Libby discovered that the radioactive isotope of carbon, 14C isotope, is formed from the nitrogen that is present in air under the effect of neutrons from the cosmic ray. The nuclear reaction is 14N(n, p)14C. (Nuclear reactions will be discussed in Chapter 6.) If the flux of the neutrons is assumed to be constant, the formation and subsequent decay of 14C result in a constant concentration of 14C. Since the living organisms continuously incorporate 14C of the carbon dioxide in the air, the 14C concentration of the living organism (i. e., the ratio of 14C/12C) is the same. When the living organism dies, the continuous uptake of 14C ends, and only the radioac­tive decay of 14C continues. Thus, the concentration and the radioactivity of 14C decrease. From the 14C/12C ratio, the time elapsed from the death of the living organism can be estimated. In radiocarbon dating, 5570 years is traditionally used as the half-life of 14C (the actual half-life is 5736 years). This means that dating is feasible if the living organism lived between 250 and 35,000 years ago. The activ­ity of 14C in the carbon dioxide of the air and the living organisms is 16 dpm/g car­bon. However, more accurate results can be obtained using modern mass spectrometry equipments to determine the 14C/12C isotope ratio.

An interesting application of the 14C/12C ratio of tooth enamel for the estimation of the age of individuals born after 1943 was published. Tooth enamel is formed at well-determined times of childhood and contains 0.4% carbon. After the formation, there is no exchange between the carbon in the enamel and the carbon dioxide in the air. Thus, the 14C/12C of the tooth enamel reflects the ratio in the air at the time of the enamel formation. The 14C/12C ratio in the air was nearly constant until 1955, when aboveground nuclear bomb tests raised it signifiantly. After the Limited Test Ban Treaty in 1963, atmospheric 14C began to drop exponentially,
and it did not return to the level before 1955 until recently. Thus, if the 14C/12C ratio of the tooth enamel is determined, the time of the enamel formation, and from here the age of the individuals, can be estimated.

Besides radiocarbon, tritium is also formed of nitrogen in the air and neutrons in the nuclear reactions 14N(n,3 4He)T and 14N(n, T)12C. Similar to 14C, tritium iso­topes can participate in continuous exchanges between the hydrogen in the air and living organisms. The half-life of tritium is 12.35 years, so it should be suitable for dating in the interval of 10—80 years (e. g., for the dating of wine in bottles). However, thermonuclear explosions in the atmosphere significantly increased the natural tritium concentration, so the dating on the basis of tritium concentration has become quite limited. Tritium activity can be used for the dating of glacier and polar ice in layers, however, because in these cases, the effect of the nuclear explo­sions is neglible.