The $N=28$ shell closure has been investigated via the ${}^{46}\mathrm{Ar}(d,p){}^{47}\mathrm{Ar}$ transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron $p_{3/2}$, $p_{1/2}$, and $f_{5/2}$ states in ${}^{47}\mathrm{Ar}$ were determined and compared to those of the ${}^{49}\mathrm{Ca}$ isotone. We deduced a reduction of the $N=28$ gap by 330(90) keV and spin-orbit weakenings of $\simeq 10(2)$ 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.

• Received 6 April 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.092501