This technique is used for the determination of the age of a plant or an animal died millions of years ago and was developed in 1960 by an American chemist Prof. Willard F.Libby and he was awarded Noble Prize for developing this technique. This technique involves the determination of radioactivity in radioactive carbon (C-14) which is an essential constituent of Plants and animals which they acquire in the following way.
The nitrogen present in the upper atmosphere is bombarded by neutrons produced by cosmic rays of the sun as a result of which some of nitrogen atoms change into radioactive carbon i.e.
7N14 + 0n1 —————-> 6C14 + 1H1
This radioactive carbon combines with atmospheric oxygen forming radioactive carbon dioxide which mixes with non-radioactive CO2 present in the atmosphere and is being continuously absorbed by Plants during metabolism. Since animals eat plants, it ultimately reaches their system and when an animal or plant dies, the intake of radioactive carbon (C-14) comes to stop and C-14 already present in it also starts decaying i.e.
6C14 —————–> 7N14 + -1e0 ; t1/2 = 5760 years
By comparing the C-14 content in the dead plant or animals with that of the living one in the same region, the age of the Plant or animal can be determined. Since half-life period of C-14 is 5760 years, which means that after 5760 years the concentration of C-14 reduces to 50% and if concentration reduces to 25% then 11520 years have been passed since the object died.