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Nanodosimetric measurements with deuterons

31.01.2011

An essential task of experimental nanodosimetry is the investigation of track structures of ionizing radiation in nanometric biological volumes, e.g. in DNA segments, with respect to the generation of radiation damage which can be attributed to direct ionization events. The essential characteristic quantity of such a track structure is the frequency distribution of the size of ionization clusters, the so-called nanodosimetric spectrum. A volume element, which can be regarded as suitable for this kind of investigation, has the size of a DNA segment and consists of approx. 20 base pairs (i.e. a cylinder with a diameter of approx. 4 nm, a height of approx. 8 nm at a density of 1 g/cm3). Nanodosimetric 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.

The stopping power of a substance depends on the type charge state and the energy of the particle being stopped. In the case of ions which have an identical atomic number Z, but a different mass number A - such as, for example, protons (Z = 1, A = 1) and deuterons (Z = 1, A = 2) - the stopping power is identical if the ions have identical velocities. The first moment of the frequency distribution of the size of ionization clusters - the mean ionization cluster size M1(Q) - is proportional to the stopping power. Therefore, M1(Q) should also be identical for ions which have a different mass number A, but an identical atomic number Z if the particle velocity and the charge state are the same.

At the accelerators of PTB, nanodosimetric spectra were measured in monoenergetic proton and deuteron beams in the energy range from 0.1 MeV to 2.5 MeV in C3H8 and N2 at a pressure of 1.2 mbar. The figure shows the comparison of the mean ionization cluster size M1(Q) derived from the measured frequency distribution of the ionization cluster size for primary proton and deuteron beams of different particle velocities vion for the two measurement gases. For both gases, the mean ionization cluster size shows a comparable dependence on the particle velocity. Compared to the values for protons, the measurement data for deuterons are, however, systematically shifted towards smaller values.

Figure : Mean ionization cluster size M1(Q) for the measured frequency distribution of the ionization cluster size for primary proton and deuteron beams of different particle velocities vion for the two measurement gases C3H8 and N2 at a pressure of 1.2 mbar.