Class 12 Physics Modern Physics Nuclei



The normal activity of living carbon-containing matter is found to be about 15 decays per minute for every gram of carbon. This activity arises from the small proportion of radioactive  present with the stable carbon isotope . When the organism is dead, its interaction with the atmosphere (which maintains the above equilibrium activity) ceases and its activity begins to drop. From the known half-life ( 5730 years) of , and the measured activity, the age of the specimen can be approximately estimated. This is the principle of  dating used in archaeology. Suppose a specimen from Mohenjodaro gives an activity of 9 decays per minute per gram of carbon. Estimate the approximate age of the Indus-Valley civilisation.
Solution: Suppose that the number of atoms per gram where at (initially) when its activity was decays per minute per gram. Today ie., after time (Age of once living organism), the number of C-14 atom per gram left is and shows an activity of 9 decays per minute per gram. Now, activity of a radioactive substance is given by




Now decay constant


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