Asymptotic and near-target direct breakup of Li 6 and Li 7

Background: Li6,7 and Be9 are weakly bound against breakup into their cluster constituents. Breakup location is important for determining the role of breakup in above-barrier complete fusion suppression. Recent works have pointed out that experimental observables can be used to separate near-target and asymptotic breakup. Purpose: Our purpose is to distinguish near-target and asymptotic direct breakup of Li6,7 in reactions with nuclei in different mass regions. Method: Charged particle coincidence measurements are carried out with pulsed Li6,7 beams on Ni58 and Zn64 targets at sub-barrier energies and compared with previous measurements using Pb208 and Bi209 targets. A detector array providing a large angular coverage is used, along with time-of-flight information to give definitive particle identification of the direct breakup fragments. Results: In interactions of Li6 with Ni58 and Zn64, direct breakup occurs only asymptotically far away from the target. However, in interactions with Pb208 and Bi209, near-target breakup occurs in addition to asymptotic breakup. Direct breakup of Li7 into α-t is not observed in interactions with Ni58 and Zn64. However, near-target dominated direct breakup was observed in measurements with Pb208 and Bi209. A modified version of the Monte Carlo classical trajectory model code platypus, which explicitly takes into account lifetimes associated with unbound states, is used to simulate sub-barrier breakup reactions. Conclusions: Near-target breakup in interactions with Li6,7 is an important mechanism only for the heavy targets Pb208 and Bi209. There is insignificant near-target direct breakup of Li6 and no direct breakup of Li7 in reactions with Ni58 and Zn64. Therefore, direct breakup is unlikely to suppress the above-barrier fusion cross section in reactions of Li6,7 with Ni58 and Zn64 nuclei.