Decay of a πh11/2 ⊗ νh11/2 microsecond isomer in 13661Pm75

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 Fe-54 on a Mo-92 target at an energy of 315 MeV. Analysis of these data using recoil-isomer tagging has established that the yrast pi h(11/ 2) circle times nu h(11/ 2), J(pi) = (8(+)), bandhead state in Pm-136 is isomeric with a half-life of 1.5(1) mu s. This isomeric state decays via a 43-keV, probable-E1 transition to a J(pi) = (7(-)) state. Consideration of the theoretical Nilsson orbitals near the Fermi surface suggests that the J(pi) = (8(+)) state has a nu h(11/ 2)[505]11(-)/2 circle times pi h(11/2)[532]5(-)/2 configuration, which decays to the J(pi) = (7(-)) state with a nu h(11/2)[505]11(-)/2 circle times pi d(5/2)[411]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(pi) = (8(+)) isomeric state. The non-observation of other 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(pi) = (7(-)) state by internal conversion or even beta decay.