Proton Shell Evolution below Sn 132: First Measurement of Low-Lying β -Emitting Isomers in Ag 123,125

6 pags., 5 figs.
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.
This work was carried out at the RIBF operated by RIKEN Nishina Center, RIKEN and CNS, University of Tokyo. Part of the WAS3ABi was supported by the Rare Isotope Science Project which is funded by MSIP and NRF of Korea. This work was supported by the National Key R&D Program of China (Contract No. 2018YFA0404403), the Natural Science Foundation of China under Grants No. 11775003, No. 11675003, No. 11775316, and No. 11875075, the National Research Foundation of Korea (No. NRF-2016R1A5A1013277 and No. NRF-2013M7A1A1075764), and the Spanish Ministerio de Economía y Competitividad under Contract No. FPA2017-84756-C4-2-P. This work was partially supported by JSPS KAKENHI (Grants No. 25247045 and No. 17H06090). Work at ANL is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357.