|MadSci Network: Earth Sciences|
Carbon-14 is a radionuclide created by neutrons from cosmic rays through a nuclear reaction as follows. 14N+ n --> 14C+p 14C is a radioactive isotope. It decays by emitting beta rays. It has a half-life of about 5500 years. The proportion of 14C is about 1 in 1,000,000,000,000 of natural 12C. Living systems, both animals and plants continuously uptake carbon atoms in the form of gaseous CO2 and food. There exists equilibrium between production of 14C, its uptake by living system and its decay. However, when an animal or tree dies, the uptake stops but the decay continues. The rate of decay of 14C gives an indication about concentration of 14C in the sample. With such a long half-life and very low concentration in archeological samples, it is difficult to determine the concentration. Background suppressed spectrometers with sample size of few grams are used for this purpose. Typically, about 1 – 3 days of counting time is required. In spite of many uncertainties, this method gives fairly good idea about the age of the sample up to about 60000 years. As you can guess, due to many nuclear tests since last few decades, this technique may fail to give us proper indication in future.
Try the links in the MadSci Library for more information on Earth Sciences.