Shell structure of the A = 6 ground states from three-body dynamics

Three-body (..cap alpha..NN) models of the /sup 6/He and /sup 6/Li ground states are used to investigate their shell structure. Three models for each nucleus are considered: simple, full (nn), and full (np) for /sup 6/He, and simple, full (0%), and full (4%) for /sup 6/Li. The full models in both cases are obtained by including the S/sub 1/2/, P/sub 1/2/, and P/sub 3/2/ partial waves of the ..cap alpha..N interaction, whereas the simple model truncates to only the strongly resonant P/sub 3/2/ wave. The /sup 6/He full models distinguish between use of the nn or np parameters for the /sup 1/S/sub 0/ NN interaction, while the /sup 6/Li full models have either a pure /sup 3/S/sub 1/ NN interaction (0%) or a /sup 3/S/sub 1/-/sup 3/D/sub 1/ interaction that leads to a 4% d-wave component in the deuteron (4%). These models are used to calculate the probabilities of the orbital components of the wave functions, the configuration-space single-particle orbital densities, and the configuration-space two-particle wave function amplitudes in j-j coupling with the nucleon coordinates referred to the alpha particle as the ''core'' or ''center of force.'' The results are then compared with those from phenomenological and realistic-interaction shell models. Major findings of the comparison are the following: None of the shell models considered have a distribution of orbital probabilities across shells like that predicted by three-body models; the orbital rms radii from three-body models indicate an ordering of the orbits within shells, i.e., p/sub 1/2/ outside p/sub 3/2/, unlike oscillator shell models with a single oscillator parameter where the p-shell orbitals have the same shape; and, as expected, three-body orbital densities decay at large radial distances as exponentials rather than the too compact Gaussian falling off of oscillator shell models.