Electron-capture-delayed fission of 228Np

The electron-capture-delayed fission from a 228Np precursor has been studied. 228Np was produced at the LBL 88-Inch Cyclotron by the 233U(p,6n) reaction utilizing a stack of 23 233U targets. 2373 pairs of coincident fission fragments were recorded, giving an asymmetric fission mass distribution, and an average total kinetic energy of 169±6 MeV. The half-life of 228Np was found to be 61.4±1.4 s. By measuring a-decay chains, we found the 228Np α-decay branch to be 60% and the electron-capture decay branch to be 40%. The delayed fission probability was found to be (2.0±0.9)x10-4. A measurement of K-capture X-rays coincident with fission fragments resulted in less than 25% of the expected coincidences. This can be explained by the lack of a second minimum (fission isomer) in the 228U fission barrier, allowing the fission to proceed before the K-vacancies from the 228Np electron capture fill