WEAK-COUPLING AND DIPOLE BANDS IN PB-191

Excited states in the nucleus Pb-191 have been investigated by in-beam gamma-ray spectroscopic techniques using the Gammasphere array. The r eaction Yb-173(Mg-24,6n) at a beam energy of 134.5 MeV was used to pop ulate states of Pb-191. The level scheme of Pb-191 has been extended u p to 5.1 MeV excitation energy and spin 45/2. The lower part (below ap proximate 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 th e upper part of the level scheme two dipole bands are observed with ba ndhead energies of similar to 2.512 and 2.291 MeV. A comparison of dip ole band 1 with the strongest negative-parity dipole bands in the heav ier 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 in terpreted 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)o rbital. [S0556-2813(98)02504-7].