Model of Tracks of Ionizing Radiations for Radical Reaction Mechanisms

The string of beads model for the track of an ionizing particle as proposed by Samuel and Magee (1) has, in the past, been widely used for the discussion of diffusion-controlled reactions of radicals in water and aqueous solutions. In this model radicals are supposed to be formed in spherical spurs, each having a fixed amount of energy deposit (-40 ev).2 The fixed spacing between adjacent spurs is made to agree with the LET of the ionizing particle. Sibling radicals from a spur diffuse throughout the medium, mingling with radicals from other spurs and solute; at all times they have finite probabilities of reaction, expressed by appropriate rate constants. Ganguly and Magee (2) extended the foregoing idea by explicitly recognizing the slowing down of the ionizing particle and the variation of the LET along the track. In the latter model, although the energy deposit (and hence the number of radicals) is the same in each spur, their spacing is random. The average spacing is such that the local LET turns out to be correct for the residual energy of the particle in that specific part of the track. Figures 1 and 2 show track pictures according to Samuel and Magee and according to Ganguly and Magee. The modified string of beads model introduced by Ganguly and Magee was used in explanation of yields due to radical recombination and scavenger reaction in the presence of a solute. The theoretical results agreed with experimental determinations except perhaps at very high solute concentration. Notwithstanding the success of the string of beads models on the whole, important questions remain unresolved as to details. For example, it is easily understood that constancy of spur size is an idealization; in reality, there must be a dis-

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