Nucleon coalescence in the interaction of 16 O with 59 Co at an incident energy of 250 MeV

Full text: In reactions induced by light nuclei one observes quite a copious production of projectile-like fragments. One mechanism responsible for such emissions is the binary fragmentation (or break-up) of the projectile. Fragments produced by this mechanism are observed at forward angles and display a broad peak at about the beam velocity. At rather small projectile energies, below about 10 MeV/amu, the spectra of these fragments are symmetric and have approximately gaussian shapes. These features do not change with increasing emission angle. This scenario changes notably with increasing projectile energy. It was found long ago that pure break-up spectra are observed only at the most forward angles and are asymmetric showing a quite steep fall off on the high energy side and a longer tail extending up to the Coulomb barrier on the low energy side. This behaviour is confirmed by the data discussed in this paper concerning the interaction of 12 C with 59 Co at an incident energy of 250 MeV. At larger emission angles (Θ ≥ 20 degrees) the characteristic shape corresponding to a pure break-up mechanism disappears. The average energy of the emitted fragments decreases quite rapidly with increasing Θ and, for Θ ≥ 30 degrees, the spectra are dominated by quite a sharp peak at about the Coulomb barrier energy. This low energy component in the spectra of IMFs was attributed by Hussein et al. to some complex pre compound process in no way related to the projectile's break up. However, to our knowledge, a quantitative description of this contribution was not proposed. In this paper we deal with the theoretical interpretation of this contribution to the measured spectra and suggest that it may be satisfactorily reproduced assuming that it is due to the coalescence of nucleons into the observed fragment during the thermalization of the composite nuclei produced both in the complete fusion of the projectile with the target nucleus and in the fusion of a fragment produced in the projectile's break-up. (Author)