There are cases when more than one radionuclide is present at the same time. The radioactivity, as well as the activity— time function, depends on all the radionu­clides that are present. The identification of each nuclide requires the mathematical decomposition of the activity—time function into components. Then, the identifica­tion of the radionuclides present can be done on the basis of the type of decay, the energy, and the half-life of the emitted particles.

When the decays of the radioactive nuclei present are independent, the radioac­tivities of the mixed nuclides are the sum of the radioactivities of all nuclides. Consequently, the activity—time function cannot be described by the kinetic of the simple radioactive decay (Eq. (4.8)). This means that the radioactivity—time curve must be decomposed. In principle, the decomposition could be done easily by using computing techniques. However, since the functions have to be fitted to the experi­mental activity—time function, it has some limitations; for example, the activity/ intensity and the half-life of the individual isotopes can be determined only by the decomposition of the activity/intensity—time function if there is at least one order of magnitude difference in the half-lives, and if the isotope mixture does contain only a limited number of different radioactive isotopes. If these conditions are not met, adding or neglecting additional nuclides does not improve the accuracy of the mathematical decomposition of the activity—time function.