Collisions induced by 9;10;11 Be on a 64 Zn target at the same c.m. energy were studied. For the first time, strong effects of the 11 Be halo structure on elastic-scattering and reaction mechanisms at energies near the Coulomb barrier are evidenced experimentally. The elastic-scattering cross section of the 11 Be halo nucleus shows unusual behavior in the Coulomb-nuclear interference peak angular region. The extracted total-reaction cross section for the 11 Be collision is more than double the ones measured in the collisions induced by 9;10 Be. It is shown that such a strong enhancement of the total-reaction cross section with 11 Be is due to transfer and breakup processes. A hundred years after Rutherford's scattering experiment [1], heavy-ion-elastic-scattering angular distributions (AD) are usually plotted as a ratio of the Rutherford cross section (i.e., pure Coulomb scattering). Such representation usually shows a decrease of the elastic cross section with the angle due to absorption at small impact parameters by nonelastic processes, and an oscillatory behavior. The latter, using the language of optics, is described in terms of refraction by nonabsorbing lenses (Coulomb rainbow model) or diffraction by sharp-edged, nonrefracting apertures (Fraunhofer or Fresnel diffraction model). However, the refraction or diffraction descriptions are oversimplifications of the realistic process; rather, the nucleus behaves as a ''cloudy crystal ball.'' The elasticscattering AD may show a peak resulting from the interference between the Coulomb and nuclear amplitudes (Coulomb-nuclear interference peak) [2], which, in analogy with the Coulomb rainbow model, is sometimes called the rainbow peak. Since elastic scattering is a peripheral process, it does not give information on the interior region of nuclei. It probes the tail of the wave function, and hence one can learn about surface properties, such as size of nuclei and surface diffuseness. Therefore, elastic scattering is an ideal tool to study peculiar nuclear structures as, for example, the nuclear halo. Such structure originates when very weakly bound nucleon(s) can tunnel into the classically forbidden region, giving rise to a diffuse tail surrounding a well-bound core. The behavior of the system in nuclear reactions is mostly determined by the tail of the wave function [3]. The reaction mechanisms may also be affected by the weak binding: at energies around the Coulomb barrier, couplings between the entrance channel and the continuum [4][5][6][7][8], as well as to the various reaction channels [9][10][11][12], are expected to be very important. Direct reactions, such as breakup or transfer, may be favored owing to the low binding energy, the extended tail of halo nuclei, and the large Q values for selected transfer channels.Almost all elastic-scattering and reaction mechanism studies around the barrier with halo nuclei have been performed with the 2n halo nucleus 6 He. All authors agree that, due to the 6 He structure, one has an enhancement of the total-reaction (T...
