Effect of oblate deformation on E 3 strengths in light lead and polonium isotopes G. D. Dracoulis, 1 T. Kibe ´ di, 1 A. P. Byrne, 1,2 A. M. Baxter, 2 S. M. Mullins, 1 and R. A. Bark 1 1 Department of Nuclear Physics, RSPhysSE, Australian National University, Canberra ACT 0200, Australia 2 Department of Physics and Theoretical Physics, Faculty of Science, Australian National University, Canberra ACT 0200, Australia Received 30 October 2000; published 1 May 2001 The multipolarities of the main transitions deexciting yrast isomers in 190 Pb have been measured using conversion electron spectroscopy with pulsed beams. The enhanced E 3 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 E 3 decay from the corresponding 11 - state in 192 Pb, has resulted in the identification of a new branch and a significantly different lifetime from that reported previously. The abnor- mally large strength of the 11 - to 8 + transitions in both 190 Pb and 192 Pb 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 =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. DOI: 10.1103/PhysRevC.63.061302 PACS numbers: 21.10.Tg, 21.10.Hw, 27.80.+w, 21.90.+f Shape coexistence in the neutron-deficient lead nuclei is calculated to take the form of a potential well with three minima at low spin 1,2 corresponding to spherical, oblate, and prolate shapes, dependent on the occupation of specific orbitals, and excitations across the Z =82 core 2. A variety of spectroscopic evidence for the existence of such minima has been suggested for 186 Pb, 188 Pb, and 190 Pb 3–5 and involves, in 190 Pb, long-lived yrast isomers associated with the proposed 11 - oblate and 12 + spherical configurations 4. To substantiate these assignments, conversion coeffi- cients for the main decays have now been measured, exploit- ing pulsed beam techniques to obtain high sensitivity. It was noted earlier that the enhanced E 3 nature of the main decay 407 keV from the 11 - isomer was only partially consistent with its proposed configuration 4, being considerably more enhanced than expected, consequently independent measure- ments of the 11 - and 12 + state lifetimes have also been carried out. Since the properties of the equivalent E 3 transi- tion 440 keV in 192 Pb extant in the literature 6,7 are also problematic corresponding to a transition strength of 220 W.u., a value which is clearly unsupportable, a new study of that nucleus has also been carried out, to confirm or correct the level scheme as necessary, and to determine more pre- cisely the lifetime and branching ratios for the 11 - isomer. The implications of the E 3 enhancements finally deter- mined are reexamined, especially since equivalent transition- strength anomalies have been observed, but not explained, in the polonium isotopes of similar neutron number 8,9. We propose that the abnormal enhancement is likely a manifes- tation of oblate deformation in the 11 - and 8 + two-proton states, and provides, correspondingly, evidence for the pres- ence of shape coexistence. The measurements for 190 Pb were carried out using the superconducting solenoidal electron spectrometer described by Kibe ´ di et al. 10. In its lens-mode of operation, electrons of a specific momentum range are transported to a cooled SiLi detector, and the magnetic field is ramped between upper and lower limits to maximize the efficiency for the energy range of interest, in this case to encompass electrons corresponding to conversion of the main delayed transitions in 190 Pb. The target arrangement used was a 0.9 mg/cm 2 foil enriched in 166 Er placed on a 1.9 mg/cm 2 gold foil, with its plane inclined at an angle of 30° to the beam axis. The solenoid axis is fixed at 90° to the beam axis and electrons are observed through the rear of the gold foil. The target and backing foil thicknesses were chosen to minimize energy straggling of the emerging electrons. Gamma rays were de- tected in a Compton-suppressed Ge detector placed at about 125° to the beam axis. The 140 MeV 28 Si beam from the ANU 14UD Pelletron was chopped to produce 8.5  s beam bursts separated by 115 s. Both electrons and  rays were recorded together with their times with respect to the beam obtained by interrogating an 8192-channel ADC, counting cycles of a 4 MHz clock. Gamma rays and electrons within the beam bursts were blocked with a fast veto. The event-by- event data, which included a measure of the instantaneous magnetic field, were subsequently sorted to produce  -time or electron-time matrices for analysis. The upper panel of Fig. 1 shows the spectrum of  rays in a 0–50  s period after each beam burst, with  rays in the subsequent 50  s period subtracted, conditions which are effective in isolating transitions from the decay of the two long isomers in 190 Pb. The lower panel of Fig. 1 shows the corresponding electron spectrum constructed in the same way, with the K-, L-, and M-lines marked. Conversion coef- ficients were obtained from the ratios of the efficiency- corrected intensities observed in these spectra. One impor- tant result can be deduced, essentially by inspection of Fig. 1; the K-conversion coefficient for the 407 keV line is, in comparison with the 455 keV line which is shown later to be E 2, a factor of 3 larger. Further, the L-conversion line for the 407 keV transition is as strong as its K-conversion line, a feature characteristic of E 3 transitions in this energy regime. Quantitative results are shown in Table I. Although not listed, M-conversion coefficients were also extracted. Limits on the total conversion coefficient for the 407 keV transition and consideration of the possible transition strengths were already sufficient to eliminate M 2 or M 3 or higher multi- RAPID COMMUNICATIONS PHYSICAL REVIEW C, VOLUME 63, 061302R 0556-2813/2001/636/0613025/$20.00 ©2001 The American Physical Society 63 061302-1