Tracks in condensed systems

Track structure is the spatial and temporal organization of atomic and molecular events that result from the interaction of fast charged particles with matters. The physical explanation of tracks according to (1) point heat theory, (2) target experiment and (3) the observation that a single particle can kill a living cell was developed. The chemical explanation is based on the works of the decomposition of water, and on the studies of biochemistry and biology, influenced by free radicals. Systematic understanding of the effects of track structure may be obtained by correlating the physical events of energy deposition with the chemical and biochemical changes. Wide range of linear energy transfer (LET) values may help understanding of the gross features of the track effects. The molecular action of the tracks of particles of intermediate LET usually decreases with increasing LET. This is true for the inactivation of protein enzymes and for the yield of free radicals measured by EPR techniques in irradiated dry enzyme preparation. In the high LET irradiation of crystalline and amorphous solids, the effects different qualitatively from those at intermediate LET are observed. There is current biological interest in the thermophysical effects in heavy particle tracks. The lesions in mammalian retinas showed similar appearance to the lesions in plastics or in mica. The retinal rods appeared to be pushed aside to produce tunnel lesions. These lesions were produced by the wholebody exposure of mice to the low dose of accelerated iron particles of about 500 MeV/amu; these particles have a LET value of more than 20 eV/A in tissues. (Yamashita, S.)