Disintegration locations in Li 7 → Be 8 transfer-triggered breakup at near-barrier energies

Background: At above-barrier energies, complete fusion cross sections in collisions of light weakly bound nuclei with heavy target nuclei are suppressed when compared to well-bound nuclei. Breakup of the projectilelike nucleus was proposed to be the cause. In addition to direct breakup, breakup following transfer was shown to be substantial. Purpose: We investigate breakup in reactions with Li7, triggered by sub-barrier proton pickup to unbound states in Be8, which subsequently separate into two α particles. Method: Measurements of sub-barrier disintegration of Li7 on a Ni58 target were made using the Heavy Ion Accelerator Facility at the Australian National University. Combining the experimental results with classical simulations of post-breakup acceleration, we study the sensitivity of α-α energy and angle correlations to the proximity of disintegration to the target (proton donor) nucleus. Results: The simulations indicate that disintegration as the colliding nuclei approach each other leads to large angular separations θ12 of the α fragments. The detectors allow for a maximum opening angle of θ12=132, such that the present experiment is largely insensitive to breakup occurring when the collision partners approach each other. The data are consistent with disintegration of (a) the 0+Be8 ground state far from the targetlike nucleus, and (b) the 2+Be8 resonance near the targetlike nucleus when the Be8 is receding from the targetlike nucleus. Conclusions: The present results shed light on the near-target component of transfer-induced breakup reactions. The distribution of events with respect to the opening angle of the α particles, and the orientation of their relative velocity with respect to the velocity of their center of mass, gives insights into their proximity to the target at the moment of breakup. Further measurements with larger angular coverage and more complete simulations are required to fully understand the influence of breakup on fusion.