Yields of single-strand breaks induced by60 Co ? or pulse irradiation in double-stranded calf thymus DNA have been measured in${\rm N}_{2}{\rm O}\text{-saturated}$ aqueous solution as a function of the concentration of tert-butanol. The yields were found to be dependent on dose rate. The experimental data were analyzed using a theoretical model based on non-homogeneous scavenging kinetics. It is concluded from this analysis that after60 Co ? irradiation in the absence of oxygen, aside from breaks caused by hydroxyl radicals, additional breaks occur which are initiated by hydrogen atoms and secondary radicals of tert-butanol. The efficiency of hydrogen atoms in causing single-strand breaks in double-stranded calf thymus DNA was determined to be 2.3%, while the rate constant for the reaction of tert-butanol radicals with DNA and their efficiency in causing single-strand breaks was determined to be$4.1\times 10^{3}\ {\rm dm}^{3}\ {\rm mol}^{-1}\ {\rm s}^{-1}$ and 2%, respectively.

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