As a result of their metabolism, living organisms can uptake natural and artificial radioactive isotopes from the environment. The degree of uptake depends both on the radioactive isotope/ion and on the living organisms, and it is characterized by so-called discrimination factors (DFs) or observed ratios (ORs). The use of these factors is based on the similar biological properties of potassium and 137Cs ions, or calcium and 90Sr ions, respectively, and indicates how different living organisms or soil can accumulate the radioactive isotopes. For example, the discrimination of

Sr-90 can be expressed as Sr 90, where NSr-90 is the activity of 90Sr (for example)

NCa

in a 1 g sample and NCa is the quantity of calcium ion in a 1 g sample.

The uptake of these radioactive isotopes (137Cs and 90Sr) is expressed by the transfer factors (TFs), which form the ratio of the DFs. For example, the TFs for

The activity of radioactive isotopes in living organisms decreases in two ways: by radioactive decay and biological secretion. The radioactive decay and secretion are characterized by the physical half-life of the radioactive isotope (f1/2fiz) and the biological half-life of the isotope in the living organism (t1/2biol). The net effect of the radioactive decay and secretion is expressed by the effective half-life (f1/2eff):

1

1 1

— 1

f1/2eff f1/2fiz f1/2biol

The physical, biological, and effective half-lives of several radioactive isotopes are shown in Table 13.4.

As seen in Table 13.4, the physical and biological half-lives range from a couple of days to thousands of years. The effective half-life is determined by the shorter of these half-lives. For example, the physical half-life of 137Cs is rather long (30 years), but the biological half-life is only 17 days; thus, the effective half-life is 17 days.