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

To test a multicluster approach for halo nuclei, we give a unified des cription for the ground states of He-6 and He-8 in a model comprising an alpha-cluster and single-neutron clusters. The intercluster wave fu nction is taken a superposition of terms belonging to different arrang ements, each defined by a set of Jacobi coordinates. Each term is then a superposition of products of gaussian functions of the individual J acobi coordinates with different widths, projected to angular momenta l = 0 or 1. This prescription defines a trial function with linear var iational parameters. For He-6, we were able to use enough terms to pro duce ground-state energies that are virtully exact within the subspace s defined by the arrangements and l-values, and used this nucleus as a testing ground for stochastic methods to choose trial functions. Tria l functions consisting of selectively chosen random terms were found t o yield excellent numerical convergence to this ''exact'' value with s ignificantly fewer terms. For the description of He-8 to be feasible, such a dimensional reduction is indispensable. For He-8 good energy co nvergence was achieved in a state space comprising three arrangements with all l = 0, and there are indications showing that the contributio ns of other subspaces are likely to be small. The He-6 and He-8 energi es are reproduced by the same effective force very well, and the matte r radii obtained are similar to those of other sophisticated calculati ons.