A Simplified Approach to Diffusion-Controlled Radical Reactions in the Tracks of Ionizing Radiations

From quite elementary considerations, the present authors have concluded (1, 2) that the track generated by an ionizing particle in the low over-all LET region may be considered as composed of a random sequence of three types of entities created along the path of the primary particle. We have called the entities spurs, blobs, and short tracks. The ratios in which they are found are strongly dependent on the primary energy. Yields have been tabulated over a wide range of electron energies in water (1). The purpose of the present paper is to discuss diffusion-controlled radical reactions based on the resulting track model and to compare them with existing work based on previous models (3-6). No claim is made that the present treatment is superior to the standard treatments of diffusion kinetics available in the literature (especially reference 6) as far as a single entity is concerned; however, the necessity and the possibility of discussing all the different entities are demonstrated in a sufficiently simple framework. In the low over-all LET limit an obvious simplification is available because in no stage of the radical diffusion process do the radicals of one particular entity overlap with radicals originating from the other entities. In other words, every individual entity develops independently as its radicals react either with each other or with scavengers. In the approximation of the model, this implies that certain spurs (and/or blobs) overlap right from the beginning (as do those comprising the short tracks), but others remain essentially isolated. Since radical reactions are practically complete by the time entities grow one order of magnitude in linear dimension, this approximation appears to be a useful one. Even the length of a typical short track is an order of magnitude less than the average interspur distance (1).