Absolute Measurement of the Uranium-235 Fission Cross Section from 0.2 to 1.2 MeV

The absolute ²³⁵U neutron-induced fission cross section has been measured at the U.S. National Bureau of Standards (NBS) 3-MV Van de Graaff Laboratory from 0.2- to 1.2-MeV neutron energy. The mass of the ²³⁵U contained in a large volume multiplated fission ionization chamber was measured relative to the NBS fissionable isotope mass standards. Pulsed beam time-of-flight techniques were used with neutrons from the ⁷Li(p, n)⁷ Be reaction while the neutron flux was monitored with a large plastic scintillator whose efficiency was both calculated and measured with the associated-particle technique. The cross sections, which were measured with a typical uncertainty of 2.3%, are ∼2% lower than the ENDF/B-V evaluation.