Effectiveness of 0·3 keV Carbon Ultrasoft X-rays for the Inactivation and Mutation of Cultured Mammalian Cells

Abstract

Carbon K characteristic ultrasoft X-rays of energy 0·278 keV were found to be effective in inducing inactivation and mutation to thioguanine resistance in cultured V79 Chinese hamster cells and human diploid fibroblasts. These X-rays act as a probe of the sensitive sites within the cells since they produce low-energy photoelectron tracks of range about 7 nm; this is an order of magnitude smaller than those produced by the 1·5 keV aluminium X-rays used in previous studies. A detailed interpretation of the results requires assumptions to be made about the positions of the sensitive sites within the cells but, for any reasonable set of assumptions, the carbon X-rays are found to be more effective than γ-rays and are probably at least as effective as long tracks of helium ions of similar LET. These observations extend the conclusions previously drawn from the observed effectiveness of aluminium X-rays regarding the sizes of the subcellular sites involved in inactivation and mutation. They imply that the sensitive sites are smaller than about 7 nm, and that highly localized energy depositions consisting of ≲ 14 ionizations are sufficient to produce biological effects. These results are also in contradiction to models of radiation action which require relatively large sites, such as the usual form of the ‘theory of dual radiation action’.