Weak coupling and dipole bands in 191Pb

Excited states in the nucleus Pb-191 have been investigated by in-beam gamma-ray spectroscopic techniques using the Gammasphere array. The reaction Yb-173(Mg-24,6n) at a beam energy of 134.5 MeV was used to populate states of Pb-191. The level scheme of Pb-191 has been extended up to 5.1 MeV excitation energy and spin 45/2. The lower part (below approximate to 2.3 MeV) presents the general features common in all odd-mass Pb isotopes, i.e., spherical states built on the 13/2(+) isomer, with bandlike structures that follow weak-coupling expectations. In the upper part of the level scheme two dipole bands are observed with bandhead energies of similar to 2.512 and 2.291 MeV. A comparison of dipole band 1 with the strongest negative-parity dipole bands in the heavier odd-mass Pb isotopes reveals striking similarities. The spherical states are interpreted as the coupling of the i(13/2) neutron hole to the yrast states in the even-mass Pb-192 Core. The dipole bands are interpreted as M1 cascades which arise from the coupling of two protons in high-j orbitals above the Z=82 gap to neutron holes in the i(13/2)orbital. [S0556-2813(98)02504-7].