Motivation

The main goal of track structure studies is to understand radiation action and radiation damage, especially of heavy ions, on the microscopic level, i.e. on a micrometer or even nanometer level. This is in particular important for radiation damage in biological systems. The most sensitive structures are the two DNA strands separated by only a few nanometers. In addition, also radiation transport in non-biological systems like dosimeters, ionization chambers and thin foils can be described on a microscopic level by taking into account each single interaction of a charged particle with an atom or molecule of the target material.

Calculational Method

The problem of radiation transport on the single interaction level is most conveniently handled by Monte Carlo (MC) methods. This allows to model the stochastic nature of the radiation action. The most important inputs for such calculations are:
the primary cross sections (double differential, in angle and energy) for delta-electron creation (as obtained e.g. by the Binary Encounter Approximation)
semi-empirical cross sections (for elastic scattering, ionization and excitation) of low-energy electrons to handle the diffusion and slowing down of electrons once they are created by a heavy ion.

Results

At first, the Monte Carlo calculations allow to illustrate the paths of heavy ions and their delta-electrons traversing matter.

Dose distributions as a function of the radial distance from the ion path are more quantitative results which allow comparison with experiments. The figure shows experimental data (symbols) measured in gas and scaled to liquid water density together with MC results (green histogram). The agreement is very good. In addition the results from calculations according to the Katz and the Chatterjee model are shown too. All models show good agreement with each other as well as with the experiment at medium distances. Here the dose falls off like 1/r**2. At very small and very large distances, however, the MC results are closer to experiment.

A real advantage of Monte Carlo techniques is the possibility to obtain event-by-event correlations. Therefore average "event sizes", i.e. the mean dose deposition in small volumes can be determined. The picture below shows measurements (symbols, Toburen et al) as well as calculations (curve) for small volumes corresponding to 0.5 micrometer diameter in liquid water. The long range tail due to delta-electron events can clearly be seen in experiment as well as in the simulation.

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