Electron-capture delayed fission was positively identified for the first time in ${}_{}{}^{228}{}_{}{}^{}\mathrm{Np}$ produced via the ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$(50-MeV p,6n${)}^{228}$Np reaction. The half-life of the observed fission activity was measured to be 61.4±1.4 s. The ECDF mode was verified by an x-ray–fission coincidence experiment. Only about 25% of the expected x-ray–fission coincidences were detected which indicates that ${}_{}{}^{228}{}_{}{}^{}\mathrm{U}$ does not have a fission isomer populated by the electron-capture and that the overall lifetime of the high-energy states populated by the electron-capture is less than about ${10}^{\mathrm{-}17}$ s. The fission activity was assigned to ECDF of ${}_{}{}^{228}{}_{}{}^{}\mathrm{Np}$ based on chemical separation and observation of known progeny activities. The ratio of electron-capture to alpha decay is 1.5±0.4 for ${}_{}{}^{228}{}_{}{}^{}\mathrm{Np}$. The production cross section for ${}_{}{}^{228}{}_{}{}^{}\mathrm{Np}$ is 35±10 μb and the delayed fission probability is (2.0±0.9)×${10}^{\mathrm{-}4}$. The kinetic energies of coincident fission fragments were measured and a pre-neutron emission, most probable total kinetic energy, of 169±6 MeV was obtained. The mass-yield distribution was found to be predominantly asymmetric.

• Received 22 February 1994

DOI:https://doi.org/10.1103/PhysRevC.50.2288