A new phenomenon: shifted identical yrast bands in neighboring even-even nuclei

Abstract

Identification of the levels in 160Sm and 162Gd in spontaneous fission studies led to the discovery of a new phenomenon, shifted identical bands (SIB). SIBs are yrast bands in neighboring nuclei (a,b) with moments of inertia which are identical when shifted by a constant amount [kappa] = -[Delta]J1/J1, so J1a(1+[kappa])= J1b, from 2+ to 8+ and higher to 16+. Building on that work, an analysis of yrast bands in even-even proton to neutron rich Xe to Pb nuclei was carried out. In over 700 comparisons, fifty-five SIBs were found for ground bands in stable to the most neutron rich Ce - W nuclei with [kappa] between 1.5% and 13%, where the spread in [kappa] is less than ±1%, and only four identical bands ([kappa][approximate, equals]0). As examples, we find for 158Sm-160Gd (from 2+ to 10+), [kappa] = -3.2+0.1-0.2% (where the ± is the total spread in [kappa] from -3.1 to -3.4) and [Delta]J2/J2 = 3.0±1.2%; 158Sm-160Sm, 3.4+0.5-0.3%; and 154Nd-158Gd, -10.4±0.2%. After the shifts, these J1 and J2 values have smaller spreads than do the outstanding examples of identical bands. The J1 values were fitted to a variable moment of inertia model with parameters J0 and C whose values correlate with the SIB J1 values. The SIBs are not correlated either with deformation, E(4+)/E(2+), or the NpNn product. Some excited bands are SIBs and IBs in proton rich Pt-Pb where nuclear shape coexistence is important.