Electron-capture delayed fission was observed in 26-min ${}^{248}\mathrm{Es}$ and 7.7-min ${}^{246}\mathrm{Es},$ which were produced at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron via ${}^{249}\mathrm{Cf}\mathrm{}(p,2n)\mathrm{}$ and ${}^{249}\mathrm{Cf}\mathrm{}(p,4n)\mathrm{}$ reactions at proton energies of 18 and 37 MeV on target, respectively. The delayed fission probabilities were determined to be $(3.5\mathrm{}\pm 1.8)\times {10}^{-6}$ for ${}^{248}\mathrm{Es}$ and ${(3.7\mathrm{}}_{-3.0}^{+8.5\mathrm{}})\times {10}^{-5}$ for ${}^{246}\mathrm{Es}.$ These values were determined in separate experiments using our automated rotating wheel online collection and measurement system. Collection and measurement times were optimized for the isotope being measured. The measured delayed fission probabilities are consistent with the experimentally observed trend of increasing delayed fission probability with increasing Q value for electron capture.

• Received 16 March 2000

DOI:https://doi.org/10.1103/PhysRevC.63.037603