Considering the characteristics of nuclear data, the time needed for the investiga­tions (several days) should match the half-life of the radionuclides. For measure­ments outside the equipment wall, gamma-emitting radionuclides with relatively high energy (>300 keV) are suitable. Radionuclides emitting a high number of gamma quantums per decay are advantageous; because of their higher count rate, lower activity is necessary for the investigations (see the role of the a factor in Eq. (11.3)). Certainly an important consideration when selecting the radionuclide is simplicity of its preparation, which should take place in the research reactor through the (n, Y) nuclear reaction, favorably with a high activity yield (as discussed in Section 8.5.2).

In addition to nuclear data, physical and chemical features will determine which radionuclides can be selected for a given tracer study. Radionuclides most fre­quently used for industrial tracer studies are summarized in Table 11.1.

In the preparatory phase of the tracer study, at the preparation of the radionu­clide, and during the investigation, rules governing the handling of radioisotopes must be complied with. The fate of the radioactive isotopes used for industrial tracer studies is important to define in advance and solve in an authorized manner.

The simplest way is to store the material labeled with relatively short-lived radionuclides in a well-separated place until its radioactivity decays below the

exempted activity level. Storing time can be considerably reduced if the labeled material is diluted during the technological processes or is artificially diluted after the study. In such cases, the exempted radioactive concentration will be the precon­dition of the release. For instance, the dilution rate of a radioactive-labeled compo­nent that is introduced into a huge storing container with a great volume of nonradioactive material can even grant an exemption from separated storage.