RADIOCARBON DATING is a method of determining the age of organic material (up to about 50,000 years old) by measuring the decay of radioactive carbon-14. It was developed in 1948–9 by Willard Libby at the University of Chicago.
Some elements have more than one type of atom, called isotopes. Carbon has three main isotopes, carbon-12, carbon-13 and radioactive carbon-14. The unstable carbon-14, created in the Earth's upper atmosphere, enters the food chain along with the stable carbon isotopes. Carbon-12 accounts for something like 99% of the total, and carbon-14 a minute fraction. The carbon-14 is in a constant state of decay but, as long as an organism is alive, ingesting more carbon, the balance between carbon-12 and carbon-14 remains stable. When the organism dies, however, new carbon is not being taken in, and so, as the carbon-14 decays, the ratio of carbon-12 to carbon-14 changes. The half-life of carbon-14 is 5,730 years. This means that, after 5,730 years, half of the carbon-14 will have gone. Therefore, the year of death of an organism can be calculated from the proportion of carbon-14 left in a sample taken from its remains. Although the proportion of carbon-14 has varied significantly during the history of the Earth, calibration tables have been developed to compensate for this. In samples older than about 50,000 years, there will be insufficient carbon-14 left to provide reliable results, and, conversely, recent samples will show too little decay to provide reliable results.