Energy Spectra of Secondary Neutrons from the ²³⁸U(n, 2n) and (n, 3n) Reactions

A consistent compound nucleus theory of (n, 2n) and (n, 3n) neutron emission was applied to ²³⁸U to obtain the energy spectra of the second and third secondary neutrons. The evaluation was based on inelastic level excitation and evaporation data for ²³⁸U, ²³⁷U, and ²³⁶U. The ²³⁸U and ²³⁶U data were retrieved from ENDF/B-IV files; the ²³⁷U data were evaluated in the Soreq Nuclear Research Center using experimental information and statistical reaction theory codes. At reaction energies E₀ just above the (n, 2n) threshold energy B₂, the energy E of the second inelastic neutron has a spectrum of (E₀ - B₂ - E); above the (n, 3n) threshold, B₃, the third neutron energy has a spectrum of (E₀ - B₂ - E)³. At energies E₀, high above the thresholds, the second and third neutron spectra approach the evaporation form. A secondary neutron spectrum for any given reaction energy E₀ is approximated by a composite form where i = 2, 3 for the second and third neutrons, respectively. The temperatures T_i and blending coefficients β_i were evaluated for several energies in the range from threshold up to 15 MeV.