Electron Transport in Tissue-Like Material

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DESCRIPTION (provided by applicant): This study will provide experimental data for understanding electron transport in the heterogeneous cellular environment and enable stringent tests of charged-particle track-structure simulation at the level of the underlying physics of energy transport, i.e., prior to assumptions of the nature of chemical and bio-chemical pathways leading to biological alterations. Much of our knowledge of the biological effectiveness of radiation to create DNA damage and cellular mutagenesis is based on Monte Carlo based simulations of the initial patterns of energy deposition. Because of a lack of detailed information on cross sections for interactions in the condensed phase, Monte Carlo simulations often use interaction cross sections derived from gas phase data, or from largely untested theoretical techniques. Uncertainties in these data might have detrimental effects on the accuracy by which patterns of energy deposition can be determined at microscopic dimensions such as are involved in the production of multiply damaged sites, damage clusters, in DNA. Since these codes provide the data upon which radiobiologic interpretations of dose-response relationships are based, they are a direct link to understanding the biological effects of radiation exposures, whether these exposures are environmental, diagnostic, or therapeutic. This work focuses on measurement of the spectra of electrons produced in thin films of condensed phase material by the transit of fast protons. Spectra, differential in ejected electron energy and emission angle, are measured following electron transport and their emergence from the material surface. The transport media of interest include water, hydrocarbons, oligonucleotides, oligopeptides and DNA in thin films and/or frozen as ices. The spectra will be compared to predictions of Monte Carlo codes that incorporate gas and/or condensed phase interaction cross sections to test the sensitivity and reliability of the codes to the details of the input data. This, the first direct test of the fundamental physics of electron transport in track simulation codes commonly used in radiobiology, will provide guidance for the application of these codes in radiobiology and strengthen confidence in our evolving understanding of the energy transport pathways leading to cellular damage and mutagenesis by ionizing radiation.