Absorbed Dose in Ion Beams: Comparison of Ionisation and Fluence-based Measurements

A direct comparison measurement of fluorescent nuclear track detectors (FNTDs) and a thimble ionisation chamber is presented. Irradiations were performed using monoenergetic protons (142.66 MeV, 3*10^6 1/cm2) and carbon ions (270.55 MeV/u, 3*10^6 1/cm2). It was found that absorbed dose to water values as determined by fluence measurements using FNTDs are, in case of protons, in good agreement (2.4 %) with ionization chamber measurements, if slower protons and Helium secondaries were accounted for by an effective stopping power. For carbon, however, a significant discrepancy of 4.5 % was seen, which could not be explained by fragmentation, uncertainties or experimental design. The results rather suggest a W-value of 32.10 eV +/- 2.6 %. Additionally, the abundance of secondary protons expected from Monte-Carlo transport simulation was not observed.