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 fragment mass analyzer at the ATLAS facility has been used to unambiguously identify the mass number associated with different decay modes of the nobelium isotopes produced via 204 Pb(48 Ca, xn) 252−x No reactions. Isotopically pure (>99.7%) 204 Pb targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (t 1/2 = 3.7 +1.1 −0.8 and 43 +22 −15 µs) were deduced from a total of 158 fission events. Both decays originate from 250 No rather than from neighboring isotopes as previously suggested. The longer activity most likely corresponds to a K isomer in this nucleus. No conclusive evidence for an α branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity.

Reaction products from the interaction of 6 He with 209 Bi have been measured at energies near the Coulomb barrier. A 4 He group of remarkable intensity, which dominates the total reaction cross section, has been observed. The angular distribution of the group suggests that it results primarily from a direct nuclear process. It is likely that this transfer/breakup channel is the doorway state that accounts for the previously observed large sub-barrier fusion enhancement in this system.

The angular distribution for the breakup of 8B-->7Be+p on a 58Ni target has been measured at an incident energy of 25.75 MeV. The data are inconsistent with first-order theories but are remarkably well described by calculations including higher-order effects. The comparison with theory illustrates the importance of the inclusion of the exotic proton halo structure of 8B in accounting for the data.

Evaporation residue and fission cross sections of radioactive 132 Sn on 64 Ni were measured near the Coulomb barrier. A large subbarrier fusion enhancement was observed. Coupled-channel calculations, including inelastic excitation of the projectile and target, and neutron transfer are in good agreement with the measured fusion excitation function. When the change in nuclear size and shift in barrier height are accounted for, there is no extra fusion enhancement in 132 Sn + 64 Ni with respect to stable Sn + 64 Ni. A systematic comparison of evaporation residue cross sections for the fusion of even 112−124 Sn and 132 Sn with 64 Ni is presented. DOI: 10.1103/PhysRevC.75.054607 PACS number(s): 25.60.−t, 25.60.Pj 0556-2813/2007/75(5)/054607(9) 054607-1