Radiocarbon half-life

Radiocarbon dating (43) has probably gained the widest general recognition (see Radioisotopes). Developed in the late 1940s, it depends on the formation of the radioactive isotope and its decay, with a half-life of 5730 yr. After forms in the upper stratosphere through nuclear reactions of... 

The constant half-life of a nuclide is used to determine the ages of archaeological artifacts. In isotopic dating, we measure the activity of the radioactive isotopes that they contain. Isotopes used for dating objects include uranium-238, potassium-40, and tritium. However, the most important example is radiocarbon dating, which uses the decay of carbon-14, for which the half-life is 5730 a. 

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar also create variations in atmospheric content activity (Stuiver and Quay, 1980, 1981). Geologically important reservoirs (i.e., carbonate rocks and fossil carbon) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half-life. The distribution of is used in studies of ocean circulation, soil sciences, and studies of the terrestrial biosphere. 

The half-life (t1 ) of a radioisotope is the amount of time it takes for that isotope to undergo radioactive decay and be converted into another. It is also a measure of the stability of the isotope the shorter its half-life, the less stable the isotope. The half-life of radioisotopes ranges from fractions of a second for the most unstable to billions of years for isotopes that are only weakly radioactive. In the case of radiocarbon (carbon-14), for example, the half-life is 5730 years (see Fig. 61). 

The decay of radiocarbon (see Fig. 61) into nitrogen-14 proceeds at a constant rate, and its half-life is 5730 + 40 years (see Textbox 14). This means that in any material containing carbon, some radiocarbon atoms disintegrate before 5730 years have elapsed and others later after 5730 + 40 years have elapsed, however, only half of the original atoms of the carbon-14... 

FIGURE 61 The decay of radiocarbon. Radiocarbon is a radioactive isotope whose half-life is 5730 + 40 years. This means that half of the original amount of radiocarbon in any carbon-containing sample will have disintegrated after 5730 years. Half of the remaining radiocarbon will have disintegrated after 11,400 years, and so forth. After about 50,000 years the amount of radiocarbon remaining in any sample is so small that older remains cannot be dated reliably. 

The half-life of radiocarbon used to calculate radiocarbon dates is 5568 years, a value known to be about 3% in error with respect to the actual half-life of radiocarbon, 5730 years. This is done to avoid confusion... 

One of the limitations of radiocarbon dating artifacts is due to the half-life of the carbon-14, 5730 years. In radiochemistry, a good rule of thumb is the following when an element decays for more than about 10 times its half-life, there is very little left to measure accurately. In the case of C-14, that time is 10 x 5730 yr or 57300 years. 

Radiocarbon The radioactive isotope of C, which has a half-life of approximately 5730 y. 

There is a difference between the thermodynamic terms stable and unstable and the kinetic terms labile and inert. Furthermore, the differences between the terms stable and unstable, and labile and inert are relative. Thus, Ni(CN)4 and Cr(CN)6 are both thermodynamically stable in aqueous solution, yet kinetically the rate of exchange of radiocarbon-labeled cyanide is quite different. The half-life for exchange is about 30 sec for the nickel complex and 1 month for the chromium complex. Taube has suggested that those complexes that react completely within about 60 sec at 25°C be considered labile, while those that take a longer time be called inert. This rule of thumb is often given in texts, but is not in general use in the literature. Actual rates and conditions are superior tools for the evaluation of the kinetic and thermodynamic stability of complexes. 

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