Radiation chemistry of heavy-particle tracks. 1. General considerations

The radiation chemistry of heavy particle tracks in dilute aqueous solution is considered in a unified manner. Emphasis is on the physical and chemical phenomena which are involved rather than on the construction of models to be used in actual calculations although the latter problem is discussed. A differential segment of a heavy particle track is composed of two parts which we call 'core' and 'penumbra'; elementary considerations show that all properties of such a differential track can be uniquely specified in terms of a two-parameter system, and we choose energy per nucleon (E) and atomic number (Z) as independent parameters. The nature of heavy-particle track processes varies with the magnitude of the energy deposit (LET), and we discuss three categories of track problems, for low-, intermediate-, and high-LET cases, respectively. Scavenger reactions normally terminate radical recombination in a track, and for heavy particle tracks we find a criterion involving the scavenger concentration for a convenient separation of core and penumbra into essentially noninteracting parts which can be treated independently. Problems of the core expansion in the three regions are considered and it is found that a versatile model can be constructed on concepts previously introduced by Ganguly and Magee. A model for the penumbra, based on the authors electron track theory, is presented and discussed.