Search for fission from a long-lived isomer in No250 and evidence of a second isomer

Abstract

In the present work, a $K$ isomeric state in $^{250}\mathrm{No}$, which is more stable against fission than the ground state, was experimentally studied. The aim was to measure the fission branch of this isomeric state. In total, 780 fission events attributed to the decay of $^{250}\mathrm{No}$ were detected. Among them 133 cases were attributed to the ground-state decay with a half-life of 4.0(4) $\ensuremath{\mu}\mathrm{s}$, which was populated by the deexcitation of the isomeric state via electromagnetic transitions with a half-life of 23(4) $\ensuremath{\mu}\mathrm{s}$. In addition, in two more cases, this long-lived isomeric state was populated in the deexcitation of a hitherto unknown, yet higher-lying and short-lived isomeric state with a half-life of $0.{7}_{\ensuremath{-}0.3}^{+1.4}\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$. No direct fission from the long-lived isomeric state, i.e., with a lifetime of longer than 40 $\ensuremath{\mu}\mathrm{s}$, was identified. This results in an upper limit of 0.035 for the branching ratio for fission. This is a significantly more strict limit than the previously known value of 0.5. Nonobservation of fission branching of the long-lived isomer is discussed relative to theoretical predictions and within various semiempirical ways, which resulted in an attribution of a lower limit of ${10}^{4}$ for the fission-hindrance factor, caused by the $K$ quantum number. The presences of multiple high-$K$ isomeric states seemingly is a widespread phenomenon in deformed heavy nuclei.