Particle tracks in the cells

To determine – for radiation protection or medical radiation therapy – the effects of ionising radiation on the human body, measurands have been used up to now which are based on phenomenological processes. In nanodosimetry, the correlation with the detailed microscopic structure of the particle track – i.e. of the spatial distribution of the interaction events inside the body – is investigated. In the long run, the aim of this work is to develop new dosimetric measurands which already take the biological effects of ionising radiation into account.

To characterise the structure of the particle track, statistical measurands – such as the ionisation cluster size distribution – are used. This measurand indicates the probability that a certain number of ionisation processes is generated in a defined volume. In biological cells, e.g. in DNA segments with 10 to 20 base pairs, these are typically volumes with dimensions of a few nanometers. The effects of ionising radiation cannot be directly measured in such small volumes; they can only be determined by means of Monte Carlo simulation calculations. To link the – calculated – ionisation cluster size distributions in condensed matter (e.g. in biological cells) with the – measurable – ones in macroscopic volumes of measuring gases, a scaling procedure has been developed.

The nanodosimeter which has been developed in collaboration with the Weizmann Institute of Science in Israel is now used to experimentally verify the validity of this scaling procedure and to validate the Monte Carlo programs used for the simulations. For this purpose, the ionisation cluster size distributions are systematically investigated in different measuring gases for different radiation qualities. The density of the measuring gases is chosen in such a way that the gas mass in the detection volume corresponds to the mass of a DNA segment. Recently, these data have revealed inconsistencies in the material-specific datasets used in the programs, with the consequence that the corresponding cross sections need to be determined experimentally.

In the short run, it is planned to measure ionisation cluster size distributions for molecules which occur as constituents in the DNA and are therefore of great importance for the investigation of the relation between nanodosimetrical measurands and the biological activity of the radiation. In the medium term, the nanodosimeter is to be further developed for the spatially resolved detection of the primary particle in order to enable the 2D measurement of the parameters of the track structure.