Proton Shell Evolution below $^{132}\mathrm{Sn}$: First Measurement of Low-Lying $\beta$-Emitting Isomers in $^{123,125}\mathrm{Ag}$

The β-delayed γ-ray spectroscopy of neutron-rich Ag123,125 isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN, and the long-predicted 1/2− β-emitting isomers in Ag123,125 are identified for the first time. With the new experimental results, the systematic trend of energy spacing between the lowest 9/2+ and 1/2− levels is extended in Ag isotopes up to N=78, providing a clear signal for the reduction of the Z=40 subshell gap in Ag towards N=82. Shell-model calculations with the state-of-the-art VMU plus M3Y spin-orbit interaction give a satisfactory description of the low-lying states in Ag123,125. The tensor force is found to play a crucial role in the evolution of the size of the Z=40 subshell gap. The observed inversion of the single-particle levels around Ag123 can be well interpreted in terms of the monopole shift of the π1g9/2 orbitals mainly caused by the increasing occupation of ν1h11/2 orbitals.