  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.
