Abstract
It is suggested that primary radiation damage to a macro-molecule in a cell which may lead to observable biological effect is normally the direct effect of a single energy-loss event in the marco-molecule. This concept is applied to the production of chromosome aberrations, regarding a local region of a chromosome as a molecule of DNA, and assuming that an aberration results from an exchange between 2 damaged regions. The model is consistent with the observed dependence aberration yield on dose, LET [Linear energy transfer] and modifying agents. Over a considerable range of LET, effectiveness per unit dose is proportional to dose-average LET. Similar basic concepts appear to be applicable to cell killing.