It has long been observed that different types of ionising radiation give various product yields in the radiolysis of water. However, the detailed mechanisms for such observations still remain elusive. All ionising particles are assumed to result in the initial net decomposition of about 5.6 water molecules/100 eV of energy absorbed . The observed variation in product yields is due to the chemistry of the radicals and molecular products of water decomposition as the medium relaxes from the energy deposition. This chemistry occurs in the localised regions that make up the particle track, the geometry of which is determined by a number of particle characteristics including its charge, energy, linear energy transfer (LET), and other parameters. The exact structure of a particle track and its effect on the radiation chemistry is of central importance to a number of fundamental and practical problems. Many characteristics of the incident particle are responsible for the geometry and the subsequent chemistry in the tracks. Although it is well known that the LET does not uniquely determine product yields, it is the most commonly used parameter because it gives a general, but not an exact, representation of the concentration of reactive species in the particle track. Increasing the LET increases the intra-track reactions of the sibling radicals. Fewer radicals are available for causing radiation damage and the molecular product yields increase. However, the particle track is very dynamic. The non-homogeneous distribution produced by the initial energy deposition is constantly relaxing by diffusion and reaction of the water products. Measuring the time variation of radicals and molecular products in high-LET particle tracks is very challenging, but necessary to properly define the radiation chemistry. Here the variation in product yields in the radiolysis of water with helium ions is compared to that observed with γ rays or fast electrons. The time dependence of the yields in pure water is extracted from the observed scavenger concentration dependence.
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