Relative Frequencies of Ion Clusters Containing Various Numbers of Ion Pairs

Abstract

Along the path of an ionizing particle, secondary electrons are set free in the primary ionization acts. Some of these electrons possess insufficient energy to produce ionizations of their own account. In such cases the primary ionization usually leads to the formation of a single ion pair. Frequently, however, a primary ionization is accompanied by one or more subsidiary ionizations. We may say, therefore, that each primary ionization is signified by an ion cluster containing one or more ion pairs. The largest ion clusters are indistinguishable from short 8-rays. In very densely ionizing radiations the successive clusters overlap to form a column of ions rather than a series of discrete clusters. The number of ion pairs in an ion cluster is roughly proportional to the energy transferred from the primary particle and dissipated in the region of formation of the cluster. The question of the frequencies of occurrence of clusters containing different numbers of ion pairs is, therefore, closely related to that of the spatial distribution of radiation-induced activations. The relative frequencies of small clusters have been invoked in various attempts to analyze radiobiological phenomena. This holds for certain applications of the direct-action hypothesis, in particular as regards radiation damage to biological macromolecules (1, 2), as well as the more recent track-segment hypothesis (3, 4).1 The values which have been assumed for the relative probabilities of occurrence of small clusters containing various numbers of ion pairs were reported in 1923 by Wilson (9), as well as rough averages of his values and those observed more recently by Beekman (10). The two sets of experimental values show considerable mutual discrepancies, however, which introduce uncertainties in the quantitative analyses into which the relative frequencies of different kinds of ion cluster enter.