Effect of oblate deformation on E3 strengths in light lead and polonium isotopes

The multipolarities of the main transitions deexciting yrast isomers in 190Pb have been measured using conversion electron spectroscopy with pulsed beams. The enhanced E3 character of the 407 keV transition from the 11- isomer is confirmed and an independent value obtained for its lifetime. A new measurement of the properties of the 440 keV E3 decay from the corresponding 11- state in 192Pb, has resulted in the identification of a new branch and a significantly different lifetime from that reported previously. The abnormally large strength of the 11- to 8+ transitions in both 190Pb and 192Pb match those known in some light polonium isotopes, an effect which is suggested to be linked to a change in structure of the orbitals in the 8+ configuration when oblate deformation is established; the lowest Ω=7/2- proton Nilsson orbital changes from being, predominantly of h9/2 character at sphericity to a mixed f7/2/h9/2 parentage at oblate deformations, thus enabling an enhanced i13/2 to f7/2 octupole-coupled transition