Ering the Systematics of the Distributions and Describing Them

Toucher-event modeling will focus on discov Calculations of distributions in energy im ering the systematics of the distributions parted and also ionization number have been and describing them with simple analytic made for 10- to 200-keV protons in sites from functions, thereby completing the descrip 2 to 100 nm in diameter. These preliminary tion of bare-proton track structure up to results indicate that the distri butions are 20 MeV. more symmetric than those produced by higher energy ions. This is to be expected because the ionization cross sections are larger, at REFERENCES least above 50 keY. Hence more interactions occur on the average within a given site size Glass, W. A., and W. C. Roesch. 1972. than occur for higher-energy ions; larger "Measurement of Ionization Distributions mean values produce more symmetric Gaussian in Tissue-Equivalent Gas." Radiat. Res. like distributions. 49:477-494. Future improvements in ion track structure Gross, W., and R. C. Rodgers. 1972. "Heavy simulation will include the possibility for Ion Event Spectra." In Proceedings of the multiple ionization of the target molecule. Thi rd Symposiulll on Microdosimetry, vol. 3, However, this extension awaits more compre pp 873-887. Ed. H. G. Ebert. October 18- hensive cross-section measurements. 22, 1971, Stresa, Italy. Commission of European Communities, Luxembourg. REFERENCES Morgan, K. Z., and J. E. Turner, eds. 1973. Principles of Radiation Protection. Robert Toburen, L. H., M. Y. Nakai, and R. A. E. Kriegler Publishing Co., Huntington, Langley. 1967. "The Measurement of High New York, p. 277 ff. Energy Charge Transfer Cross Sec t ions for Incident Protons and Atomic Hydrogen in Various Gases and the K-, L-, and M-Auger, TRACK STRUCTURE FOR 10- TO 300-keV PROTONS L-Coster-Kronig, and the Conversion-Electron Spectra of Platinum in the Decay of 195Au." W. E. Wilson Thesis, ORNL-TM-1988, Oak Ridge National Laboratory, Oak Ridge, Tennessee. Simulation of ion track structures has been extended down to an ion energy of 10 keY by Toburen, L. H., M. Y. Nakai, and R. A. developing algorithms for the charge-exchange Langley. 1968. Phys. Rev. 171:114-122. cross sections (Toburen, Nakai, and Langley 1967, 1968) and the accompanying delta-ray spectra. These results must be considered MICROSCOPIC TRACK STRUCTURE OF EQUAL LINEAR preliminary, however, because of the approx ENERGY-TRANSFER HEAVY IONS imate nature of some of the code extensions. For the single differential cross section W. E. Wil son and T. L. Cri swell * (SDCS) for delta-ray emission, we have made use of a "hydrogenic approximati on" by Pro The spatial distributions of ionization and fessor M. E. Rudd. The Rudd algorithm is for energy deposition produced by heavy (HZE) low-energy ion-induced delta-ray emission ions are crucial to an understanding of their from hydrogen rather than water. We use it radiation quality as exhibited, for example, to estimate emission from each valence shell in track segment experiments of cell survival of water vapor by allowing for the different and chromosome aberrations of mammalian hinding energies, and then sum the shells to cells. The stopping power, or linear energy get a total for the eight valence electrons transfer (LET), of a high-velocity ion is in the molecule. This procedure produces an proportional to the ratio Z**2/v**2, where Z ion stopping power that is about 25% too and v are the atomic number and velocity, large around 50 to 100 keY, decreasing on respectively, of the moving ion, apart from a each side of this to become 5% to 10% too small at 10 and 300 keV. *Boeing Aerospace Corporation, Seattle, Washi ngton. 28 slowly varying logarithmic factor. The maxi tation. Consequently, the orientation de mum delta-ray energy that an ion can produce pendence of radical yields must be related to is proportional to v**2 (nonrelativistic the spatial distribution of energy deposition ally). Therefore, two HZE ions having the events rather than to the total amount of same LET, but in general different Z and v, energy absorbed in DNA. The most obvi ous will have different maximum delta-ray ener difference in the pattern of energy deposi gies and consequently will produce different tion between the two irradiation geometries spatial patterns of energy deposition along is that charged particles that traverse the their paths. To begin to explore the impli sample nearly parallel to the helical axis of cations of this fact for the microscopic DNA have a greater probability of multiple dosimetry of heavy ions, we have calculated energy transfer to the same molecule than do radial distributions in energy imparted and particles moving perpendicular to this axis. ionization for iron and neon ions of approxi However, this difference in the spatial dis mately equal LET to directly compare their tribution of energy absorption should have no delta-ray track structure. Monte Carlo tech consequence for radical production unless niques are used for the charged-particle intramolecular energy transfer greatly ex radiation transport simulation. Results of ceeds intermolecular energy transfer. Hence, these preliminary studies were reported in a it is the implication for intramolecular paper presented at the XXVI Plenary Meeting energy transfer that makes the unusual orien of the Committe on Space Research (COSPAR), tation dependence of radical yields important June 1986, Toulouse, France. for radiation biology. Although singlet and triplet excitations ap RADICAL YIELDS IN ORIENTED DNA EXPOSED TO pear to be "self trapped" (migration of more HIGH LINEAR-ENERGY-TRANSFER RADIATION than 10 base pairs is highly improbable), the presence of a conduction band in nucleic and J. H. Miller, W. E. Wilson, and the transport of vibrational excitation are C. E. Swenberg* still open questions. Enhanced recombination due to intramolecular charge transport may We are developing track structure-based account for a part of the reduction in the models to calculate the yield of free radical yield of thymine anion and guanine cation; species in oriented DNA fibers exposed to however, we postulate that the appearance of ionizing radiation with large LET. Experi TH radicals in the parallel irradiation geom ments performed at AFRRI show an unusual etry is due to intramolecular vibrational effect: radical yields detected by electron energy transport. This hypothesis is consis spin resonance (ESR) at 77 u K depend on the tent with the findings of Graslund et al. relative orientation of the axis of the DNA (1975), who investigated the formation of TH molecules to the flux of ionizing particles. by annealing of oriented DNA that was gamma When a neutron flux is incident perpendicular irradiated at 77°K. From the analysis of to the DNA, ESR spectra are consistent with their kinetic data, Graslund et al. conclude the production of roughly equal amounts of the following: (i) TH is produced by ther thymine anion and guanine cation. However, mally activated proton transfer to thymine when the sample is irradiated with the neu anion, (ii) water of hydration is the most tron flux approximately parallel to the DNA likely proton source, and (iii) the reaction axis, yields of thymine anion and guanine has a distribution of activation energies cation are reduced and a new ESR signal, between 0.2 and 0.7 eV. characteristic of the 5,6-dihydro-5-thyml (TH) radical, is seen. The possibility of radiation-induced thermal activation has been recognized for some time The amount of energy absorbed in DNA relative (Mozumder 1969). The excess temperature to that absorbed in the aqueous matrix of the induced by the isolated energy deposition fibers is primarily determined by their re events that are characteristic of sparsely spective volume and is independent of orien- ionizing radiation dissipates too rapidly to effectively induce chemical reactions; how *Armed Forces Radiobiology Research Insti ever this may not be true for high-LET radia tute, Bethesda, Maryland. tion. Assuming a large asymmetry in thermal 29 .

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