Theoretical studies of physical and chemical aspects of primary processes in fundamental radiobiology

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An understanding of radiation-induced processes in aqueous systems is vital to many areas of basic and applied radiobiological physics and chemistry, medicine, and in a variety of technological and industrial applications. If, nowadays, a general understanding of the phenomena that underline the radiation chemistry of water and aqueous solutions has been obtained in many cases, it appears that certain quantitative aspects of this radiolysis are not yet fully resolved. This is especially true forultrafast processes that link chemistry and physics in the first few picoseconds following energy deposition. Understanding this interface between radiation physics and radiation chemistry (i.e., "breaking the picosecond barrier") is of obvious relevance to fundamental radiobiology and related science as liquid water is by far the most abundant constituent of biological cells and tissue (living cells contain ~70-85% water by weight). Most importantly, it is central to a reliable description of the chemical nature and highly nonhomogeneous spatial distribution of all reactive species created on the (sub) picosecond time scale and involved as precursors of radiobiological damage. This understanding is also critical to creating reliable predictive models. Of a theoretical nature, the present project will focus on the characterization of the physicochemical stage of radiation action (<1 ps). In particular, one difficult problem that we wish to address in the next five years under an NSERC Discovery Grant is the physical behavior of the numerous low-energy (<30 eV) secondary electrons generated in the slowing of primary ionizing radiations and the pivotal role they can have, as precursors of hydrated electrons, on the transient ensuing chemistry in the radiation track development. Another aera of great importance that we wish to examine is the fact that the aqueous medium itself is not continuous on a molecular scale (all track physics programs have hitherto considered water as a continuum). We will attempt to generate a track in a manner that recognizes the molecular nature of the target medium. Using state-of-the-art stochastic Monte Carlo methods and molecular dynamics calculations in combination with the knowledge gained from current experimental efforts, the proposed research program aims to design experiment-and-theory based models to advance our knowledge of the radiolysis of (dilute and concentrated) aqueous systems for which, we feel, an early-time, molecular-level characterization of the underlying chemistry is essential to produce a complete, accurate picture of this radiolysis. It is, without doubt, part of a major challenge in fundamental radiobiology where our long-term goal is to achieve a global comprehension of the effects of radiation in biological systems and to apply this knowledge to enhance the therapeutic and diagnostic efficiency of radiation.