Fission Cross Sections of Neptunium-237 from 20 eV to 7 MeV Determined from a Nuclear-Explosive Experiment

The ²³⁷Np(n,f) cross section has been measured relative to the ²³⁵U(n,f) cross section from 20 eV to 7 MeV, including the range where no data previously existed (4 to 24 keV). Energies of the source neutrons from the Physics 8 underground nuclear explosion were determined by measuring the time-of-flight from the source to the neptunium target foil. The current signals produced in solid-state detectors by fission fragments were photographically recorded and converted to cross sections using the neutron flux measured with ²³⁵U(n,f)and ⁶Li(w,α)T reactions. Subthreshold resonances were observed in the ²³⁷Np(n,f) cross section. The maximum value of the cross section observed just above threshold is 1.9 b at 2.26 MeV. A comparison of results with some previously reported values is presented.Present address: University of Wyoming, Laramie, Wyoming 82070.