Proton Shell Evolution below Sn-132: First Measurement of Low-Lying beta-Emitting Isomers in Ag-123,Ag-325

Abstract

The β -delayed γ-ray spectroscopy of neutron-rich 123, 125Ag isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN, and the long-predicted 1/2− β-emitting isomers in 123,125Ag 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 123,125Ag. 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 123Ag 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. Show more