Radioactive Dating

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All radioactive decays follow first order kinetics. Therefore, the half-life of a radioactive element is independent of the amount of sample. With the help of half-life values of a suitable radioisotope of an element, which is present in a rock, or in an artefact, the age of the rock and the artefact can be determined. This is called radioactive dating.

Radioactive dating is used in determining the age of a dead tree or for that matter any dead organic matter. The isotope used is carbon-

Carbon-14 is present in atmosphere as a result of cosmic - ray is produced by the collision of a neutron with a nitrogen-14 nucleus.

Carbon-14 is unstable and decays by beta emissions to nitrogen. Because of the constant production of carbon-14 and its radioactive decay, a small fractional abundance of carbon-14 is maintained in the atmosphere.

Living plants, which use carbon dioxide from the atmosphere, also maintain a constant amount of carbon-14. However, once a plant is dead, the ratio of carbon-14 to carbon-12 starts decreasing by radioactive emission. Therefore, by meaning the radioactivity of carbon-14 in a wood, the age of the (approximate) can be determined.

Examples 10:

A sample of wood from the core of an ancient pine tree shows a carbon-14 content that is 53% of the atmospheric carbon-14. What is the approximate age of the tree?

Suggested answer:

It is assumed that level of carbon-14 in the atmosphere is constant. Since radioactive decay follows 1^st order kinetics, the ratio carbon-14 in the wood and the carbon-14 in the atmosphere is given by

= 4943 years

The age of the tree is 4943 years.