Heavy-ion effects on bacteria spores: the impact parameter dependence of the inactivation.

The impact parameter dependence of the inactivation of Bacillus subtilis-spores has been measured using a heavy-ion minibeam facility, which permits single-ion exposure of individual spores with a spatial accuracy of about 1 micron. The apparatus consists basically of a collimator and a microscope used to position the biological objects directly behind the collimator. Measurements were obtained for nickel, tin, and uranium ions at 1.4 MeV/u. for central hits the results show an inactivation probability of less than one with a continuous decrease in inactivation with increasing distance. Long-ranging effects which extend beyond the range of the delta electrons could not be confirmed.

Cell inactivation, repair and mutation induction in bacteria after heavy ion exposure: results from experiments at accelerators and in space.

To understand the mechanisms of accelerated heavy ions on biological matter, the responses of spores of B. subtilis to this structured high LET radiation was investigated applying two different approaches. 1) By the use of the Biostack concept, the inactivation probability as a function of radial distance to single particles' trajectory (i.e. impact parameter) was determined in space experiments as well as at accelerators using low fluences of heavy ions. It was found that spores can survive even a central hit and that the effective range of inactivation extends far beyond impact parameters where inactivation by delta-ray dose would be effective. Concerning the space experiment, the inactivation cross section exceeds those from comparable accelerator experiments by roughly a factor of 20. 2) From fluence effect curves, cross sections for inactivation and mutation induction, and the efficiency of repair processes were determined. They are influenced by the ions characteristics in a complex manner. According to dependence on LET, at least 3 LET ranges can be differentiated: A low LET range (app. < 200 keV/micrometers), where cross sections for inactivation and mutation induction follow a common curve for different ions and where repair processes are effective; an intermediate LET range of the so-called saturation cross section with negligible mutagenic and repair efficiency; and a high LET range (>1000 keV/micrometers) where the biological endpoints are majorly dependent on atomic mass and energy of the ion under consideration.