Decay of aπh11/2⊗νh11/2microsecond isomer in61136Pm75

An experiment has been performed to populate several extremely neutron-deficient nuclei around the mass-140 region of the nuclear chart, using a beam of $^{54}\mathrm{Fe}$ on a $^{92}\mathrm{Mo}$ target at an energy of 315 MeV. Analysis of these data using recoil-isomer tagging has established that the yrast $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2},{J}^{\ensuremath{\pi}}=({8}^{+})$, bandhead state in $^{136}\mathrm{Pm}$ is isomeric with a half-life of 1.5(1) \ensuremath{\mu}s. This isomeric state decays via a 43-keV, probable-$E1$ transition to a ${J}^{\ensuremath{\pi}}=({7}^{\ensuremath{-}}$) state. Consideration of the theoretical Nilsson orbitals near the Fermi surface suggests that the ${J}^{\ensuremath{\pi}}=({8}^{+})$ state has a $\ensuremath{\nu}{h}_{11/2}[505]{\frac{11}{2}}^{\ensuremath{-}}\ensuremath{\bigotimes}\ensuremath{\pi}{h}_{11/2}[532]{\frac{5}{2}}^{\ensuremath{-}}$ configuration, which decays to the ${J}^{\ensuremath{\pi}}=({7}^{\ensuremath{-}})$ state with a $\ensuremath{\nu}{h}_{11/2}[505]{\frac{11}{2}}^{\ensuremath{-}}\ensuremath{\bigotimes}\ensuremath{\pi}{d}_{5/2}[411]{\frac{3}{2}}^{+}$ configuration. Differences in the shape-driving effects for these two configurations is reasoned to be responsible for the long half-life of the ${J}^{\ensuremath{\pi}}=({8}^{+})$ isomeric state. The non-observation of other $\ensuremath{\gamma}$ rays in prompt or delayed coincidence with the 43-keV transition suggests that this transition may feed another, longer lived isomeric state with a half-life of the order of milliseconds or greater. However, the present experiment was not sensitive to the decay of this new ${J}^{\ensuremath{\pi}}=({7}^{\ensuremath{-}}$) state by internal conversion or even $\ensuremath{\beta}$ decay.