The normal activity of living carbon containing matter is found to be about 15 decays /min for every gram of carbon . This activity arises from the small proportion of radioactive $_6C^{14}$ present with the ordinary $_6C^{12}$ 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 $_6C^{14}$ and the measured activity , the age of the specimen can be approximately estimated the approximate age of the Indus valley civilization
