A universal fate for spin-orbit partners in the weak-binding regime?

A growing body of experimental data on single-particle energies in weakly bound systems has materialized over the past decade or so. The most illuminating of these data has been for the neutron-rich nuclei around N = 20 and 28. A smooth decrease in the separation of the 2p3/2-2p1/2 spin-orbit splittings is observed as they approach zero binding, which is at odds with the well-accepted scaling, with mass, of measured spin-orbit splittings across the chart of nuclides for well-bound states as established by Mairle [Phys. Lett. B 304, 39 (1993)]. In studying the contrast between experimental data, the mean-field descriptions, and the trends established by Mairle for both well-bound and weakly bound systems across the nuclear chart, with a focus on neutron spin-orbit partners, a seemingly universal behavior emerges that could prove predictive. Many of the regions explored have connections to other prominent topics in nuclear physics, such as r-process nucleosynthesis, where the ordering of single-particle energies near threshold in weakly bound systems plays a role in reaction rates. Throughout, I will highlight one of the principal techniques used to extract such data: the solenoidal spectrometer technique, now used at ATLAS, CERN, and FRIB. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract Number DE-AC02-06CH11357.