  At the Heidelberg Ion-Beam Therapy Center, the track structure of carbon ions
of therapeutic energy after penetrating layers of simulated tissue was
investigated for the first time. Measurements were conducted with primary
carbon ion beams of different energies and polymethyl methacrylate (PMMA)
absorbers of different thicknesses to realize different depths in the phantom
along the pristine Bragg peak. Ionization cluster size (ICS) distributions
resulting from the mixed radiation field behind the PMMA absorbers were
measured using an ion-counting nanodosimeter. Two different measurements were
carried out: (i) variation of the PMMA absorber thickness with constant primary
carbon ion energy and (ii) combined variation of PMMA absorber thickness and
primary carbon ion energy such that the primary carbon ion's energy in the
target volume is constant. The data analysis revealed unexpectedly high mean
ICS values compared to stopping power calculations and the data measured at
lower energies in earlier work. This suggests that in the measurements the
primary carbon ion energies behind the PMMA absorber may have deviated
considerably from the expected values obtained by the calculations. In
addition, the results indicate the presence of a marked contribution of nuclear
fragments to the measured ICS distributions, especially if the carbon ion does
not cross the target volume.
