RELATION BETWEEN TRACK STRUCTURE AND LET EFFECT ON FREE-RADICAL FORMATION FOR ION BEAM-IRRADIATED ALANINE DOSIMETER

The yield and local concentration of free radicals generated from alanine (alpha-aminopropionic acid) by irradiation with 3 MeV H+ and He+ ions were examined by means of electron spin resonance (ESR) and ESR power saturation methods at room temperature. The G-value of the radical formation showed a marked dependence on linear energy transfer (LET) of the ions. The G-value for the H+ ion (average LET: 28 eV/nm) was almost the same as that for gamma-irradiation and it was smaller by a factor of 1/4.7 for the He+ ion (average LET: 225 eV/nm). Combining the local concentration of the free radicals along the ion tracks with the G-values and the reported ion range, the radius of a track filled with free radicals was estimated to be 4 approximately 5 nm by assuming a simple rod-shaped track with a constant radius and homogeneous distribution of the free radicals in it. The track radius scarcely depends on the LET within the range examined. The radiation energy deposited in the core region of the ion track was concluded to spread over the rod to generate free radicals.