Core excitation in three-body systems: Application to Be-12

Core excitation is included in the three-body hyperspherical formulation for systems of the type (core + n + n). We show that core excitation generates a three-body attraction, improving on the common underbinding problem present in many inert-core models. The Pauli principle significantly influences both the binding energies and the wave functions. A study of the effect of the different terms of the nn interaction on the three-body system is presented. The model is applied to Be-12, where one would expect that the inert-core approximation would not be appropriate. A deformed pi-core interaction is determined by the properties of the subsystem Be-11. Results for the binding energy, r.m.s. radius, structure and momentum distribution of the ground state of Be-12 are presented, and very good agreement with experimental data is found. A full comparison with the inert-core model is made. The results with 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 model 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 excited states is calculated and compared with the measured values and some discrepancies are found. Following a recent proposal for an experiment, predictions for the Be-12(p,d) transfer cross sections are given.