Direct and compound reactions induced by unstable helium beams near the Coulomb barrier

Abstract

Reactions induced by radioactive $^{6,8}\mathrm{He}$ beams from the SPIRAL facility were studied on $^{63,65}\mathrm{Cu}$ and $^{188,190,192}\mathrm{Os}$ targets and compared to reactions with the stable $^{4}\mathrm{He}$ projectiles from the Mumbai Pelletron. Partial residue cross sections for fusion and neutron transfer obtained from the measured intensities of characteristic in-beam $\ensuremath{\gamma}$ rays for the $^{6}\mathrm{He}+^{63,65}\mathrm{Cu}$ systems are presented. Coincidence measurements of heavy reaction products, identified by their characteristic $\ensuremath{\gamma}$ rays, with projectilelike charged particles, provide direct evidence for a large transfer cross section with Borromean nuclei $^{6}\mathrm{He}$ at 19.5 and 30 MeV and $^{8}\mathrm{He}$ at 27 MeV. Reaction cross sections were also obtained from measured elastic angular distributions for $^{6,8}\mathrm{He}+\mathrm{Cu}$ systems. Cross sections for fusion and direct reactions with $^{4,6}\mathrm{He}$ beams on heavier targets of $^{188,192}\mathrm{Os}$ 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 $\ensuremath{\gamma}$-ray measurements for reaction studies near the Coulomb barrier with low intensity isotope separation on-line beams is highlighted.