Energy spectra of secondary neutrons from the U-238 (n.2n) and (n. 3n) reactions

A consistent compound nucleus theory of (n,2n) and (n,3n) neutron emission was applied to U-238 in order to obtain the energy spectra of the second and third secondary neutrons. The evaluation was based on inelastic level excitation and evaporation data for U-238, U-237 and U-236. The U-238 and U-236 data were retrieved from ENDF/B-IV files; the U-237 data had been evaluated using experimental information and statistical reaction theory codes. At reaction energies E0 closely above the (n,2n) threshold energy B2 the energy E of the second inelastic neutron has a spectrum of (E0-B2-E); closely 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. The experimental effective temperature for all secondary neutrons from U-238 (n,n'), (n,2n) and (n,3n) at 14 MeV is compared with the value calculated (by performing a least squares fit to our data). The results are shown to be consistent with the experimental data. (B.G.)