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The fusion of 6 He with a 209 Bi target has been studied at energies near to and below the Coulomb barrier. Despite the weak binding of the valence neutrons in 6 He, little evidence is found for suppression of fusion due to projectile breakup. Instead, a large enhancement of sub-barrier fusion is observed. It is suggested that this enhancement may arise from coupling to positive Q value neutron transfer channels, resulting in "neutron flow" between the projectile and the target. [S0031-9007 (98)07674-1] PACS numbers: 25.60.Pj, 25.70.JjRecent theoretical studies of near-barrier and subbarrier fusion of the exotic "neutron halo" system 11 Li with 208 Pb (see, e.g., [1][2][3][4][5]) have generated a considerable amount of interest and controversy. The 11 Li nucleus contains two valence neutrons that are only very weakly coupled to a relatively tightly bound 9 Li core. This unusual composition manifests itself in both the structure of the nucleus, as in the existence of the neutron halo and of low-lying E1 modes [6], and also in reactions with other nuclei. Furthermore, neither the n-9 Li nor the n-n subsystems of 11 Li are bound, so that particle stability in this nucleus is achieved via three-body interactions. Systems of this kind, referred to as "Borromean" nuclei [7], provide an unusual opportunity to study three-body interactions in the nucleus.It has been known for some time that sub-barrier fusion of stable nuclei can be enhanced by several orders of magnitude beyond expectations from simple one-dimensional barrier penetration calculations. A qualitative understanding of this phenomenon has been achieved in terms of couplings to internal degrees of freedom of the target and projectile [8], resulting in a lowering of the effective fusion barrier. This dynamical effect is a very sensitive probe of the nuclear structure of the colliding partners. A lowering of the barrier, by 20% or more, is also a general feature in the results for 11 Li 1 208 Pb fusion presented in [1][2][3][4][5], but the leading effect that was calculated in this case is a static one, resulting from the larger radius of the 11 Li "halo" wave function which allows the attractive nuclear force to act at longer distances. However, additional dynamical enhancement was obtained from the coupling to the soft E1 mode [1,2]. The role played by projectile breakup channels, which are possibly important due to the weak binding of the valence neutrons, is considerably more controversial. Several groups [2][3][4] have reported that coupling to these channels reduces the fusion cross section near the barrier, leading to intriguing structure in the excitation function in this region. However, this point of view has been criticized by Dasso and Vitturi [5] who suggest that it results from a misunderstanding of the nature of multidimensional quantum-mechanical tunneling processes. They report only enhancement of the fusion yield, even in the presence of strong breakup channels. It is important to resolve this controversy since the competition between project...

The neutron unbound ground state of (25)O (Z=8, N=17) was observed for the first time in a proton knockout reaction from a (26)F beam. A single resonance was found in the invariant mass spectrum corresponding to a neutron decay energy of 770_+20(-10) keV with a total width of 172(30) keV. The N=16 shell gap was established to be 4.86(13) MeV by the energy difference between the nu1s(1/2) and nu0d(3/2) orbitals. The neutron separation energies for (25)O agree with the calculations of the universal sd shell model interaction. This interaction incorrectly predicts an (26)O ground state that is bound to two-neutron decay by 1 MeV, leading to a discrepancy between the theoretical calculations and experiment as to the particle stability of (26)O. The observed decay width was found to be on the order of a factor of 2 larger than the calculated single-particle width using a Woods-Saxon potential.

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