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We investigate the influence of the neutron halo and the breakup channel on the total 6 He+ 238 U fusion cross section at near-barrier energies. To include static effects of the 2n-halo in 6 He nuclei, we use a single-folding potential obtained from an appropriate nucleon-238 U interaction and a realistic 6 He density. Dynamical effects arising from the breakup process are then included through coupled-channel calculations. These calculations suggest that static effects dominate the cross section at energies above the Coulomb barrier, while the coupling to the breakup channel is more important at sub-barrier energies. The comparison of our calculations with recent data suggests that the coupling to other channels may be influencing the cross section at very low energies.

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Some commonly used approximations for complete fusion and breakup transmission coefficients in collisions of weakly bound projectiles at near barrier energies are assessed. We show that they strongly depend on the adopted classical trajectory and can be significantly improved with proper treatment of the incident and emergent currents in the WKB approximation.

We describe a schematic coupled channels transfer calculation for the reaction 6 He + 238 U at near-barrier energies. We also present a simple semiclassical DWBA calculation of the two neutron transfer. Both calculations are meant to supply under conditions at which the transfer cross section becomes much larger than the complete fusion one at sub-barrier energies. It seems that a feasible mechanism is the incoherent contributions of two or more processes with quite different Q-values. PACS numbers: Valid PACS appear hereCross-sections for fusion reactions with neutron halo nuclei in the energy region below the Coulomb barrier are necessary for calculating the thermonuclear reaction rates in massive stars. In addition, such reactions provide useful information about the shape of the nuclear potential on the inner side of interaction barrier. Furthermore, interpretation of such cross-sections may bring possible gain information on the influence of the distribution of nuclear matter and the nuclear reaction dynamics, especially for those energies where penetrability effects are important [1].Recently, nuclear reactions involving the neutron-rich nucleus 6 He have attracted considerable attention. In particular, very interesting experimental data on the fusion of He isotopes with 238 U have been obtained. These data show no enhancement of the 6 He+ 238 U fusion cross section, but a very high transfer cross section has been observed [2,3]. The physical process leading to this result has not yet been established. The natural candidates are the coupling with the breakup and transfer channels. However, understanding the effect of the neutron halo on fusion has been controversial, since the weakly bound neutrons in 6 He are expected to influence the fusion cross section in two ways. Firstly, by the static effect of barrier lowering due to the existence of a halo. Secondly through the coupling with the breakup channel. Also, in neutron halo nuclei reaction, the neutron transfer cross section should play an important role in sub-barrier fusion of heavy nuclei, due to the small binding energy of neutrons halo and the positive Q-value.Furthermore, reactions with stable nuclei, at energies below Coulomb barrier are caracterized by large enhancements in the fusion cross section with respect to calculations based on one dimensional barrier penetration models. It has been quite well understood that these enhancements are due to the coupling to different degres of freedom acting on the tunneling process, mainly static deformations and surface vibrations of nuclei. The role of transfer channels is however still unclear.In this short paper we describe a schematic coupled channels transfer calculation for the reaction 6 He + 238 U at near-barrier energies. We also present a simple semiclassical DWBA calculation of the two neutron transfer. Both calculations are meant to supply under conditions under which the transfer cross section becomes much larger than the complete fusion one at sub-barrier energies. In fact, such a situation s...

We study nuclear reactions in collisions of unstable projectiles with heavy targets. For this purpose, we use a simple approximation for the breakup channel and treat two-neutron halos as a single particle. We then evaluate cross sections for collisions of 6 He projectiles with targets of 238 U and 209 Bi. Comparing our results with recently measured fusion cross sections, we conclude that the large enhancement observed in 6 He+ 238 U fusion at sub-barrier energies cannot be explained by the coupling with the breakup channel. The effects of the halo on other reaction channels are also investigated. Coulomb-nuclear interference in the breakup channel is discussed.

We discuss the derivation of an equivalent l -independent polarization potential for use in the optical Schrödinger equation that describes the elastic scattering of heavy ions. Three diffferent methods are used for this purpose. Application of our theory to the low energy scattering of the halo nucleus 11 Li from a 12 C target is made. It is found that the notion of l -independent polarization potential has some validity but can not be a good substitute for the l -dependent local equivalent Feshbach polarization potential.