A Concise History of Science in India c9.s2.3

c9.s2.3

b Carbon-14 (C¹⁴) is a radioactive isotope of carbon (or radiocarbon) with an atomic weight of 14, the normal atomic weight of carbon being 12. The radiocarbon is being produced in nature as a result of the interaction of the fast-moving neutrons (which are themselves produced from primary cosmic rays on entering the earth’s atmosphere) on N¹⁴ atoms of the air. The radiocarbon combines with oxygen forming gaseous carbondioxide (C¹⁴O₂) and the latter gets into the vegetation as a result of plant metabolism and also into the animals which feed on vegetation. When carbon-14 disintegrates, nitrogen is formed and beta particle is emitted. In 5570 ± 40 years, known as the half-life period of carbon-14, half of a given number of atoms of the radiocarbon undergoes radioactive decay and, with the lapse of time, the radioactivity goes on decreasing. It has been calculated that in about 70,000 years, practically all its radioactivity will be lost. The quantity of radiocarbon in contemporary materials is too small and far below the sensitivity limit of an isotope-measurement device such as the mass spectrograph. However, its radioactivity can be measured and, by comparing the radioactivity of a particular carbon remnant (like plant remains, charcoal, marine shells and carbonate deposits) with that of carbon of the present time, the time elapsed in relation to the carbon remnant can be arrived at mathematically. It is generally assumed that all living organisms possess the same proportion of carbon-14/carbon-12 and that the radiocarbon disintegrates at the same rate after their death. These assumptions are not found to be strictly correct and hence a certain amount of error creeps in the reckoning of dates by the radiocarbon method. (For details *see* McGraw-Hill Encyclopedia of Science and Technology, 1960, Vol. II, pp. 291–96).