Measurements of Absolute Delayed Neutron Yield and Group Constants in the Fast Fission of ²³⁵U and ²³⁷Np

The delayed neutron activity resulting from the fast induced fission of ²³⁵U and ²³⁷Np has been studied. The six-group decay constants, relative abundances, and absolute yield of delayed neutrons from fast fission of ²³⁵U and ²³⁷Np were measured using the Godiva IV fast assembly at the Los Alamos Critical Experiments Facility. The absolute yield measured for ²³⁵U was 0.0163 ± 0.0008 neutron/fission. This value compares very well with the well-established Keepin absolute yield of 0.0165 ± 0.0005. The absolute yield value measured for ²³⁷Np was 0.0126 ± 0.0007. The measured delayed neutron parameters for ²³⁵U are corroborated with period (e-folding time) versus reactivity calculations.