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

Identification of the levels in Sm-160 and Gd-162 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 = -DeltaJ(1)/J(1), so J(1a)(1+kappa)= J(1b), 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 congruent to 0). As examples, we find for Sm-158 - Gd-160 (from 2(+) to 10(+)), kappa = -3.2(-0.2)(+0.1)% (where the +/- is the total spread in kappa from -3.1 to -3.4) and DeltaJ(2)/J(2) = 3.0+/-1.2%; Sm-158 - Sm-160, 3.4(-0.3)(+0.5)%; and Nd-154 - Gd-158, -10.4+/-0.2%. After the shifts, these J(1) and J(2) values have smaller spreads than do the outstanding examples of identical bands. The J(1) values were fitted to a variable moment of inertia model with parameters J(0) and C whose values correlate with the SIB J(1) 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.