Speaker
Description
The neutron-removal reaction $^{36}$S($p,d$)$^{35}$S was studied at iThemba LABS up to an excitation energy of E$_x = 16$ MeV to investigate the effectiveness of the $N = 20$ shell closure and examine the neutron $d_{5/2} - d_{3/2}$ spin-orbit splitting in $^{36}$S. An unexpected and pronounced $j$-dependence of the cross-section angular distributions at forward angles enabled the study of spin-orbit splitting using cross section measurements alone.
The results indicate that the splitting between the reconstructed $d_{5/2}$ and $d_{3/2}$ single-particle spin-orbit partners increases from $^{36}$S to $^{40}$Ca, contrary to the generally observed trend predicting a decrease of approximately $\sim$450 keV. This atypical splitting offers a valuable test case for exploring the role of the tensor force, particularly because the neutron-proton tensor force counterbalances the spin-orbit force as protons occupy the $1d_{3/2}$ orbital. These findings provide critical data to constrain state-of-the-art theoretical models, especially in evaluating the proton-neutron tensor component's impact.
