Direct and compound reactions induced by unstable helium beams near the Coulomb barrier
Résumé
Reactions induced by radioactive 6,8He beams from the SPIRAL facility were studied on 63,65Cu and 188,190,192Os targets and compared to reactions with the stable 4He projectiles from the Mumbai Pelletron. Partial residue cross sections for fusion and neutron transfer obtained from the measured intensities of characteristic in-beam gamma rays for the 6He+63,65Cu systems are presented. Coincidence measurements of heavy reaction products, identified by their characteristic gamma rays, with projectilelike charged particles, provide direct evidence for a large transfer cross section with Borromean nuclei 6He at 19.5 and 30 MeV and 8He at 27 MeV. Reaction cross sections were also obtained from measured elastic angular distributions for 6,8He + Cu systems. Cross sections for fusion and direct reactions with 4,6He beams on heavier targets of 188,192Os at 30 MeV are also presented. The present work underlines the need to distinguish between various reaction mechanisms leading to the same products before drawing conclusions about the effect of weak binding on the fusion process. The feasibility of extracting small cross sections from inclusive in-beam gamma-ray measurements for reaction studies near the Coulomb barrier with low intensity isotope separation on-line beams is highlighted.