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

Abstract

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 γ rays for the ⁶He+^63,65Cu systems are presented. Coincidence measurements of heavy reaction products, identified by their characteristic γ rays, with projectile like 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 γ-ray measurements for reaction studies near the Coulomb barrier with low intensity isotope separation on-line beams is highlighted.