Three-quasiparticle isomer in Ta 173 and the excitation energy dependence of K -forbidden transition rates

Using the Er168(B10,5n) reaction at a beam energy of 68 MeV, new data have been obtained for the population and decay of a T1/2=148 ns, Kπ=21/2- three-quasiparticle isomer at 1717 keV in Ta173. Revised decay energies and intensities have been determined, together with newly observed members of a rotational band associated with the isomer. By comparison with other isomers in the A≈180 deformed region, the Ta173 isomer properties help to specify the key degrees of freedom that determine K-forbidden transition rates. In particular, when all three quasiparticles are of the same nucleon type, there is a strong dependence of the E2 reduced hindrance factor on the isomer excitation energy.