H. Esbensen

Pair correlations near the neutron drip line

Abstract A theory of pairing in weakly bound nuclei is presented. The nucleus is treated as a three-body system consisting of two interacting nucleons together with a structureless core. The pairing interaction is modelled by a density-dependent contact interaction. It is constrained to the free nucleon interaction at low density. Numerically, the Hamiltonian equation is solved by a two-particle Greens function method in coordinate space. Given the single particle resonance energy of 10 Li, the theory reproduces the marginal binding of 11 Li. The large electric dipole stength in 11 Li found experimentally is also accounted for. The nucleus 14 Be is also found to be bound.

Breakup reactions of the halo nuclei Be-11 and B-8

We calculate the nuclear induced breakup of

Effects of E2 transitions in the Coulomb dissociation of 8B

^{11}\mathrm{Be}

Soft dipole excitations in 11Li

and

Nuclear breakup of Borromean nuclei

^{8}\mathrm{B}

The (p,n) reaction and the nucleon-nucleon force

using a more realistic treatment of the diffraction and stripping processes than in previous work. The breakup is treated in the eikonal approximation with a profile function calculated from a realistic optical potential at low energies and from free nucleon-nucleon cross sections at high energies. This treatment gives a good description of measured breakup cross sections, as well as the longitudinal momentum distribution of the corelike fragments, which is narrower than predicted in the transparent limit. The real part of the potential is found to be significant and enhances the diffractive breakup at low energies. \textcopyright{} 1996 The American Physical Society.

Hindrance of heavy-ion fusion due to nuclear incompressibility

We examine the Coulomb dissociation of aB in a model that has both E1 and E2 matrix elements. We find that the interference between El and E2 amplitudes produces large asymmetries in the angular and momentum distributions of the emitted protons and 7Be fragments. By measuring these asymmetries one may be able to put constraints on the E2 component and thereby improve the accuracy of the E1 strength that can be extracted from Coulomb dissociation experiments. We also investigate the effect of higher-order dynamical processes and find that they reduce the asymmetries in reactions on high-Z targets. They also reduee the effect of E2 transitions on the dissociation probability and on the peak height of the decay energy spectrum compared to predictions of first-order perturbation theory.

Higher-order dynamical effects in Coulomb dissociation

Abstract A three-body model of 11Li is extended to include all interactions in unbound states in the continuum. We use a Green function technique to solve the three-body hamiltonian equation, and study the continuum dipole states produced by electromagnetic excitations of the ground state. The final-state interaction modifies the dipole strength function substantially, making it similar to an independent-particle strength function, but the total strength is enhanced by 50% due to ground-state correlations. The dipole strength is concentrated in a peak just above threshold, and the strength distribution is consistent with the measured beam energy dependence of the Coulomb dissociation cross section. This threshold peak also gives a narrow component in the neutron and the residual nucleus momentum distributions. The angular distributions of the neutrons emitted in Coulomb-induced reactions show a surprising anticorrelation, favoring emission with a large opening angle between the directions of the two neutrons in the rest frame of 11Li.

Barkas effect in a dense medium: Stopping power and wake field

We study the eikonal model for the nuclear-induced breakup of Borromean nuclei, using {sup 11}Li and {sup 6}He as examples. The full eikonal model is difficult to realize because of six-dimensional integrals, but a number of simplifying approximations are found to be accurate. The integrated diffractive and one-nucleon stripping cross sections are rather insensitive to the neutron-neutron correlation, but the two-nucleon stripping does show some dependence on the correlation. The distribution of excitation energy in the neutron-core final state in one-neutron stripping reactions is quite sensitive to the shell structure of the halo wave function. Experimental data favor models with comparable amounts of s and p waves in the {sup 11}Li halo. {copyright} {ital 1998} {ital The American Physical Society}

Influence of Nuclear Structure on Sub-Barrier Hindrance in Ni + Ni Fusion

The (p,n) charge exchange reaction is a powerful tool of nuclear structure physics, with spectroscopic characteristics that are closely related to the free interaction between nucleons. At proton energies in the range of 150-500 MeV, the interaction probes the spin dynamics in the charge exchange process and is particularly sensitive to nuclear pionic fields. At low energies, say less than 50 MeV bombarding energy, the reaction also probes the isovector density. An outstanding success of the reaction as a structural probe is the elucidation of the Gamow-Teller strength function in the nuclear excitation spectrum. However, the total strength found falls short of sum rule predictions by about 40%. Explanations of this quenching have been advanced along two lines, based on subnuclear degrees of freedom or on configuration mixing into high continuum states. Detailed theoretical arguments support the importance of configuration mixing. The subnuclear degrees of freedom may be comparable, but a decisive test is lacking.