Radioactive isotopes and their uses

Examples 1 C, 1 C and gC are three of the isotopes of carbon. About 98.9% of naturally occurring carbon is in the stable JC form. 1 is also a stable isotope but it only occurs at 1.1% natural abundance. Trace amounts of radioactive 1gC are, found naturally this Is a negatron-emitting radioisotope (see Table 35.2). 

The decay of,22Na by positron emission S ) leads to the production of a -ray when the positron is annihilated on 

The isotopes of a particular element have the same number of protons in the nucleus but a different number of neutrons, giving them the same proton number (atomic number) but a different nucleon number (mass number, i.e. number of protons + number of neutrons). Isotopes may be stable or radioactive. Radioactive isotopes (radioisotopes) disintegrate spontaneously at random to yield radiation and a decay product. 

There are three forms of radioactivity (Table 35.1) arising from three main 

Radiation Range of maximum energies (MeV ) Penetration range in air (m) Suitable shielding material 

Note from the above that more than one type of radiation may be emitted when a radioisotope decays. The main radioisotopes used in chemistry and their properties are listed in Table 35.2. 

Each radioactive particle or ray carries energy, usually measured in electron volts (eV). The particles or rays emitted by a particular radioisotope 

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Equation does not contain the concentration of A, so the half-life of a first-order reaction is a constant that is independent of how much A is present. The decomposition reactions of radioactive isotopes provide excellent examples of first-order processes, as Example illustrates. We describe the use of radioactive isotopes and their half-lives to determine the age of an object in more detail in Chapter 22. 

Even nowadays the application of radioactive isotopes is the most sensitive method for the analysis of biomolecules or their reaction products. Besides the low detection limits, the replacement of a naturally overbalancing stable isotope by its radioactive analogue does not interfere with the physical or chemical properties of the enzyme (with some exceptions for hydrogens). Figure 6 lists some frequently used radioactive isotopes and their half-life periods. 

The use of stable isotopes in the elucidation of pathways of biosynthesis is char-actised by two particular features. In many cases, NMR, and in particular C-NMR, has been the method of isotope detection employed. Furthermore, in many cases the use of radioactive isotopes and their ease of detection has been combined with the structural dehnition achievable by the use of stable isotopes in conjunction with NMR. Although much of the reported work has utilised carbon-13 in combination with carbon-14 labelling, there are, nevertheless many instances of the use of nitrogen-15 and oxygen-18 in the study of biosynthetic pathways. 

Chapters 16 (waste disposal). Chapter 17 (radioactive materials transportation), and Chapter 18 (decontamination and decommissioning) are related to many radioactive processes and materials. Radioactive waste is generated wherever radioactive materials are handled and used. This includes medical applications of radioactive isotopes and their production, as well as the facilities and processes involved in nuclear power. This waste must be stored and/or disposed of in a way that isolates it from the environment until the radioactive materials decay. 

The selective uptake of iodide ion by the thyroid gland is the basis of radioiodine treatment in hyperthyroidism, mainly with although various other radioactive isotopes ate also used (40,41). With a half-life of eight days, the decay of this isotope produces high energy P-particles which cause selective destmction within a 2 mm sphere of their origin. The y-rays also emitted are not absorbed by the thyroid tissue and are employed for external scanning. 

The decay of long-lived radioactive isotopes was initially used by geochemists exclusively for the measurement of geologic time. As noted in the introduction, their use as tracers of mantle processes was pioneered by Hurley and co-workers in the early 1960s. The decay... 

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