Abstract
B. subtilis spores, E. coli BB-1 and E. coli B/r were bombarded with α-particles and heavy ions of carbon, nitrogen and oxygen accelerated in the IPCR Cyclotron. The RBE versus LET∞ curve for B. subtilis spores showed a maximum peak at ?? 120 keV/μm, while those for E. coli BB-1, and E. coli B/r declined without any maximum as LET∞ values increased. In the region of α-particles, the effective inactivation cross section (Seff) for these three strains increased with increasing LET∞ and the rates of increase in Seff in the LET region from ?? 30 to ?? 150 keV/μm were 15.0, 1.5 and 2.5 times for B. subtilis spores, E. coli BB-1, and E. coli B/r, respectively. In the case of B. subtilis spores, Seff values for heavy ions were almost independent of their energies, but the other two strains showed a considerable dependence upon beam energy. The charcteristic LET dependence of Seff observed in this study was fairly well explained by the target theory based on microdose concept.
