Structure of Light Neutron-Rich Nuclei and Important Roles of Tensor Interaction

Structure of light neutron-rich nuclei are studied by shell model calculations. Our shell model Hamiltonian for the p-shell, SFO [1], which takes into account important roles of the tensor interaction, can explain well the Gamow-Teller (GT) transitions in 14C and 12C and magnetic moments of p-shell nuclei as well as shell evolutions toward drip-lines [2]. Its modified version, SFO-tls [3], is applied to study structure of neutron-rich nuclei such as C isotopes. Characteristic behavior of the effective neutron single-particle energies of the1s1/2, 0d5/2 and 0d3/2 orbits as well as low-lying states, that is, the change of the shell structure toward the neutron-rich region is discussed. The anomalous quenching of the Ml transition strength in 17C [4] is found to be well explained by taking full account of the tensor interaction [3]: the tensor components of the π + ρ meson exchange model are used for the p-sd cross shell matrix elements. Corrections are made also for the monopole terms in the isospin T= 1 channel. We discuss and compare with the monopole terms from three-nucleon forces induced by Δ excitations. GT transitions in 19C → 19N are also studied. Relatively large strengths found in the energy region as low as Ex = 6∼7 MeV in 19N [5] are rather well explained by SFO-tls. The halo nature of the neutron 1s1/2 orbit in 19C is investigated from the GT strength of the retarded transition to the 1/2+ (2.14 MeV) state in 19N [5]. The halo is shown to be consistent with the one with the separation energy of 500∼600 keV obtained from the Coulomb dissociation of 19C [6]. Structure and transitions in neutron-rich B isotopes will be also discussed. Recently, a reduction of the tensor force due to the medium modifications based on the Brown-Rho scaling is suggested from the shell model study of the extremely small GT strength in the transition, 14C → 14N (1+gs) within the two-hole configurations [7]. We will show, on the contrary, based on our Hamiltonians that the strong tensor force of the π + ρ meson exchanges is indispensable to get the vanishing of the GT strength. The tensor interaction, thus, plays important and essential roles in the structure of both neutron-rich and stable nuclei.(author)