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...

In this paper we analyze the nuclear fusion rate between equal nuclei for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime). The rate is determined by Coulomb barrier penetration in dense environments and by the astrophysical S-factor at low energies. We evaluate previous studies of the Coulomb barrier problem and propose a simple phenomenological formula for the reaction rate which covers all cases. The parameters of this formula can be varied, taking into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the ^{12}C+^{12}C fusion reaction. This reaction is very important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovae, and in accreting neutron stars. The S-factor at stellar energies depends on a reliable fit and extrapolation of the experimental data. We calculate the energy dependence of the S-factor using a recently developed parameter-free model for the nuclear interaction, taking into account the effects of the Pauli nonlocality. For illustration, we analyze the efficiency of carbon burning in a wide range of densities and temperatures of stellar matter with the emphasis on carbon ignition at densities rho > 10^9 g/cc.Comment: 22 pages, 6 figures, accepted for publication in PR

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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.

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