Progress on seniority isomers in the nuclei near ¹³²Sn and <italic>N</italic>=82 shell evolution

The investigation of shell evolution far from the β-stability line has been a forefront research topic in nuclear physics. It has been found that the magic numbers are not immutable in some mass regions, where some of the magic numbers disappear, meanwhile new ones appear. Recently, the south vicinity of 132Sn (Z=50, N=82) has attracted particular interest not only in the purely shell structure perspective but also in its strong astrophysical implications, especially in improving our understanding of the A=130 peak of the solar r-process abundance distribution. Theoretical studies have predicted a significant reduction or even complete disappearance of the N=82 shell closure as nuclei approach the neutron drip line. In experiments, isomeric states and their decay mechanism are often used to obtain information on shell gaps. Particularly, the existence of seniority isomers is an indication of a significant shell gap. The low energy of the deexciting transitions, as well as the B(E2) values following the characteristic U-shaped behavior, are good indications that a high-j orbital is well separated from the other ones. This paper reviews the progress in the study of seniority isomers observed in the nuclei 136,138Sn, 128Pd, 130Cd, 128Ag, and the evolution of the N=82 shell in the 132Sn region, which were performed at the Radioactive Isotope Beam Factory of RIKEN with the EURICA project. The 8+ seniority isomers (g9/22, v=2) were identified in 130Cd and 128Pd with B(E2) values following the expected U shape, which manifests that the N=82 shell closure in the neutron-rich Cd and Pd isotopes. The spectroscopic studies of the very neutron-rich odd-odd nucleus 128Ag have been performed by our group for the first time. A new seniority isomer with a half-life of 1.60(7) μs has been identified and is proposed to have a spin-parity of 16− with a maximally aligned configuration comprising three proton holes in the g9/2 orbital and one neutron hole in the h11/2 orbital. Shell model calculations have been performed with the state-of-the-art monopole-based universal interaction VMU plus a spin-orbit force from M3Y and using the nucleon-pair approximation. The new level structure in 128Ag is quite well described by shell model calculations without invoking excitations across the Z=50 and N=82 shell gaps, and presents a good case of seniority scheme in odd–odd nuclei in the south vicinity of the double-magic nucleus 132Sn. With a classification of various components of the proton–neutron interaction, the inversion of lowest-lying 9− and 10− states between 128Ag and its neighboring isotone 130In is found to be dynamically ascribed to the seniority-nonconserving proton–neutron interaction components in terms of rank-1 quasispin tensor operators. The structure above 10− up to the 16− isomer in 128Ag shows remarkable similarities to seniority structures in the semimagic nuclei 128Pd and 130Cd. These spectroscopic features in 128Ag indicate that the N=82 shell closure is still robust in silver isotopes and provide a new insight into the role played by the proton–neutron interaction in the structure of nuclei adjacent to the magic shell. To probe the evolution of the neutron N=82 shell further below 132Sn and the underlying mechanism, the seniority pattern in the lighter odd-Z nuclei near the N=82 is of particular interest in future studies.