Spatial patterns of ionization in charged-particle tracks

Abstract

The physical structure of charged-particle tracks plays an important role in determining the response of a stopping material to the absorption of energy following irradiation by charged particles and neutrons. For biological systems, and for muelectronic circuits, the response is sensitive to the energy deposited in submun-size volume elements. Because of the stochastic nature of energy deposition in and near charged particle tracks, there may be wide variations in the actual quantities of energy deposited in critical volume elements. Since direct measurements of energy deposition in condensed material (solids or liquids) are not technologically feasible, the effects of charged-particle track structure are usually estimated from one of several model calculations; the most commonly used are homogeneous track structure models. Recent interest in the radiation biology of high Linear-Energy-Transfer (LET) radiation has spurred interest in testing these model descriptions of the structure of high-energy heavy-ion tracks. As one means of providing such tests, experiments were recently conducted at the GSI-Darmstadt, UNILAC accelerator, to measure energy deposition in small volumes as a function of the radial distance from the path of fast, heavy ions. These measurements, conducted in collaboration with research groups of Dr. G. Kraft at GSI and Prof. Schmidt-Böcking of the University of Frankfurt, were made for simulated tissue volumes 0.5 and 1.0 μm in diameter, located from 0 to 10 μm from the path of Ge ions having energies from 13.0- to 17.2-MeV/amu. Excellent agreement was observed between model calculations and measured dose distributions for radial distances up to a few mumeters in simulated tissue. At greater distances the actual measured dose in irradiated volumes was much more than the calculated average value. These differences reflect the stochastic nature of energy deposition in which a large fraction of the volume elements receive no dose from a given particle traversal, but elements which do receive energy receive relatively large amounts. This may have important consequences for effects which occur with a nonlinear or threshold energy response.