Wesley E. Bolch

Radiation Interactions and Energy Transport in the Condensed Phase

We review the state of knowledge about inelastic interactions of swift charged particles with condensed matter. Emphasis is placed on the properties of the dielectric response function and its representation for biologically interesting materials. Progress toward the goal of modeling electron transport and track structure in these materials is described.

Interactions of Low-Energy Electrons with Condensed Matter: Relevance for Track Structure

Here we review the state of knowledge about the transport of subexcitation electrons in water. Longitudinal optical phonon and acoustical phonon interactions with an electron added to the system can give rise to an increase in the effective mass of the electron and to its damping. We generalize the pioneering treatment of thermalization given by Frolich and Platzman to apply to both long-range, polarization-type interactions, and as well to short-range interactions of low-energy electrons in water. When an electron is ejected from a molecule in condensed matter quantum interference may take place between the various excitation processes occuring in the medium. We have estimated the magnitude of this interference effect, together with the effect of Coulombic interactions on the thermalization of subexcitation electrons generated in the vicinity of a track of positive ions in water.

Monte Carlo Track-Structure Calculations for Aqueous Solutions Containing Biomolecules

Detailed Monte Carlo calculations provide a powerful tool for understanding mechanisms of radiation damage to biological molecules irradiated in aqueous solution. This paper describes the computer codes, OREC and RADLYS, which have been developed for this purpose over a number of years. Some results are given for calculations of the irradiation of pure water. Comparisons are presented between computations for liquid water and water vapor. Detailed calculations of the chemical yields of several products from X-irradiated, oxygen-free glycylglycine solutions have been performed as a function of solute concentration. Excellent agreement is obtained between calculated and measured yields. The Monte Carlo analysis provides a complete mechanistic picture of pathways to observed radiolytic products. This approach, successful with glycylglycine, will be extended to study the irradiation of oligonucleotides in aqueous solution.

Radiation damage to a biomolecule: New physical model successfully traces molecular events

For the first time, a complete computer simulation of physical and chemical reactions at the molecular level has been used to calculate the yield of a chemical species resulting from irradiation of a biological molecule in aqueous solution. Specifically, when a solution of glycylglycine is irradiated anaerobically, an ammonia molecule is released by the action of a hydrated electron, which is produced by irradiation of water. In the computations, Monte Carlo techniques are used to simulate the statistical progression of molecular events as they are assumed to occur. These include the initial physical ionization and excitation of water molecules along a particle track in the liquid; the subsequent formation of free radicals and other species: and the random diffusion and chemical reactions of the species with each other, the solvent, and solute molecules. We have calculated and measured the yield of ammonia from irradiation of glycylglycine with 250 kVp x-rays as a function of glycylglycine concentration between 0.01 and 1.2 M. Excellent agreement is obtained between predicted and measured results. The literal simulation of events, combined with specific experimental measurements, offers a powerful new tool for studying mechanisms of radiation action and damage at the molecular level.

Considerations of beta and electron transport in internal dose calculations

Ionizing radiation has broad uses in modern science and medicine. These uses often require the calculation of energy deposition in the irradiated media and, usually, the medium of interest is the human body. Energy deposition from radioactive sources within the human body and the effects of such deposition are considered in the field of internal dosimetry. In July of 1988, a three-year research project was initiated by the Nuclear Engineering Department at Texas A M University under the sponsorship of the US Department of Energy. The main thrust of the research was to consider, for the first time, the detailed spatial transport of electron and beta particles in the estimation of average organ doses under the Medical Internal Radiation Dose (MIRD) schema. At the present time (December of 1990), research activities are continuing within five areas. Several are new initiatives begun within the second or third year of the current contract period. They include: (1) development of small-scale dosimetry; (2) development of a differential volume phantom; (3) development of a dosimetric bone model; (4) assessment of the new ICRP lung model; and (5) studies into the mechanisms of DNA damage. A progress report is given for each of these tasks within the Comprehensive Report. In each use, preliminary results are very encouraging and plans for further research are detailed within this document. 22 refs., 13 figs., 1 tab.