E3 strength of the 11- to 8+ isomeric decays in Pb194 and Pb196 and oblate deformation

The E3 decays of the 11- isomers in the isotopes Pb194 and Pb196 have been studied experimentally and evaluated in terms of configuration mixing due to the development of oblate deformation. New results include a remeasurement of the lifetime of the 11- isomer in Pb194 and clarification of the intensities of its main decay branches including the known 496 keV E3 branch. Its intensity is an order of magnitude weaker than previously reported, leading to an E3 transition strength of 29(4) W.u. Limits are placed on possible E3 decays of the 11- isomer in Pb196 to a previously assigned 8+ two-proton state. Neither the branch, nor the state is observed. An alternative 562 keV transition to a new state at 2630 keV is proposed, with an E3 strength of 26(2) W.u. The approximately constant E3 strength for the range of even-even isotopes Pb190-196 is consistent with an oblate deformation for the 11- state of similar magnitude. This is supported by K-constrained potential-energy-surface calculations for the 11- and 8+ configurations.