Complete and Incomplete Fusion Processes in the 12C + 12C System at an Energy of 16.7 MeV/nucleon

An increasing demand for comprehensive knowledge of the processes involved in the interaction of two light ions is emanating, not only from basic research, but also from applications in fields such as hadrontherapy or radiation protection studies on space missions. Especially important seems to be the study of the reactions induced by 12C on the elements making up biological tissue. Spectra of the intermediate mass fragments (IMF) produced in the 12C + 12C interaction at 200 MeV are presented here. The experiment was performed at iThemba LABS, South Africa. The IMF spectra were measured at angles ranging from 8o to 20o with a silicon detector telescope which allowed a good mass and charge separation. The observed IMFs range from 7Be up to 20F, i.e. those expected from 12C fragmentation up to considerably heavier ones than 12C. Especially important is the observation of positron emitters produced with sizeable cross sections at an energy which corresponds to the Bragg Peak region (BPR) of a C-beam inside the body. In addition to 11C, also 13N, 15O and 18F are observed. In hadrontherapy production of positron emitters is of the uttermost importance for visualizing the beam during irradiation through PET techniques, which consequently results in a safer irradiation of the tumour volume [1]. Fragments considerably heavier than 12C produced in the BPR increase the radiobiological effectiveness of the beam thus making their measurements of great relevance. The observation of these heavy fragments seems to confirm the important role played by complete fusion and/or break-up-fusion reactions which may produce these fragments as evaporation residues, as was found in a recent investigation of 12C + 27Al interaction [2]. In a preliminary analysis the Boltzmann Master Equation theory, interfaced with the transport code FLUKA [3], is used to evaluate the decay by Fermi break-up of very light equilibrated nuclei. These are produced after the cascade of N-N interactions through which the nuclei created in complete fusion and break-up fusion reactions thermalize [4]. It seems that these mechanisms, in addition to inelastic scattering and few nucleon transfer reactions, mainly applicable to fragments with mass and charge close to 12C, may explain the yield and the angular and energy dependence of the measured spectra.