House, London.
Physics and Archaeology, by M. J. Aitken. Interscience Publishers, London.

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