BOUND-STATE PROPERTIES OF BORROMEAN HALO NUCLEI - HE-6 AND LI-11

The nuclei He-6 and Li-11 which exhibit pronounced halo-structures wit h two loosely bound valence neutrons, are currently being explored as secondary-beam projectiles. These nuclei are Borromean, i.e. while the y are bound (only one bound state) they have, considered as three-body systems, no bound states in the binary subsystems. We argue that a th ree-body description is the natural one for central properties of such exotic loosely bound nuclei, and give the state of the art by compari ng fully blown three-body calculations for He-6 (and neighboring A = 6 nuclei) with a range of measured observables. We restrict this review to bound state properties, with emphasis on genuine three-body featur es. The bound state is the initial stage of the various reaction scena rios that now are being studied experimentally and a main objective of these studies. Currently used procedures for solving the three-body b ound state problem are outlined, with emphasis on expansions on hypers pherical harmonics and also the coordinate space Faddeev approach. Alt hough strict calculations can also be carried out for Li-11, they are inconclusive concerning the details of the structure since the availab le information on the binary neutron-Li-9(core) channel is insufficien t. Calculations for a number of plausible model interactions, includin g treatments of the Pauli principle, are presented. They all reproduce the binding energy and halo characteristics such as valence one-parti cle density and give about the same internal r.m.s. geometry for Li-11 . In spite of this, the wave functions have pronounced differences in their spatial correlations. The same ambiguity is also present in othe r inclusive observables, such as momentum distributions. We also demon strate that candidates for the nuclear structure can be explored withi n an approximate scheme COSMA. Predictions of exclusive observables ar e discussed, and quantities such as momentum correlations in complete measurements are found to be more sensitive to the detailed features o f the nuclear structure of the bound state.