Weak coupling and dipole bands in191Pb

Abstract

Excited states in the nucleus ${}^{191}\mathrm{Pb}$ have been investigated by in-beam \ensuremath{\gamma}-ray spectroscopic techniques using the Gammasphere array. The reaction ${}^{173}\mathrm{Yb}{(}^{24}\mathrm{Mg},6n)$ at a beam energy of 134.5 MeV was used to populate states of ${}^{191}\mathrm{Pb}.$ The level scheme of ${}^{191}\mathrm{Pb}$ has been extended up to 5.1 MeV excitation energy and spin $45/2.$ The lower part (below $\ensuremath{\approx}2.3\mathrm{MeV}$) 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 $\ensuremath{\sim}2.512$ 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 ${}^{192}\mathrm{Pb}$ 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.