Ion-pair clusters and excited-state yields in radiolysis

Abstract

WHEN β or γ rays are absorbed in matter, sparse tracks of ion pairs are produced: the secondary electrons sometimes have enough energy to produce further ion pairs in small clusters. Distributions of cluster size in a cloud chamber were measured by Wilson as long ago as 1923 (ref. 1); reinterpretation² of his and other results shows that nearly two thirds of the ion pairs occur in clusters. Similar behaviour should occur in other phases, but little is yet known of the role of clusters in radiolysis. In hydrocarbons, the geminate recombination of radical ions is a major process: most theoretical descriptions have dealt only with single ion pairs³. Since most of the ion pairs recombine very quickly, the clusters will rapidly become single pairs, so the approximation is probably quite good for scavenging studies. If we consider recombination products, especially the yields of singlet and triplet excited states, cluster formation must be taken into account⁴.