A new phenomenon—shifted identical bands and new region of stable octupole deformation

Summary From γ-γ-γ coincidence measurements in the spontaneous fission of 252 Cf, level energies were established for spins 14 + to 20 + in many neutron-rich nuclei. New isotopes 160 Sm and 162 Gd were identified. The yrast bands of 152,154,156 Nd, 156,158,160 Sm, and 160,162 Gd exhibit remarkably near-identical transition energies and moments of inertia which are shifted by the same constant amounts for every spin state from 2 + to 12 + or 14 + for nuclei differing by 2n, 4n, 2p, 4p, α, α+2p, and α+2n. These shifted identical bands (SIB) are a new phenomenon. Analysis of all known even-even proton- to neutron-rich nuclei from Ba ( Z = 56) to Os ( Z = 76) reveal no SIB for proton-rich nuclei and only a few additional cases of SIB for the most neutron-rich pairs around N = 98–102 separated by 2n and 2p. The levels in 8107 Mo were extended and levels in 109 Mo identified for the first time. Conversion coefficient measurements and level patterns help establish a new region of stable octupole deformation for 107,109 Mo with N = 65, 67, associated with the ν ( d 5/2 - h 11/2 ) orbitals. This is the first region based on only one pair of orbitals.