Energy deposition clusters in the nanometer scale have been investigated systematically in electron, proton, and α-particle tracks. The mean absolute frequency distributions of ionization clusters in nanometer sites have been computed. For locally multiply damaged sites and especially for double-strand breaks, the threshold numbers of ionizations in a site of a given diameter necessary for their induction have been estimated. At least three to six ionizations localized in sites of 1 to 4 nm diameter, respectively, are required to produce the double-strand break.

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