Physics XII Notes for Physics Notes

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Chain Reaction

Chain reaction is also a nuclear reaction in which once the fission started until and unless all the fissionable nuclei are present. During chain reaction, large amount energy is released. In fig, when a neutron is bombarded with a uranium it releases 3 neutron. These neutron are further utilized for bombardment of uranium. The process goes on continuously until and unless all the uranium nuclei undergo fission as shown in the figure. The uncontrolled fission of chain reaction is the principle of atom bomb.
In atmosphere, large no. of mixture of gases are in continuous motion. Among them nitrogen is one of the major gas. When nitrogen combines with neutron, it gives radioactive isot of carbon (6C14) according to the following equation:


Radioactive isotopes of carbon are slightly soluble in atmospheric air. As a result it reaches in plants during photosynthesis (with CO2). Besides that is also reaches to animal’s body during respiration is well but in animals 6C14 reaches through the food which are obtained from plants. In this way, radioactive isotope reaches in plants and animal’s body . This isotope goes on continuous accumulation until the death of plants and animals i.e. throughout their life. After the death of plants and accumulation of 6C14 stops and slowly it disintegrate as the half life of 6C14 is 5600 years, by studying the activity at any instant of time. Age of plants and animals after their death can be estimated. This is known as carbon dating. The most important thing is carbon dating is that it requires carbon containing material (sample contain from plant and animal only).

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