Reduction of the Spin-Orbit Splittings at the N=28 Shell Closure

Abstract : The N=28 shell closure has been investigated via the 46Ar(d,p)47Ar transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron p3/2, p1/2, and f5/2 states in 47Ar were determined and compared to those of the 49Ca isotone. We deduced a reduction of the N=28 gap by 330(90) keV and spin-orbit weakenings of ~=10(2) and 45(10)% for the f and p states, respectively. Such large variations for the f and p spin-orbit splittings could be accounted for by the proton-neutron tensor force and by the density dependence of the spin-orbit interaction, respectively. This contrasts with the picture of the spin-orbit interaction as a surface term only.