Elucidation of the energy absorption spectra of the (11)C beam in a plastic scintillator

Heavy ion therapy using the energetic (12)C beam is successfully under way at HIMAC, Japan. The method is more advantageous than traditional radiation therapy in dose concentration owing to the Bragg peak and high relative biological effectiveness. A research study using the (11)C beam for heavy ion therapy in the future has been carried out in order to develop the capability of monitoring the dose distribution. Our group has examined the total energy absorption spectrum of the (11)C beam in a plastic scintillator. We could clearly observe the total absorption peak of (11)C in the energy spectrum and, in addition, we found a broad bump structure was associated with the peak. The bump area occupies 37% of the total spectrum and it probably affects the dose calculation for an accurate treatment planning. We elucidated the mechanism that leads to the structure of the total energy absorption spectra given by (11)C and (12)C in a block of plastic scintillator. This paper describes the method in detail and gives experimental analysis results which deal with the bump structure. We could explain the bump structure using the energy spectra caused by the fragmentation reactions.