Neutron hole states in 131Sn and spin-orbit splitting in neutron-rich nuclei

Abstract In atomic nuclei, the spin-orbit interaction originates from the coupling of the orbital motion of a nucleon with its intrinsic spin. Recent experimental and theoretical works have suggested a weakening of the spin-orbit interaction in neutron-rich nuclei far from stability. To study this phenomenon, we have investigated the spin-orbit energy splittings of single-hole and single-particle valence neutron orbits of 132 Sn. The spectroscopic strength of single-hole states in 131 Sn was determined from the measured differential cross sections of the tritons from the neutron-removing 132 Sn(d, t) 131 Sn reaction, which was studied in inverse kinematics at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The spectroscopic factors of the lowest 3 / 2 + , 1 / 2 + and 5 / 2 + states were found to be consistent with their maximal values of ( 2 j + 1 ) , confirming the robust N = 82 shell closure at 132 Sn. We compared the spin-orbit splitting of neutron single-hole states in 131 Sn to those of single-particle states in 133 Sn determined in a recent measurement of the 132 Sn(d, p) 133 Sn reaction. We found a significant reduction of the energy splitting of the weakly bound 3 p orbits compared to the well-bound 2 d orbits, and that all the observed energy splittings can be reproduced remarkably well by calculations using a one-body spin-orbit interaction and a Woods–Saxon potential of standard radius and diffuseness. The observed reduction of spin-orbit splitting can be explained by the extended radial wavefunctions of the weakly bound orbits, without invoking a weakening of the spin-orbit strength.