As mentioned previously, radioactive tracers can be used to determine the local, time, and concentration distribution of the substances, so they are appropriate to use for studying transport processes, including migration, diffusion, and self-diffusion. As discussed in Section 3.2, diffusion is a suitable means of separating isotopes, especially for the isotopes of the light elements. Even heavy isotopes, such as 235U and 238U, were successfully separated for military purposes by diffusion in the United States between 1945 and 1948.

In this chapter, the application of radioactive tracers will be discussed in some arbitrarily chosen solid/gas, solid/liquid, and solid/solid systems. In addition, self­diffusion will be investigated. In the diffusion studies, the application of radioactive isotopes is only one of the available methods, though it is one of the cheapest and most elegant. In the self-diffusion studies, however, the radiotracer method is the only simple possibility. In the diffusion studies, the mixing entropy is maximal; the process is directed by the concentration gradient. The specific activity is con­stant, meaning that the activity is proportional to the concentration. In the self­diffusion studies, the specific activity changes, and the process is directed by the increase in the mixing entropy.