Anisotropy of Prompt Fission Neutron Spectra 232Th(n, F) and 238U(n, F)

The angular anisotropy of the neutron emission spectra (NES) of the Th-232 + n and U-238 + n reactions is due to the angular dependence of elastic scattering, direct excitation of collective levels, and preequilibrium emission of the 1st neutron in (n, nX)(1) reaction, as well as collective excitations in the continuous spectrum with U approximate to 1 similar to 6 MeV. In the case of the prompt fission neutron spectra (PFNS), the anisotropy is caused by the anisotropy of the exclusive spectra of prefission neutrons in (n, xnf)(1) reactions. To correctly estimate the excitation of the Th-232 and U-238 levels in continuous spectrum with excitation energy U approximate to 1 similar to 6 MeV, the coupling of the ground-state band levels with J(pi) = 0(+), 2(+), 4(+), 6(+), 8(+) (in the rigid rotator model) is considered, while the coupling of the gamma-band levels with K-pi = 0(+), 2(+) and the levels of the octupole bands with K-pi = 0(-) in the soft rotator model (levels with excitation energies U approximate to 0 similar to 1 MeV). The Th-232 + n and U-238 + n NES at E-n similar to 6, similar to 12, similar to 14, similar to 18 MeV are described in detail. An adequate description of the doubly differential emission spectra and angular distributions of the (n, nX)(1) neutrons in the range of the continuous spectrum corresponding to excitation energies U approximate to 1 similar to 6 MeV of residual nuclei has been achieved. In the case of PFNS, the anisotropy of Th-232 + n and U-238 + n reactions is shown to be caused by a part of the (n, nX)(1) neutrons manifested in exclusive spectra of prefission neutrons, as in the Pu-239(n, xnf) reaction. For Th-232(n, xnf)(1, ... x) and U-238(n, xnf)(1, ... x) , the PFNS demonstrate different sensitivity to (n, xnf)(1) neutron emission into the forward and rear hemispheres relative to the incident neutron beam. The mean energy of PFNS and (n, xnf)(1) neutrons depends on the neutron emission angle theta, as well as fission cross section, mean number of prompt fission neutrons and the total kinetic energy (TKE) of fission fragments (products). The exclusive spectra of (n, xnf)(1, ... x) neutrons at theta similar to 90 degrees are consistent with the observed cross sections of the Th-232(n, F)(U-238(n, F)) fission reactions and Th-232(n, xn)(U-238(n, xn)) neutron emission reactions. For Pu-239(n, xnf), the approximated distribution omega(theta) is consistent with the Th-232 + n and U-238 + n NES at E-n approximate to 6 similar to 18 MeV. The angular anisotropy of the (n, xnf)(1) neutrons for Th-232 + n and U-238 + n spectra is manifested in the ratios of the mean energies of PFNS and exclusive (n, xnf)(1, ... x) spectra of prefission neutrons. A correlation is found between the angular anisotropy of secondary (n, xnf)(1) neutrons with the contribution of the (n, nf) emission/fission process to the observed fission cross section and NES angular anisotropy.