MICROSCOPIC MULTICLUSTER DESCRIPTION OF NEUTRON-HALO NUCLEI WITH A STOCHASTIC VARIATIONAL METHOD

To test a multicluster approach for halo nuclei, we give a unified description for the ground states of He-6 and He-8 in a model comprising an alpha-cluster and single-neutron clusters. The intercluster wave function is taken a superposition of terms belonging to different arrangements, each defined by a set of Jacobi coordinates. Each term is then a superposition of products of gaussian functions of the individual Jacobi coordinates with different widths, projected to angular momenta l = 0 or 1. This prescription defines a trial function with linear variational parameters. For He-6, we were able to use enough terms to produce ground-state energies that are virtully exact within the subspaces defined by the arrangements and l-values, and used this nucleus as a testing ground for stochastic methods to choose trial functions. Trial functions consisting of selectively chosen random terms were found to yield excellent numerical convergence to this ''exact'' value with significantly fewer terms. For the description of He-8 to be feasible, such a dimensional reduction is indispensable. For He-8 good energy convergence was achieved in a state space comprising three arrangements with all l = 0, and there are indications showing that the contributions of other subspaces are likely to be small. The He-6 and He-8 energies are reproduced by the same effective force very well, and the matter radii obtained are similar to those of other sophisticated calculations.