Anisotropy of Prompt Fission Neutron Spectra of 239Pu(n, F) and 235U(n, F)

Abstract

Angular anisotropy of secondary neutrons is evidenced in neutron emission spectra (NES) and prompt fission neutron spectra (PFNS). In case of NES that is due to angular dependence of elastic scattering, direct excitations of discrete collective levels and pre-equilibrium/direct emission of (n, nX)¹ neutrons, while in case of PFNS it is because of these neutrons are evidenced in exclusive pre-fission neutrons in (n, xnf)¹. In ²³⁹Pu(n, xnf)^1,...x and ²³⁵U(n, xnf)^1,...x PFNS demonstrate different responses to forward and backward (n, xnf)¹ neutrons emission with respect to the incident neutron momentum. Average energy of (n, nf)¹ neutrons depends on the emission angle θ, i.e., fission cross section, prompt neutron number and total kinetic energy depend on θ as well. Exclusive neutron spectra (n, xnf)^1,...x at θ ~ 90° are consistent with ²³⁵U(n, F)(²³⁵U(n, xn)) and ²³⁹Pu(n, F)(²³⁹Pu(n, xn)) reaction cross sections within E_n ~ 0.01–20 MeV energy range. Exclusive neutron spectra of (n, xnf)^1,..x, (n, nγ) and (n, xn)^1,...x are calculated within Hauser–Feshbach formalism alongside with (n, F) and (n, хn) reaction cross sections, angular dependence of first neutron (n, nX)¹ emission \(\omega (\theta )\) being included. Approximation obtained for \(\omega (\theta )\) fits the measured NES at 14 MeV. The correlation of angular dependence of (n, xnf)¹ neutron emission with emissive fission (n, xnf) contribution to the observed fission cross section and angular anisotropy of NES. The ratios of mean PFNS energies \(\left\langle E \right\rangle \) for forward and backward emission of ²³⁵U(n, xnf) and ²³⁹Pu(n, xnf) pre-fission neutrons are consistent with measured data.