FORMATION OF PLASMID DNA STRAND BREAKS INDUCED BY LOW-ENERGY ION-BEAM- INDICATION OF NUCLEAR STOPPING EFFECTS

Plasmid pGEM 3zf(+) was irradiated by nitrogen ion beam with energies between 20 and 100 keV and the fluence kept as 1 x 10(12) ions/cm(2). The irradiated plasmid was assayed by neutral electrophoresis and quan tified by densitometry. The yields on DNA with single-strand and doubl e-strand breaks first increased then decreased with increasing ion ene rgy, There was a maximal yield value in the range of 20-100 keV. The r elationship between DNA double-strand breaks (DSB) cross-section and l inear energy transfer (LET) also showed a peak-shaped distribution. To understand the physical process during DNA strand breaks, a Monte Car lo calculation code known as TRIM (Transport of Ions in Matter) was us ed to simulate energy losses due to nuclear stopping and to electronic stopping. It can be assumed that nuclear stopping plays a more import ant role in DNA strand breaks than electronic stopping in this energy range. The physical mechanisms of DNA strand breaks induced by a low-e nergy ion beam are also discussed.