Halo structure of 14Be in a microscopic 12Be+n+n cluster model

The 14Be nucleus is investigated in the three-cluster generator coordinate method, involving several 12Be+n+n configurations. The 12Be core nucleus is described in the harmonic oscillator model with all possible configurations in the p shell. We present the theoretical energy spectrum of 14Be up to 5 MeV excitation energy, and show that the matter densities support a halo structure of the ground state. A strong enhancement of the rms radius with respect to the 12Be core is obtained, in agreement with experiment. Our calculation indicates that the 12Be (g.s.)+n+n configuration represents 66% only of the total wave function, and that core excitations cannot be neglected. A comparative study of the 12Be, 13Be, and 14Be nuclei is performed with identical conditions of calculation. We also analyze dipole and quadrupole excitations of the 14Be ground state, and show that a significant part of the sum rules for soft modes is exhausted at low excitation energies