CORE EXCITATION IN 3-BODY SYSTEMS - APPLICATION TO BE-12

Core excitation is included in the three-body hyperspherical formulati on for systems of the type (core + n + n). We show that core excitatio n generates a three-body attraction, improving on the common underbind ing problem present in many inert-core models. The Pauli principle sig nificantly influences both the binding energies and the wave functions . A study of the effect of the different terms of the nn interaction o n the three-body system is presented. The model is applied to Be-12, w here one would expect that the inert-core approximation would not be a ppropriate. A deformed pi-core interaction is determined by the proper ties of the subsystem Be-11. Results for the binding energy, r.m.s. ra dius, structure and momentum distribution of the ground state of Be-12 are presented, and very good agreement with experimental data is foun d. A full comparison with the inert-core model is made. The results wi th core excitation show a very small component of (p(1/2))(2) neutrons in the ground-state wave function, in contrast to the inert-core mode l where the Be-12 ground state is mainly (p(1/2))(2) neutrons relative to the core. We point out the possible implications of this fact for heavier Be isotopes, especially Be-13. The spectrum of Be-12 bound exc ited states is calculated and compared with the measured values and so me discrepancies are found. Following a recent proposal for an experim ent, predictions for the Be-12(p,d) transfer cross sections are given.