Evidence for collective oblate behavior in ${}_{}{}^{196}{}_{}{}^{}\mathrm{Pb}$ is presented. One irregular and two regular bands of M1 transitions have been observed following the ${}_{}{}^{170}{}_{}{}^{}\mathrm{Er}$${(}^{30}$Si,4n) and ${}_{}{}^{176}{}_{}{}^{}\mathrm{Yb}$${(}^{26}$Mg,6n) reactions. Transitions linking the most intense regular band to the low-lying negative-parity yrast levels are observed, establishing excitation energies, spins, and probable parities of the band members. In contrast, no such transitions have been found for the irregular band and the weaker regular band. The bands are interpreted as corresponding to collective oblate rotation, arising mainly from deformation-aligned high-j, shape-driving quasiproton excitations across the Z=82 shell gap, coupled to rotation-aligned quasineutrons.

• Received 23 December 1992

DOI:https://doi.org/10.1103/PhysRevC.47.R1337