Shell evolution in neutron-rich nuclei: the single particle perspective

The isospin dependence of spin-orbit (SO) splitting has become increasingly important as N/Z increases in neutron-rich nuclei. Following the initial independent-particle strategy towards explaining the occurrence of magic numbers, we have systematically investigated the isospin effect on the shell evolution of neutron-rich nuclei within the Woods-Saxon (WS) mean-field potential plus the spin-orbit term. It is found that new magic numbers N = 14 and N =16 may emerge in neutron-rich nuclei if one changes the sign of the isospin-dependent term in the spin-orbit coupling while the traditional magic number N = 20 may disappear. The magic number N = 28 is expected to be destroyed despite the sign choice of the isospin part in spin-orbit splitting, corresponding to the strength of the SO coupling term. Meanwhile, the N = 50 and 82 shells may persist within the single particle scheme, although there is the decreasing trend for their gaps towards the extreme proton-deficient nuclei. Besides, an appreciable energy gap appears at N = 32 and 34 in neutron-rich Ca isotopes. All these results are more consistent with those of the interacting shell model, when enhancing the strength of the SO potential in the independent particle model. The present study may provide a more reasonable starting point for not only the interacting shell model but also other nuclear many-body calculations towards the neutron-dripline of Segre chart.