Progress on nuclear shell evolution in neutron-rich region around 132Sn

The disappearance of traditional magic numbers and the generation of new magic numbers in weakly-bound nuclei have always been the frontier of physics with radioactive beam. In recent years, the research based on large scientific devices around the world has got fruitful results. In the light neutron-rich region far away from the beta-stable line, it has been found that the traditional neutron magic numbers 8, 20, 28 disappear and the new neutron magic numbers 14, 16, 34 appear. In the medium and heavy neutron-rich regions, experimental and theoretical studies have shown that the traditional neutron magic numbers 50 and 82 tend to weaken or disappear with the increase of the neutron/proton ratio. For example, recent experimental studies have shown that the N=82 shell closure in Sn-132, Cd-130, and Pd-128 isotones is still robust, but a significant reduction of the N=82 gap was suggested to occur between Sn and Zr as a consequence of the absent Z=40 subshell gap. With the Z=47, the neutron-rich odd-A Ag isotopes are naturally of great interest. Their valence protons are assumed to fill the pi g(9/2) and pi p(1/2) orbitals, between which the Z=40 subshell gap is formed. The energy difference between the lowest lying 9/2(+) and 1/2(-) states in these neutron-rich odd-A Ag isotopes provides direct information on the Z=40 subshell gap. Therefore, to explore the N=82 shell evolution in Ag isotopes, the beta-delayed gamma-ray spectroscopy of neutronrich Ag-123,Ag-125 isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN in the framework of the Euroball RIken Cluster Array project. The long-predicted 1/2(-) beta-emitting isomers in Ag-123,Ag-125 are identified for the first time. Shellmodel calculations have been performed using the KSHELL code with the state-of-the-art monopole-based universal interaction V-MU plus a spin-orbit force from M3Y (V-MU+LS), and give an overall satisfactory description of experimental levels in Ag-123,Ag-125, particularly for the low-lying levels. The calculations indicate that, as approaching N=82, the wave functions from the pi g(9/2) and pi p(1/2) orbitals dominate low-lying states of Ag isotopes. With the newly observed 1/2(-) isomeric states in Ag-123,Ag-125, the systematic energy difference between the lowest 9/2(+) and 1/2(-) states along Ag isotopic chain indicates an increasing trend beyond 125Ag, which reveals that the Z=40 subshell gap starts to be restored toward N=82. Extrapolating the trend toward N=82, a considerable diminishment of the Z=40 subshell gap is expected. To get more insight into the microscopic origin of shell evolution in this region, the effective single-particle energies are calculated for the proton orbitals in the region of N=68-82 with the V-MU+LS interaction. It is found that, if only the central + spin-orbit parts are considered, the pi p(1/2) orbital lies below the pi g(9/2) orbital in the whole region of N=68-82. In contrast, with the inclusion of the tensor part (especially the pi g(9/2)-nu h(11/2) monopole), the pi g(9/2) orbital is affected much more than the pp1/ 2 orbital, and the spacing between pi g(9/2) and pi p(1/2) orbitals is notably reduced, consequently, resulting in the inversion of these two orbitals at N similar to 74, which is compatible with the experimental inversion position. The tensor force manifests its crucial role in the modification of the order of the proton orbitals and the size of the Z=40 subshell gap in Ag isotopes mainly through the pi g(9/2)-nu h(11/2) monopole. Vice versa, our calculations indicate that this tensor force will also influence the behavior of the nu h(11/2) orbital, which is important for the N=82 shell gap.