Isomeric states in Rf256

Abstract

The question of the number and origin of isomeric states in $^{256}\mathrm{Rf}$ arose from two independent experiments but remained unanswered for a decade. To shed light on this puzzle, we studied isomeric decay in $^{256}\mathrm{Rf}$ by measuring conversion electrons with fast fully digital electronics. $^{256}\mathrm{Rf}$ was produced in the fusion-evaporation reactions of $^{50}\mathrm{Ti}+^{207}\mathrm{Pb}$ and $^{50}\mathrm{Ti}+^{208}\mathrm{Pb}$ at the gas-filled recoil separator TransActinide Separator and Chemistry Apparatus. Among a total of 120 decays of $^{256}\mathrm{Rf}$, we detected 22 and 12 decays proceeding through one and two isomeric states. Half-lives of the low- and higher-lying states were assigned to be ${T}_{1/2}={14}_{\ensuremath{-}4}^{+6}$ and ${10}_{\ensuremath{-}3}^{+5}\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$, respectively. Population rates of these isomeric states were estimated to be $\ensuremath{\approx}18%$ and $>10%$, which are similar to those for two-quasiparticle $K$ isomeric states in this region of nuclei. The results, thus, confirm an earlier claim on the existence of multiple $K$ isomeric states in $^{256}\mathrm{Rf}$ and show their population rates are as high as in No isotopes. Suggestively, $K$ remains a good quantum number in isotopes of heavier elements like Sg and Hs, where yet unknown high-$K$ isomeric states still could exist. The present experimental results demonstrate that the ``triggerless'' measurement guarantees an efficient detection of delayed conversion electron signals. Thus, it is a very efficient method for the identification of isomeric states in experiments with one nucleus-at-a-time production rates.