ANOMALY IN THE N-15, O-16, F-19-54, FE-56 FUSION CROSS-SECTIONS AROUND THE COULOMB BARRIER ENERGY(, FE)

Fusion cross sections have been determined by detecting evaporation re sidues from six systems formed by projectiles of N-15, O-16 and F-19 a nd target nuclei of Fe-54 and Fe-56 at incident energies around the Co ulomb barrier. The comparison of the experimental results with the pre diction from the one-dimensional barrier-penetration model revealed th at the O-16 reactions were not much enhanced, whereas the F-19 fusion reactions at sub-barrier energies are more than ten times higher than the model calculation. The fusion cross sections for N-15 projectiles showed a medium enhancement. The one-dimensional model has reproduced to some extent the experimental data of the N-15 + Fe-54 and O-16 + Fe -54, Fe-56 systems by making corrections for the quadrupole deformatio n of the reacting nuclei. However, the F-19 data cannot be described b y this model. In order to explain uniformly the cross sections of the six systems we have performed coupled-channel calculations. The N-15 a nd O-16 data have been reproduced within the experimental error by ope ning channels for inelastic excitation in the reacting nuclei and thos e for one-nucleon transfer. For better explanation of the F-19 data, h owever, an additional channel for the triton transfer was required. [G RAPHICS]