Characterisation of the PTB ion counter nanodosimeter's target volume and its equivalent size in terms of liquid H2O

For the first time a dedicated investigation of the target size of a nanodosimeter device has been carried out in order to investigate to what extent measured ionisation cluster size distributions can serve as benchmark data for modelling approaches, with particular focus on the target size in terms of liquid H2O. To this end, measurements with alpha particles from a 241Am source were carried out using three different target gases, H2O, C3H8 and C4H8O. For each of the three target gases, three different drift-time windows were applied to realise three different target sizes. A method has been developed to determine the dimensions of the simulated nanometric target volume in liquid H2O for cylindrical and spherical shape, as often used in approaches to model radiation effects to DNA. Simulations with nanometric targets of dimensions determined with this method agree very well with the corresponding measurements. Scaling of the spatial distribution of the extraction efficiency for different target gases and drift-time windows, which corresponds to the nanodosimeter's target volume, in terms of liquid H2O using ($\rho\lambda_{ion}$)-ratios has also been investigated and proved to yield an estimate of the target volume in liquid H2O. Thus, it can be concluded that ionisation cluster size distributions measured with a nanodosimeter device are suited as benchmark data for approaches that model radiation induced damage to DNA in nanometric volumes of liquid H2O in simple geometries such as cylinders or spheres, provided that the nanodosimeter's target volume is characterised accordingly. By proper selection of drift-time window length as well as target gas and density a wide range of target volume dimensions in terms of liquid H2O can be realised with the PTB Ion Counter nanodosimeter according to specific requirements of modelling approaches.