ブラックピーク近傍の炭素イオン線によるイイン固有DNA損傷の研究

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We determined the action cross-sections for the induction of single- (ssb) and double-strand breaks (dsb) in plasmid DNA and lethal effect in T1 phage after irradiation with low energy He, C,Ar, Fe and Xe ion beams at different positions along with the Bragg curve. We developed the new system for the irradiation of ions around the Bragg peak at the Medium-Energy Beam Course at the NIRS (Chiba). Ions with 6 MeV/n were extracted into air through a thin metal foil, and their energy loss was measured by an ionization chamber in air at different distances form the exit window ; the Bragg peak was observed at 136 mm for Cions. The fluence was determined with the CR39 solid state track detector. Results are as follows : (1) Dry DNA : the cross-sections for ssb and dsb induction were determined after irradiation with He, C,Ar, Fe and Xe ions : those increased from He to Fe and then decreased to Xe. The cross-sections decreased from the Bragg peak to the track end more rapidly for ssb than dsb. The dsb/ssb ratio increased toward the track end and the maximum value was 1.7, demonstrating that dsb was induced more than ssb. (2) DNA in solution : using a new method making a 10-μm thin films of agarose gel containing plasmid DNA, DNA in solution was irradiated with Cions, which have a very short range of the order of 100μm in water. Results were similar to those for dry DNA.The maximum value of the dsb/ssb ratio was 2.0. (3) Dry T1 phage : the cross-sections for T1 phage inactivation were determined after C,Ar and Xe ion irradiation. The cross-sections monotonously decreased toward the ion track end. The RBE decreased with the increase in LET : RBE was 0.036 at the highest LET, 8783keV/μm (Xe).