Fission Cross Section of Plutonium-238

The neutron-induced fission cross section of ²³⁸Pu was measured from 18 eV to 3 MeV. The nuclear explosion Persimmon provided an intense, single-pulse neutron source spanning an energy range of more than five decades. Neutrons from this underground source were sorted by time of flight over a 304-m vertical path, and interacted with a ²³⁸ Pu sample and with flux monitors at ground level. The neutron flux was measured relative to the reactions ³He(n,p), ⁶Li(n, αt), and ²³⁵U(n,f). Fission cross-section areas are reported for 49 resonances below 500 eV. A number of previously unobserved resonances are reported, yielding an average level spacing of 9.5 ± 0.7 eV. There is strong evidence for intermediate structure in the fission cross section with an average spacing of ∼1 keV.