Reduction of the spin-orbit splittings at the N=28 shell closure

The N 28 shell closure has been investigated via the 46 Ar d;p 47 Ar transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron p 3 = 2 , p 1 = 2 , and f 5 = 2 states in 47 Ar were determined and compared to those of the 49 Ca 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