Experimental determination of the dependence of OH radical yield on photonenergy: A comparison with theoretical simulations

On the basis of simulations of water radiolysis, it has been postulated tha t the yield of OH radicals, which become homogeneously distributed, is depe ndent upon energy when irradiated with low-energy photons. The aim of this study is to determine experimentally the dependence of the yield of OH radi cals, which escape intratrack recombination, on photon energy of incident r adiation using plasmid DNA as a probe. The yields of single strand breaks ( ssb) induced in plasmid DNA (pUC18) when irradiated with photons varying in energy from 0.28 keV to 1.25 MeV was determined and, when normalized relat ive to the yield for Co-60 irradiation, may be used as a measure of the yie ld of OH radicals escaping radiation tracks. As the photon energy decreases from 1.25 MeV to 1.5 keV the OH radical yield decreases from 0.290 to 0.07 2 mu mol J(-1), respectively, in line with an increased ionization density of the radiation and hence an increasing probability of radical recombinati on. However, with a further decrease in photon energy from 1.5 to 0.28 keV there is an upturn in OH radical yields. Carbon-K X-rays are found to have a significantly higher yield of 0.257 mu mol J(-1) than that associated wit h the higher energies. The experimental dependence is compared with a numbe r of theoretical calculations, which predict an upturn in OH radical yields between 0.1 and 1 keV. Such a dependence of OH radical yields on energy pr ovides experimental data suitable for direct comparison with simulations, a iding their refinement and development.