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Experimental testing of the scalability of track structures of ionizing radiation in nanometric volumes

29.09.2008

A fundamental task of experimental nanodosimetry is the examination of track structures of ionizing radiation in nanometric biological volumes, such as, e.g., in DNA segments, with regard to the generation of radiation damage which is directly caused by ionizing events. The essential characteristic quantity of such a track structure is the frequency distribution of the size of ionizing clusters, the so-called "nanometric spectrum". A volume element which can be considered to be suitable for this kind of test is of the size of a DNA segment consisting of approx. 20 base pairs (i.e. a cylinder with a diameter of approx. 4 nm and a height of approx. 8 nm at a density of 1 g/cm3). Nanometric spectra which are to be assigned to such a volume element can be measured in an ion-counting nanodosimeter which is filled with a suitable gas and in which the ions produced by the ionizing radiation are detected after their drift through the gas.

As a cylinder of liquid water serves as a replacement for a DNA segment, a method is needed to rescale the nanometric spectra measured in gaseous media in such a way that they are equivalent to the nanometric spectra of a target of liquid water with nanometric dimensions. Such a scaling method is based on the ratio of the mean free path lengths of the ionizing particles in the respective medium. The mean free path length of a particle in a material is, in turn, inversely proportional to the cross section. The procedure was described in [1] and verified with Monte Carlo simulations.

At the PTB accelerator facilities, nanometric spectra were measured in monoenergetic proton and alpha particle beams in the energy range from 0.1 MeV to 20 MeV. C3H8 and N2 served as target gases. The mean free path lengths resulting from the cross sections of C3H8 and N2 yield pressure ratios of 0.55 mbar C3H8 and 1.2 mbar N2 for alpha particles, and 0.5 mbar C3H8 and 1.2 mbar N2 for protons. In the experiment, the best agreement is achieved for pressure ratios of 0.46 mbar C3H8 and 1.2 mbar N2 for alpha particles, and 0.425 mbar C3H8 and 1.2 mbar N2 for protons. The reason for this difference lies in the large uncertainties of the cross sections for alpha particles and protons - currently available in the literature - in the gases used.

Figure : Mean ionization cluster size M1(T) for the measured frequency distribution of the ionization cluster size for primary proton and alpha particle beams of different energies T for the respective combination of the two measuring gases.

The figure shows the comparison of the mean ionization cluster size M1(T) for the measured frequency distribution of the ionization cluster size for primary proton and alpha particle beams of different energies T for the respective combination of the two measuring gases.

Literature

  1. Großwendt, B.:
    Nanodosimetry, the metrological tool for connecting radiation physics with radiation biology,
    Radiat. Prot. Dosim. (2006), Vol. 122, No. 1 - 4, p. 404 - p. 414.