La méthode des indicateurs radioactifs: principe, avantages et limites

Abstract

Radioactive substances have two generally quite distinct types of application according to whether they are used as radioactive tracers or as sources of ionising radiations. The tracer method is a general experimental technique which allows the evolution of a population of specific elements similar to each other to be observed by a process of labelling certain elements only. The indispensable conditions for the success of this method are discussed; the various ways of labelling are presented and compared. The advantages resulting from the the choice of isotopic radioactive tracers are such that the radioactive tracer method is of great interest in the most diverse fields of science, medicine and industry. The exceptional sensitiveness of this technique makes it particularly adapted for use in analytical chemistry and also for the study of the behaviour of traces.
The existence of isotopic effects constitutes a fundamental limitation of the radioactive tracer method which may be capable of compromising its results. The physical isotopic effects which are at the basis of all the others are first explained ; the different categories of chemical isotopic effects are then described and finally their incidence in radiochemistry is examined. The radiolytic action of radiation emitted by the radiotracers represents a second fundamental limitation to be considered by the radiochemist; the chemical transformations of the labelled molecules which occur at the very moment of disintegration constitute a third, not always negligible limitation; these phenomena are described and their importance evaluated.
Several experimental contingencies may also play a part in limiting the use of the radioactive tracer method; it is often essential to have available radio-isotopes of high specific activity; the radio-isotopic, radiochemical and chemical purity of the radioactive substances exercise an important influence. The choice of radio-isotope (half-life, nature and energy of the radiation emitted), when this is possible, can have a decisive influence on the success of the tracer method. Finally, the problem of radiation protection of the operators and the imperatives prescribed by it is touched upon.