Yrast 6 + seniority isomers of 136,138 Sn

Delayed γ-ray cascades, originating from the decay of (6+) isomeric states, in the very neutron-rich, semimagic isotopes 136,138Sn have been observed following the projectile fission of a 238U beam at RIBF, RIKEN. The wave functions of these isomeric states are proposed to be predominantly a fully aligned pair of f7/2 neutrons. Shell-model calculations, performed using a realistic effective interaction, reproduce well the energies of the excited states of these nuclei and the measured transition rates, with the exception of the B(E2;6+→4+) rate of 136Sn, which deviates from a simple seniority scheme. Empirically reducing the νf7/22 orbit matrix elements produces a 41+ state with almost equal seniority 2 and 4 components, correctly reproducing the experimental B(E2;6+→4+) rate of 136Sn. These data provide a key benchmark for shell-model interactions far from stability.