Further Data on DNA Strand Breakage by Various Radiation Qualities

Abstract

SummaryThin films of dry DNA (∼ 30 per cent water content) from T7 bacteriophage were irradiated by charged particles with track segment LET∞ values up to 255 keV/µm. Mean numbers of single- and double-strand breaks per molecule were determined by boundary sedimentation in an analytical ultracentrifuge. For both types of break, the number per unit dose increased only slightly up to about LET∞ = 100 keV/µm, but more rapidly beyond that point; the ratio of the numbers of the two types of breaks did not change systematically throughout the whole range of LET∞. The data have been analysed to deduce the separate contributions of the primary track cores and the delta-rays to total dose and to total single- or double-strand breakage. Breakage by the primary cores per unit dose from cores begins to increase markedly above a core LET100 around 50 keV/µm. The increase could be explained by effective cooperation between two or more events of small size occurring together at high LET but which would be rather ineffec...