2 TYPES OF DOUBLE-STRAND BREAKS IN ELECTRON AND PHOTON TRACKS AND THEIR RELATION TO EXCHANGE TYPE CHROMOSOME-ABERRATIONS

Yields of DNA double-strand breaks (dsb), i.e. the average number of d sb, (N) over bar, per relative molar mass, M(r)., and dose, D, produce d by electrons and photons in the energy range 50 eV-1 MeV were calcul ated. The experimental data of dsb induction by ultrasoft x-rays and b y photons agree well with the calculated yields of dsb as a function o f photon energy. The dsb are classified into simple and complex ones. Energy transfers of less than about 200 eV producing at least two ioni zations generate mainly simple dsb, while low-energy electrons with an initial energy between 200 and 500 eV induce preferentially complex d sb. Assuming that dsb is the main DNA lesion leading to exchange-type chromosome aberrations (etca), three different mechanisms have to be c onsidered: 1) complex dsb on its own; 2) interaction between two dsb i nduced by the same primary particle; and 3) interaction between two ds b induced by different primary particles. Mechanisms 1) and 2) produce a linear term, whereas mechanism 3) leads to a quadratic term for the yield of etca. The sum of contributions I) and 2) to the yield of dic entrics describes fairly well the non-trivial structure of the experim ental data. The results suggest that interaction between complex dsb d oes not contribute significantly to the formation of dicentrics via me chanism 3).