Mechanism of the induction of DNA strand breaks by low energy radiation

Most energy of ionizing radiation transfers to many low energy secondary electrons，and then they produce most radiobiological effects.Vacuum-and far-UV photons provide an unique tool to control the energy absorbing processes，excitation and ionization。Monochromatic low energy electrons and photons，thus，are essential to elucidate mechanism of radiobiological effects，but，however，they are scarcely used in radiation biology because of lack of convenient sources.Our project is an international cooperation to elucidate mechanism of the induction of DNA strand breaks using two unique radiation sources：monochromatic low energy electrons(Gray Laboratory，UK)and monochromatic vacuum-UV photons(Photon Factory，Japan)。Major results on the strand break induction in plasmid DNA are as follows；(1)electrons as low as15Ev induced ssb and dsb linearly with the fluence，indicating that the threshold energy for the induction of ssb and dsb is below15Ev；experimental determination of the value for dsb is one of the major result of this project。(2)The ratio of dsb/ssb was about0.01，which was similar to that for I D 160锡D 1 Coγ-rays.(3)Low energy photons down to 4.3 Ev(The Lowest Energy Tested)induced ssb and dsb with very low quantum yield。This result indicated that the threshold energy was lower than 4.3Ev in this sense，but that defined usual was about10Ev.(4)We irradiated plasmid DNA in buffer solution by vacuum-UV photons using the method that was developed in the Gray Laboratory；monochromatic7-Ev photons induced ssb and dsb.(5)These threshold energies are crucially important to simulation calculation of models for DNA strand break。

Most energy of ionizing radiation transfers to many low energy secondary electrons, and then they produce most radiobiological effects. Vacuum-and far-UV photons provide an unique tool to control the energy absorbing processes, excitation and ionization. Monochromatic low energy electrons and photons, thus, are essential to elucidate mechanism of radiobiological effects, but, however, they are scarcely used in radiation biology because of lack of convenient sources. Our project is an international cooperation to elucidate mechanism of the induction of DNA strand breaks using two unique radiation sources : monochromatic low energy electrons (Gray Laboratory, UK) and monochromatic vacuum-UV photons (Photon Factory, Japan). Major results on the strand break induction in plasmid DNA are as follows ; (1) electrons as low as 15 Ev induced ssb and dsb linearly with the fluence, indicating that the threshold energy for the induction of ssb and dsb is below 15 Ev ; experimental determination of the value for dsb is one of the major result of this project. (2) The ratio of dsb / ssb was about 0.01, which was similar to that for ィイD160ィエD1Co γ-rays. (3) Low energy photons down to 4.3 Ev (the lowest energy tested) induced ssb and dsb with very low quantum yield. This result indicated that the threshold energy was lower than 4.3 Ev in this sense, but that defined usual was about 10 Ev. (4) We irradiated plasmid DNA in buffer solution by vacuum-UV photons using the method that was developed in the Gray Laboratory ; monochromatic 7-Ev photons induced ssb and dsb. (5) These threshold energies are crucially important to simulation calculation of models for DNA strand break.