Inactivation Cross Sections of Yeast and Bacteria Exposed to Heavy Ions of Low Energy (less than 600 keV/u)

Abstract A haploid yeast mutant of Saccharomyces cerevisiae and spores of Bacillus subtilis were exposed to heavy ions with energies of maximal 600 keV.u-1 down to energies associated with ranges which are just high enough to penetrate yeast cells and bacteria. The experiments were performed at the UNILAC at GSI (Darmstadt, FRG). The primary energy of 1.4 MeV.u-1 was reduced by a titanium window and by Hostaphan foils of different thickness. Generally the inactivation cross section decreases when the velocity of the primary particle is reduced due to the shrinking dimensions of the d electron penumbra. The results of the experiments with Kr, Pb and U ions demonstrate that the inactivation cross sections for yeast as well as for bacterial spores do not change significantly when the energy is reduced stepwise below 600 keV.u-1. Close to the end of the range of the ions, however, in all cases an increase of 10 to 30% in total is observed. These results indicate that at the very end of the range of the particles, i.e. at particle energies of around 100 keV.u-1, other interactions than electronic stopping may play a more important role.