β-delayed fission fromEsm256and the level scheme ofFm256

The 7.6-h isotope ${}_{}{}^{256}{}_{}{}^{}\mathrm{Es}_{}^{\mathrm{m}}{}_{}{}^{}$ was produced from a 2.5-μg/${\mathrm{cm}}^2$ target of ${}_{}{}^{254}{}_{}{}^{}\mathrm{Es}$ by the (t,p) reaction. The reaction products were separated radiochemically, and the decay properties of ${}_{}{}^{256}{}_{}{}^{}\mathrm{Es}_{}^{\mathrm{m}}{}_{}{}^{}$ were determined via β-γ, γ-γ, and β-fission correlation techniques. From these measurements we were able to assign 57 γ rays to 26 levels in the daughter ${}_{}{}^{256}{}_{}{}^{}\mathrm{Fm}$. An isomeric level was observed at 1425 keV and assigned a spin and parity of $7^{\mathrm{-}}$. This level has a $t_{1/2}$ of (70±5) ns and we observed two β-delayed fissions with delay times in the proper time range to be associated with fission from this level. This gives a β-delayed fission probability of 2×${10}^{\mathrm{-}5}$ for this level and a partial fission half-life of ${0.8}_{\mathrm{-}0.7}^{+8.8\mathrm{}}$ ms at the 95% confidence level.