Important role of the spin-orbit interaction in forming the 1/2+ orbital structure in Be isotopes
Abstract
The structure of the second 0+ state of 10Be is investigated using a microscopic α+α+n+n model based on the molecular-orbit (MO) model. The second 0+ state, which has dominantly the (1/2+)2 configuration, is shown to have a particularly enlarged α-α structure. The kinetic energy of the two valence neutrons occupying along the α-α axis is reduced remarkably due to the strong α clustering and, simultaneously, the spin-orbit interaction unexpectedly plays important role to make the energy of this state much lower. The mixing of states with different spin structure is shown to be important in negative-parity states. The experimentally observed small-level spacing between 1- and 2- (~ 300 keV) is found to be an evidence of this spin-mixing effect. 12Be is also investigated using α+α+4n model, in which four valence neutrons are considered to occupy the (3/2-)2(1/2+)2 configuration. The energy surface of 12Be is shown to exhibit similar characteristics, that the remarkable α clustering and the contribution of the spin-orbit interaction make the binding of the state with (3/2-)2(1/2+)2 configuration properly stronger in comparison with the closed p-shell (3/2-)2(1/2-)2 configuration.
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