Fission Cross Section of Californium-249

The neutron-induced fission cross section of ²⁴⁹Cf was measured from 13 eV to 3 MeV. Neutrons from the Physics-8 underground nuclear explosion traversed a 240-m vertical evacuated flight path and interacted at ground level with a ²⁴⁹Cf sample and with neutron flux monitors. Abundant fission was observed throughout the neutron energy region studied, although the several-MeV cross section was lower than expected on the basis of systematics. Forty-three resonances between 15 and 70 eV were parameterized using a multilevel R-matrix formalism. In this energy region, the average level spacing, corrected for five postulated unobserved levels, was 1.07 ± 0.14 eV, both spin states of the compound nucleus being taken together. Assuming both spin states to have the same properties, the s-wave neutron strength function per spin state ⟨⟩/⟨D⟩ was (1.5 ± 0.3) × The average reduced neutron width ⟨⟩ was 0.31 ± 0.08 meV. For 35 well-defined resonances between 15 and 70 eV, the average fission width ⟨Γ_ƒ⟩ was 180 meV.