Electron-capture delayed fission properties ofNp228

Abstract

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\ifmmode\pm\else\textpm\fi{}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 ${10}^{\mathrm{\ensuremath{-}}17}$ 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\ifmmode\pm\else\textpm\fi{}0.4 for $^{228}\mathrm{Np}$. The production cross section for $^{228}\mathrm{Np}$ is 35\ifmmode\pm\else\textpm\fi{}10 \ensuremath{\mu}b and the delayed fission probability is (2.0\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. The kinetic energies of coincident fission fragments were measured and a pre-neutron emission, most probable total kinetic energy, of 169\ifmmode\pm\else\textpm\fi{}6 MeV was obtained. The mass-yield distribution was found to be predominantly asymmetric.