Anisotropy of Prompt Fission Neutron Spectra from 233U(n, F)

Abstract

The angular anisotropy of secondary neutrons in neutron emission spectra (NES) of even–even and even–odd target nuclei and in observed prompt fission neutron spectra (PFNS) of the even–odd ²³⁹Pu and ²³⁵U target nuclei is caused by the preequilibrium emission of neutrons in the reaction (n, nX)¹ and its effect on the prefission neutron spectrum from (n, xnf) reactions. In the case of PFNS, the average energy of (n, nf)¹ neutrons depends on the angle of emission with respect to the incident neutron beam θ; consequently, the fission cross section, the average number of prompt fission neutrons (PFN), and the total kinetic energy (TKE) of fission fragments also depend on θ. High sensitivity to emission of (n, xnf)¹ neutrons in the forward and backward hemispheres is predicted for the reaction ²³³U(n, F). Exclusive prefission neutron spectra from reactions ²³³U(n, xnf)^1,...x and exclusive neutron spectra from (n, nγ) and (n, xn)^1,...x are calculated within the Hauser–Feshbach formalism together with the cross sections for the (n,F) and (n,хn) reactions with allowance for the angular dependence of the first neutron emission \(\omega (\theta )\) in ²³³U(n, nX)¹. Calculated exclusive neutron spectra of ²³³U(n, xnf)^1,...x for θ ~ 90° agree with the consistent description of the observed ^{233, 232}U(n, F) fission cross sections for neutrons with an energy of E_n ~ 0.01–20 MeV. An approximation of the angular PFNS anisotropy \(\omega (\theta )\) obtained for ²³⁹Pu(n, F) and ²³⁵U(n, F) allows correlating the angular anisotropy of ²³³U(n, xnf)¹ neutrons with the contribution of the ²³³U(n, xnf) emission fission to the observed ²³³U(n, F) fission cross section. The ratio of average PFNS energies \(\left\langle E \right\rangle \) is obtained for forward and backward emission of (n, xnf)¹ in the reaction ²³³U(n, xnf), which is much larger than for the ²³⁹Pu(n, F) and ²³⁵U(n, F) reactions.