  For the assessment of radiation effects of clinical ion-beams, dosimetry has
to be complemented by information on particle-energy distribution or related
quantities. Fluorescence nuclear track detectors made from C,Mg-doped alumina
single crystals allow for the quantification of ion track density and energy
loss on a single-track basis. In this study, their feasibility and accuracy to
quantify fluence, linear-energy-transfer (LET) distributions, and eventually
dose for a spread-out carbon ion Bragg peak was investigated. We found that the
primary ions track densities agreed well with the reference data, but the
determination of the individual detector sensitivity represented a major source
of uncertainty in LET (and dose) assessment. While low-LET fragments in the
beam are not contributing to this dose significantly, their number of was
largely underestimated by approximately a factor three. The effect was most
pronounced for protons where the measured fluence deviates at least an order of
magnitude. We conclude that this is mainly caused by the wide angular
distribution of protons in a carbon beam. The use of a dedicated FNTD reader
device and semi-automated workflow improved outcome due to the considerably
larger amount of data available as compared to a state-of-the-art multi-purpose
confocal laser scanning microscope.
