New results on shape coexistence in the light Pb region

Experimental evidence for shape-coexisting configurations has been observed in Pb isotopes close to the neutron mid-shell. These structures intrude down to energies close to the spherical ground state and can be associated with intruder 2$p$-2$h$ and 4$p$-4$h$ proton shell-model excitations across the $Z$=82 energy gap [1], resulting in a unique shape-triplet in $^{186}$Pb [2]. In-beam $\gamma$$\gamma$ coincidence data have been collected for $^{186}$Pb by combining the JUROGAM Ge-detector array and the GREAT spectrometer with the RITU gas-filled recoil separator for Recoil-Decay Tagging measurements. In addition to the known prolate yrast band in $^{186}$Pb, these data have enabled three new bands to be identified [3]. Complementary results were provided in Recoil Distance Doppler-Shift lifetime measurements, where the electromagnetic transition properties of the lowest yrast states were measured [4]. In this contribution, the results of latest experiments employing these techniques will be presented and their implications for intruder structures will be discussed.\newline \newline [1] W. Nazarewicz, Phys.\ Lett.\ B {\bf 305}, 195 (1993).\newline [2] A. Andreyev, Nature {\bf 405}, 430 (2001).\newline [3] J. Pakarinen \textit{et. al.}, Submitted to Phys.\ Rev.C.\newline [4] T. Grahn, Private Communications.