Effects of early track structure on the radiation chemistry of water irradiated with heavy ions

Abstract

The ways in which the physical structure of heavy ion tracks influences the observed radiation chemistry of water are reviewed. Hydrated electrons are produced in the same initial yield as in low LET radiolysis, but they disappear more rapidly during diffusion of the denser tracks. LET increases with increased charge and with decreased ion velocity, but radii of the core and penumbra regions of the track depend only on ion velocity. Consequently, radicals can be produced at very different spatial densities with ions of similar LET. Homogeneous radical, molecular and initial total water decomposition yields reflect these differences. The HO 2 radical is produced in significant yield only in tracks, but unlike e - aq and OH its yield appears to be a unique function of core LET. Radical scavengers increase the effective radical yields by competing with recombination processes, most efficiently in the denser tracks. Nuclear fragmentation is expected to increase radical yields measured at depth in thick targets irradiated with high energy heavy ions, by contaminating the beam with secondaries that have less dense tracks. Recent evidence indicates that this effect may not be as large as had been thought previously.