Resonances in transfer-triggered breakup reactions near the fusion barrier

Full text: Above barrier fusion of light, weakly-bound projectiles such as ⁷Li is known to be suppressed. Single-barrier penetration model calculations, which accurately model fusion of strongly-bound projectiles such as ¹⁸O, overestimate measured fusion cross sections for weakly bound ⁷Li by 25%. Breakup reactions occurring before the fusion barrier radius is reached is thought to significantly reduce the probability for fusion of the entire projectile. Indeed, the ⁷Li ground state has a α-triton structure, with an α-t breakup threshold at just Q=- 2.467 MeV; direct breakup should expected to be strong. However, non-elastic breakup modes have been found to be very important. In particular, pickup of a proton, leading to unbound ⁸Be, is the dominant breakup mode in collisions with heavy targets such as ²⁰⁸Pb and ²⁰⁹Bi. Understanding the detail of these breakup processes is crucial: only if breakup occurs when the projectile is approaching the target could the disintegration suppress fusion. Measurements made at the ANU Heavy Ion Accelerator Facility have revealed the important role that resonant states play, and highlighted a strong sensitivity to their structure in determining where breakup occurs. Long-lived states such as the ⁸Be ground state survive until the ejectile is very far from the target, and so cannot suppress fusion. Short-lived states will decay much nearer the target, but even sub-zeptosecond (<10^-21 s) lifetimes may by be sufficient to alter where the projectile-like nucleus breaks up, and reduce the impact of breakup on fusion. Recent measurements of transfer-triggered disintegration reactions, their interpretation in terms of short-lived resonant states, and associated classical dynamical model simulations, will be discussed. (author)