LABORATORI NAZIONALI DI LEGNARO
In view of the fact that the effectiveness of ionizing radiation to induce severe damages to living cells depends on the track structure of ionizing particles, it can be assumed that the quality of ionizing radiation is strongly related to the pattern of inelastic interactions of single ionizing particles in tissue and, in particular, to the corresponding spatial distribution of ionization processes within the cell DNA. In consequence, radiation quality can be characterized by the probability distribution of the number of ions (the ionization cluster size) caused by single ionizing particles (including their δ-electrons) in nanometre-sized target volumes, as a function of impact parameter d (distance between the centre of a target volume and the axis of a narrow beam of primary particles). The measurement of such distributions can be performed at the Tandem-Alpi accelerator at LNL using the STARTRACK device, which is worldwide unique and able to count the number of ionizing processes which are induced by single primary particles in a cylindrical target volume, 20 nm in diameter at a density of 1 g/cm³, when penetrating through or passing by the target cylinder at a specified impact parameter. One of the results of previous STARTRACK experiments is the finding that the conditional cluster-size distribution in the penumbra region (the δ-electron cloud) of a needle beam of ionizing particles neither depends on the type or energy of the primary particles nor on the impact parameter d. To study this result in a greater detail, the influence of δ-electron production on the ionization cluster-size formation in nanometre-sized target volumes of liquid water is analyzed for light ions in a large LET range as a function of impact parameter d, using the results of Monte Carlo simulations. Here, special emphasis is laid on the δ-electron contribution to the cluster-size distribution due to light ions, and to the resulting mean cluster sizes M1, Fano factors M2/M1-M1, and moment ratios M2/M1.

DATA: 21-11-2011