Important role of the spin-orbit interaction in forming the 1/2(+) orbitalstructure in Be isotopes - art. no. 034301

The structure of the second 0(+) state of Be-10 is investigated using a mic roscopic alpha+alpha+n+n model based on the molecular-orbit (MO) model. The second 0(+) state, which has dominantly the(1/2(+))(2) configuration, is s hown to have a particularly enlarged alpha-alpha structure. The kinetic ene rgy of the two valence neutrons occupying along the alpha-alpha axis is red uced remarkably due to the strong alpha clustering and, simultaneously, the spin-orbit interaction unexpectedly plays an important role in making the energy of this state much lower. The mixing of states with different spin s tructure is shown to be important in negative-parity states. The experiment ally observed small-level spacing between 1(-) and 2(-) (similar to 300 keV ) is found to be evidence of this spin-mixing effect. Be-12 is also investi gated using the alpha+alpha+4n model, in which four valence neutrons are co nsidered to occupy the (3/2(-))(2)(1/2(+))(2) configuration. The energy sur face of Be-12 is shown to exhibit similar characteristics, that the remarka ble alpha clustering and the contribution of the spin-orbit interaction mak e the binding of the state with the (3/2(-))(2)(1/2(+))(2) configuration pr operly stronger in comparison with the closed p-shell (3/2(-))(2)(1/2(-))(2 ) configuration.