| The shape coexistence phenomena exist in many nuclear regions in the chart of nuclides,exhibiting the complexity of the nucleus as a quantum many-fermion system and providing abundant nuclear structure information.Based on theoretical calculations,shape coexistence is expected in the neutron-deficient nuclei around the mid-shell at N～104 in the lead region.However,due to the low production cross-sections of these nuclides,there is not much experimental information on shape coexistence,especially for nuclei with Z>82.Furthermore,theoretically,the shape coexistence phenomena of nuclides in this nuclear region are not fully understood either.In order to study shape coexistence in this region,prompt and delayed y-ray spectroscopy of the 187Pb,188Bi and 188Po isotopes produced by the fusion-evaporation reaction 50Cr+142Nd→192Po*has been performed at the Argonne Gas-Filled Analyzer.The recoil-decay tagging（RDT）technique was used to provide an unambiguous y-ray assignment to a specific nuclide.In 187Pb,a new 5.15（15）-μs isomeric state at 308 keV above the spherical 3/2ground state was identified.A strongly-coupled rotational band is observed on top of this isomer,which is nearly identical to the one built on the prolate 7/2-[514]Nilsson state in the isotone 185Hg.Based on this similarity,the new isomer in 187Pb was proposed to be prolate with Jπ=（7/2-）and classified as a shape isomer.The retarded character of the 308-keV（7/2-）→ 3/2-gs transition with a deduced B（E2）=5.6（2）× 10-4 W.u.can be well explained by the significant difference between the prolate initial and spherical final configurations,leading to the shape isomerism.The combined results of the present work and the previous α-decay and laser spectroscopy studies present the evidence for the triple spherical-oblate-prolate shape coexistence at low energy in the negative-parity configurations of 187Pb,which is well reproduced by the potentialenergy surface calculations.A new 0.25（5）-μs isomeric state and a prompt cascade have been established on top of（10-）188mBi.Based on the systematics with the neighbouring isotopes 187,189Bi,this cascade could be a candidate for a prolate rotational band in 188Bi.The first excited 2+ state in 188Po was identified,its excitation energy of 242（2）keV continues the nearly constant trend for the first 2+ states in 190,192,194Po.The new results obtained in the present work provide new information to shape coexistence on bismuth and polonium isotopes near the neutron midshell at N=104. |
