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Track structure of carbon ions of different energies characterized nanodosimetrically

03.12.2014

A fundamental task of experimental nanodosimetry is the examination of track structures of ionizing radiation in nanometric biological volumes such as 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 ionization clusters, i.e. the number of ionization interactions of a primary particle and its secondaries in the target volume. A volume element that 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 and a height of approx. 8 nm at a density of 1 g/cm3). Track structures 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. Hereby, not only the size of the target volume and the radiation quality, but also the distance of the trajectory of the primary particle from the centre of the target volume is an important parameter which can be determined by means of a position-sensitive detector.

At the accelerators of HIL in Warsaw and of INFN in Legnaro, the frequency distributions of the ionization cluster sizes for carbon ion radiation of different energies have been measured in 1.2 mbar N2 and in 1.2 mbar C3H8 within the scope of the EMRP JRP BioQuaRT [1].

 

Fig. 1: Mean ionization cluster size M1(Q, d) for 88 MeV 12C6+ ions in 1.2 mbar C3H8 as a function of the distance d of the trajectory of the primary particle from the centre of the target volume having of diameter D.

Figure 1 shows the mean ionization cluster size M1(Q, d) for the radiation quality Q of 88 MeV 12C6+ as a function of the distance d of the trajectory of the primary particle from the centre of the target volume of diameter D. The target gas used was 1.2 mbar C3H8, the area density of the target volume was Dρ = 0.26 μg/cm2. Due to the penumbra of secondary electrons that surrounds the trajectory of the primary particles, M1(Q, d) for d > D/2 is not 0, but drops continuously when d increases, as with increasing distance from the trajectory of the primary particle, fewer and fewer secondary electrons reach the target volume, so that their energy becomes, on average, ever higher – and so that they become ever less densely ionizing.

 

Fig. 2: Ionization cluster size distributions Pν(Q, d) for a few distances d of the trajectory of the primary particle from the centre of the target volume of diameter D and for the total radiation field between d = -6.6 D and d = +6.6 D.

Figure 2 shows, for the same radiation quality, besides the frequency distributions of the ionization cluster sizes Pν(Q, d) for a few distances d, also the distribution for the total, wide radiation field in the range -6.6 D ≤ d ≤ +6.6 D which is obtained if the position of the primary particle is not resolved.

References:

  1. www.ptb.de/emrp/bioquart.html