Characterization of the 32 μs isomer in Pb189 as a shears-mode bandhead

Internal conversion coefficients have been measured for transitions following the decay of the 32 μs isomer in the neutron-deficient isotope Pb189. The main branch at 337 keV to the 25/2+ excited state is shown to be of E3 multipolarity with a strength of about 24 W.u. This, together with multipolarity assignments to other branches, leads to a spin and parity assignment of Kπ=31/2- for the 2435-keV state. This also defines that state as the isomer, rather than the lifetime being due to feeding from a higher lying 33/2+ isomeric state as had been proposed previously. The isomer is suggested to arise from the partially aligned coupling of the 11- two-proton configuration with the i13/2 neutron hole. The low-lying nature of the 31/2- multiplet member is attributed to the change in quasiparticle character of the i13/2 neutron configuration as the neutron shell is depleted.