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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Atoms Molecules and Nuclei Know More

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  • Radioactive Dating:
    If one knows the half life of a given radionuclide, it is possible to use the decay of that radionuclide as a clock to measure time intervals. The decay of long lived nuclides can be used to measure the age of rocks and to determine the time that has elapsed since they were formed. The age of the earth, the moon and meteorites have been measured by these methods and have been consistently found to be about 4.5 109 years.

    The radionuclide K40 decays to Ar40 with a half life of 1.25 109 years. Thus, a measurement of the ratio K40 to Ar40 found in a particular rock can be used to measure the age of that rock. Other radioactive disintegrations such as U235 to Pb207 can be used to verify this calculation.

    For measuring shorter time intervals, radiocarbon dating is used. The radionuclide C14 (having a half life of 5,730 years) is produced at a constant rate in the upper atmosphere due to bombardment of atmospheric nitrogen by cosmic rays. This carbon mixes with the carbon normally present in the atmosphere and there is about one C14 atom per 1013 atoms of stable C12. During biological activity such as photosynthesis or breathing, the atmospheric carbon exchanges places with the carbon atoms of living creatures. Eventually an exchange equilibrium is reached and the carbon atoms in every living creature contains a fixed (tiny) fraction of the radioactive C14. This equilibrium continues during the life of the organism. When the organism dies, the exchange with the atmosphere no longer exists and the radioactive carbon decays with a half life of 5,730 years. By measuring the amount of C14 per gram, it is possible to measure the time that has passed since the organism died. Many prehistoric artifacts have been dated by this method.

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