Neutron-hole strength in N = 81 nuclei

A systematic study of neutron-hole strength in the N = 81 nuclei Ba-137, Ce-139, (141) Nd, and Sm-143 is reported. The single-neutron removal reactions (p, d) and (He-3, alpha) were measured at energies of 23 and 34 MeV, respectively. Spectroscopic factors were extracted from measured cross sections through a distorted-wave Born approximation analysis and centroids of single-particle strength have been established. The change in these centroid energies as a function of proton number have been compared to calculations of the monopole shift for the s(1/2) and h(11/2) orbitals, where the majority of the strength has been observed. Significant fragmentation of strength was observed for the d and g(7/2) orbitals, particularly for the latter orbital which is deeply bound, with summed strengths that indicate a significant amount lies outside of the measured excitation energy range.