Isomeric States In (256)Rf

The question of the number and origin of isomeric states in (256)Rf arose from two independent experiments but remained unanswered for a decade. To shed light on this puzzle, we studied isomeric decay in (256)Rf by measuring conversion electrons with fast fully digital electronics. (256)Rf was produced in the fusion-evaporation reactions of Ti-50 + Pb-207 and Ti-50 + Pb-208 at the gas-filled recoil separator TransActinide Separator and Chemistry Apparatus. Among a total of 120 decays of (256)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(-4)(+6) and 10(-3)(+5) mu s, respectively. Population rates of these isomeric states were estimated to be approximate to 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)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.