Halo structure ofBe14in a microscopicBe12+n+ncluster model

The ${}_{}{}^{14}{}_{}{}^{}\mathrm{Be}$ nucleus is investigated in the three-cluster generator coordinate method, involving several ${}_{}{}^{12}{}_{}{}^{}\mathrm{Be}$+n+n configurations. The ${}_{}{}^{12}{}_{}{}^{}\mathrm{Be}$ core nucleus is described in the harmonic oscillator model with all possible configurations in the p shell. We present the theoretical energy spectrum of ${}_{}{}^{14}{}_{}{}^{}\mathrm{Be}$ 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 ${}_{}{}^{12}{}_{}{}^{}\mathrm{Be}$ core is obtained, in agreement with experiment. Our calculation indicates that the ${}_{}{}^{12}{}_{}{}^{}\mathrm{Be}$ (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 ${}_{}{}^{12}{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{Be}$, and ${}_{}{}^{14}{}_{}{}^{}\mathrm{Be}$ nuclei is performed with identical conditions of calculation. We also analyze dipole and quadrupole excitations of the ${}_{}{}^{14}{}_{}{}^{}\mathrm{Be}$ ground state, and show that a significant part of the sum rules for soft modes is exhausted at low excitation energies.