Isomerism in the “south-east” of 132Sn and a predicted neutron-decaying isomer in 129Pd

Abstract

Excited states in neutron-rich nuclei located south-east of 132Sn are investigated by shell-model calculations. A new shell-model Hamiltonian is constructed for the present study. The proton–proton and neutron–neutron interactions of the Hamiltonian are obtained through the existing CD-Bonn G matrix results, while the proton–neutron interaction across two major shells is derived from the monopole based universal interaction plus the M3Y spin–orbit force. The present Hamiltonian can reproduce well the experimental data available in this region, including one-neutron separation energies, level energies and the experimental B(E2) values of isomers in 134,136,138Sn, 130Cd, and 128Pd. New isomers are predicted in this region, e.g. in 135Sn, 131Cd, 129Pd, 132,134In and 130Ag, in which almost no excited states are known experimentally yet. In the odd–odd 132,134In and 130Ag, the predicted very long E2 life-times of the low-lying 5− states are discussed, demanding more information on the related proton–neutron interaction. The low-lying states of 132In are discussed in connection with the recently observed γ rays. The predicted 19/2− isomer in 129Pd could decay by both electromagnetic transitions and neutron emission with comparable partial life-times, making it a good candidate for neutron radioactivity, a decay mode which is yet to be discovered.