IIT JEE 2006 Paper : CHEMISTRY Section III - Comprehension Passage 4
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14C is produced in the upper atmosphere by the bombardment of 14N by cosmic neutrons. This 14C is then circulated in the atmosphere and living organisms by absorption of 14C by plants for photosynthesis. 14C undergoes radioactive decay with a half life period of 5760 years.
In a living organism there exists an equilibrium of 14C due to its constant assimilation by the organism and its constant radioactive decay. When an organism dies, the decay continues but the assimilation of 14C stops. Hence, the concentration of 14C starts depleting which when compared to the concentration of 14C in a living organism can help to determine the approximate age of a fossil.
Whenever there is a nuclear explosion the concentration of 14C in the nearby atmosphere and surrounding living organisms increases. This method cannot be employed to determine the age of fossils more than 30,000 years old due to very less ativity of 14C after this much time.