In this report, a quantitative interpretation of mutation induction cross sections by heavy charged particles in bacterial cells is presented. The approach is based on the calculation of the fraction of energy deposited by indirect hits in the sensitive structure. In these events the particle does not pass through the sensitive volume, but this region is hit by δ rays. Four track structure models, developed by Katz (in Quantitative Mathematical Models in Radiation Biology, pp. 57-83, Springer-Verlag, 1988), Chatterjee et al. (Radiat. Res. 54, 479-494, 1973), Kiefer and Straaten (Phys. Med. Biol. 31, 1201-1209, 1982) and Kudryashov et al. (Proceedings of the First Soviet Congress on Microdosimetry, Atomizdat, Moscow, 1973), respectively, were used for the calculations. With the latter two models, very good agreement of the calculations with experimental results on mutagenesis in bacteria was obtained. Depending on the linear energy transfer (${\rm LET}_{\infty}$) of the particles, two different modes of mutagenic action of heavy ions are distinguished: "δ-ray mutagenesis," which is related to those radiation qualities that preferentially kill the cells in direct hits (${\rm LET}_{\infty}$ ≥ 100 keV/μm), and "track core mutagenesis," which arises from direct hits and is observed for lighter ions or ions with high energy (${\rm LET}_{\infty}$ ≤ 100 keV/μm).

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