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 h_9/2 character at sphericity to a mixed f_7/2/h_9/2 parentage at oblate deformations, thus enabling an enhanced i_13/2 to f_7/2 octupole-coupled transition.