Post-Bragg Peak keV-Secondary Electron Radiolysis Revealed by Track-Ends Imaging of High-Energy Protons.

Abstract

Upstream of the efficiency of proton or carbon ion beams in cancer therapy, and to optimize hadrontherapy results, we analysed the chemistry of Fricke solutions in track-end of 64-MeV protons and 1.14-GeV carbon ions. An original optical setup is designed to determine the primary track-segment yields along the last millimetres of the ion track with a sub-millimetre resolution. The Fe3+-yield falls in the Bragg peak to (4.9±0.4)×10-7 mol/J and 1.9×10-7 mol/J, under protons and carbon ions respectively. Beyond the proton Bragg peak, a yield recovery is observed over 1 mm. It is attributed to the proton beam straggling. Since a dose rate effect is ruled out in this region, keV secondary electrons are hypothesized to produce this intermediate LET effect. They might also provide DNA lesions during proton exposure, potentially at the origin of delayed radio-induced carcinogenesis.