Heavy ion effects on yeast cells: reappearance of the oxygen effect with very high LET.

Survival of a diploid and a haploid wild type and a radiation-sensitive rad52-mutant was investigated after exposure to accelerated ions in the presence or absence of oxygen. Ar, Kr, Xe, Sm, Pb and U ions were used with specific energies between 0.3 and 12 MeV/u. The results demonstrate that the oxygen enhancement ratios (o.e.r.) do not only depend on LET or Z*2/beta 2 but even more so on ion specific energy. The o.e.r.s are always higher with greater E/m values pointing to the importance of delta-electron action.

Heavy ion action on yeast cells: inhibition of ribosomal-RNA synthesis, loss of colony forming ability and induction of mutants.

The action of heavy ions (Ar to U) accelerated to specific energies up to about 10 MeV/u (u=atomic mass unit) on different functions of yeast cells was studied. Ribosomal-RNA synthesis is inhibited according to a single-hit mechanism. Inactivation cross-sections were linearly related to the ratio of the squares of the effective charge Z* and the velocity of the ions. It is concluded from the analysis that the range of the most energetic delta-electrons is larger than previously assumed. There is no such dependence for survival and induction of mutants. In both cases cross-sections increase with the ion's specific-energy indicating an important contribution of long-range delta-electrons. The analysis shows that diploid yeast is not killed by a single-hit mechanism even by very heavy ions if the track width is too small. The relative importance of the penumbral region is even more pronounced with the more sensitive strains.

Heavy ion effects on yeast cells: induction of canavanine-resistant mutants.

The induction of forward mutations (resistance to canavanine) by heavy ion bombardment was investigated in wild type haploid yeast Saccharomyces cerevisiae. Accelerated ions of argon, titanium, nickel, krypton, xenon, lead and uranium with specific energies between 1.7 and 9.25 MeV/u were obtained from the UNILAC machine at the Gesellschaft für Schwerionenforschung, Darmstadt/Germany. LET-values ranged from 1200 to about 15 000 keV/microns. There was no unequivocal dependence of mutation induction cross section on either LET or Z*2/beta 2, but also a prominent influence of ion specific energy. This is explained by the action of long-ranging delta-electrons.

Experiments on the formation of particles by chemical reactions in the dark and under influence of light.

Some experiments concerning the photochemical production of condensation nuclei are described. Preliminary measurements of filtered atomospheric air, initially free of particles yielded high concentrations of particles by reactions in the dark when the air was previously irradiated by sunlight. In further investigations a definite composition of pure gases was used. The formation of nitric acid particles from NO2 in pure nitrogen of different relative humidities in the dark and under influence of light was investigated. No particle formation was found which could be correlated to any production of nitric acid nuclei. Even within a spectral region in which photolysis of NO2 takes place no HNO3-nucleation could be found. The particles detected under certain conditions of irradiation originate from impurities in the walls of the reaction chamber. Particle growth in an irradiated mixture of N2 and NO2 with benzene is demonstrated and the mean radius of particles is calculated from measurement with a diffusion battery.