Simultaneous analysis of elastic scattering and fusion in He6+Zn64 : A transition in direct reaction mechanisms, striking threshold anomalies, and halo effects

Abstract

Using the extended optical model, a simultaneous analysis is carried out of elastic scattering and fusion reactions induced by the neutron halo $^{6}\mathrm{He}$ projectile on a $^{64}\mathrm{Zn}$ target. Dynamic polarization potentials ${U}_{F}$, ${U}_{D}$ are introduced which can be associated to fusion and direct couplings, respectively. Appropriate fitting parameters leading to dispersive potentials are found which provide a good simultaneous description of the data. An unexpected local maximum is uncovered in the energy dependence of the total reaction cross sections, indicating a transition in the involved direct reaction processes, and a possible underlying mechanism is discussed. ${U}_{F}$ is found to follow the normal threshold anomaly but with an unusually large threshold energy. ${U}_{D}$, on the other hand, exhibits a breakup threshold anomaly, but with a strikingly high value of the peak energy, which can be associated to the above-mentioned transition. The results also are discussed within the context of possible halo effects.