Biological effects of ionizing radiation: impact of protons on biomolecules

Today it is recognized that radiation damage in bio-molecules, in particular strand breaking in the DNA, is not only the result of a single interaction of the primary ionization projectile with the molecules involved, but also due to the simultaneous and consecutive action of the primary and the secondary species, including also radicals (for instance oxygen) that are produced by destruction of the water molecules surrounding the DNA. A detailed knowledge of the ionization process is a must for a full understanding of radiation damage on a microscopic level. The research program is focused on the study of swift proton impact ionization of biomolecules in the gas phase and of mixed clusters (DNA or RNA basis and water molecules). The proton energy range (30 - 150 kV) explored corresponds to a range where proton beam action in living matter is known to be maximum (Bragg peak range). Preliminary experiments are devoted to the measurement of fragmentation cross sections of a target molecule in coincidence with the post-collision charge state of the projectile. Therefore, it was possible for the first time to separate the direct ionization and the ionization via electron capture by the projectile. New insights on ionization and charge exchange processes in the interaction of protons and neutral hydrogen atoms with water and biomolecules such as DNA or RNA bases are given. Experimental results for proton ionization of water molecules based on a novel event by event analysis of the different ions produced (and lost) are presented. It was able to obtain mass analyzed product ion signals (e.g. H2O-, OH+, O+, O++, H+, and also negative ions) in coincidence with the charge-analyzed projectile, i.e., either being H+ or after single electron capture during the ionization event neutral H+ or H- after double electron capture. After proper calibration it was thus able to determine a complete set of cross sections for the ionization of a molecular target by protons (20 - 150 keV) including the total and the partial cross sections and in addition also the direct ionization and the electron capture cross sections. Similar measurements have been performed on the one hand for ionization of water by swift neutral atoms, on the other hand for proton ionization of uracil molecules (RNA Base) of which preliminary results were presented. (nevyjel)