Author of the Month


By Sean Hancock
December 2000

Principles of Radiocarbon dating

William Frank Libby and his team developed the principles of radiocarbon dating during the 1950s. By 1960 their work was complete and in December of the same year Libby was presented with the Nobel Prize for Chemistry. One of the scientists who nominated Libby for the award commented:

"Seldom has a single discovery in chemistry had such an impact on the thinking of so many fields of human endeavour. Seldom has a single discovery generated such wide interest" [2]

Libby discovered that the unstable radioactive component of carbon (C14) disintegrated at a predictable level against the stable elements of the carbon composite (C12 and C13). All three of these isotopes occur naturally in our atmosphere in the following proportions: C12 - 98.89%, C13 - 1.11% and C14 - 0.00000000010% [3].

The stable isotopes of carbon (12 and 13) were formed when all of our planet's atoms materialised --a long, long time ago [4]. C14 is formed, albeit on a miniscule level, due to bombardments of cosmic rays that hit our planet, on a day-to-day basis, and interact with our atmosphere [5]. These rays strike the earth's existing atoms and break them up leaving the neutrons of these atoms to float around our atmosphere.

A carbon 14 isotope is formed when one of these floating neutrons merges with the nucleus of a nitrogen atom [6]. Radiocarbon, therefore, is a kind of Frankenstein isotope, a fusion between different atomic elements. These rogue carbon 14 isotopes, which are produced at a steady rate, are then oxidized and absorbed into the biosphere through the process of photosynthesis and the natural food chain [7].

Consequently all living things incorporate the atmospheric ratio of C14 to C12 in their geographical area, which is maintained by their metabolic rate [8]. Once dead, however, living organisms stop absorbing carbon and it is the behaviour of C14 after this point that is interesting. Libby discovered that radiocarbon decays with a half-life of 5568 years [9]. This means that after 5568 years or so, half of the original amount of C14 would have disintegrated from the sample. After another 5568 years, again, half of what is left dies.

Therefore, with the original amount of C14 to C12 being a geological constant [10], the age of a sample can be determined by measuring the residual C14 present. For example, if one quarter of the original amount of C14 is present then the organism in question died two half lives ago (5568 + 5568), which equates to an age of 10, 146 years.

Herein lie the basic backbone principles of radiocarbon dating as a tool of science and archaeology. It is a fact that radiocarbon is absorbed into the biosphere. It is a fact that when an organism dies no more C14 is absorbed. It is a fact that C14 spontaneously dies after this point. It is a fact that this process can be measured.

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  1. Quoted in Taylor 1987
  2. Higham, Thomas,
  3. Hedges, Robert, E. M,
  4. Renfrew, Colin, Before Civilization (1973), Pg 255
  5. Ibid
  6. Gove, H. E, Carbon Dating The Turin Shroud, 1996, 11
  7. Taylor, R. E, Radiocarbon Dating, 2 (useful illustration)
  8. The actual duration of the "Half Life" has never been universally accepted, Taylor 4, Renfrew, 261-262
  9. Levine, Mary Ann, Accommodating Age - Archaeology of Eastern North America, Volume 18, 1990, 33. This will be discussed later on

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