[en] In order to be able to calculate the probability that a specific type of radiation induces a DNA double strand break in one radiation event it is necessary to have information on the spatial distribution of the primary events occurring in an ionizing track. Because there are several indications that the radiation induced OH radicals are mainly responsible for DNA damage, an attempt has been made to derive an equation for the stopping power of water which incorporates a measure of the track dimension and which covers the whole range of electron and ion velocities of interest. The excitation spectrum of Platzman and the known electron ionization potentials in water have been used to calculate the degradation spectrum for a specific type of radiation to obtain the relative contributions of the different ionizing particles, generated by the primary radiation, to the total dose. Using the degradation spectra together with the stopping power equation the mean response of a one-hit water equivalent detector has been calculated for different radiation types. A comparison is made between the experimental data on the dependence of the induction of DNA strand breaks on radiation quality and that calculated using this track model taking into account the structure of the DNA target