Angular anisotropy of secondary neutron spectra in $^{232}$Th+n

Abstract

Neutron emission spectra (NES) of $^{232}$Th+n interaction provide strong evidence of angular anisotropy of secondary neutron emission, another evidence might be predicted in $^{232}$Th%(n,F)% prompt fission neutron spectra (PFNS). In case of NES observed angular anisotropy is presumably due to angular dependence of elastic scattering, direct excitation of collective levels and preequilibrium emission of (n,nX) neutrons. In $^{232}$Th+n direct excitation data analysis, ground state band levels are coupled within rigid rotator model, while those of beta bands, gamma bands and octupole band theta are coupled within soft deformable rotator model. NES of $^{232}$Th+n at En of 6, 12, 14, 18 MeV exhaustively described. The net effect of these procedures for En up to 20 MeV is the adequate approximation of angular distributions of $^{232}$Th$(n,nX)$ first neutron inelastic scattering in continuum, which corresponds to U of 1.2-6 MeV excitations of $^{232}$Th. The contribution of $^{232}$Th$(n,F)$ PFNS to the NES is exceptionally low. PFNS anisotropy occurs because some portion of $(n,nX)$ neutrons might be involved in exclusive prefission neutron spectra. In $^{232}$Th$(n,xnf)$ reactions PFNS demonstrate different response to forward and backward (n,xnf) neutron emission relative to the incident neutron momentum, when compared with $^{235}$U$(n,xnf)$ or 239Pu(n,xnf) reactions. Average energy of (n,xnf) neutrons depends on the neutron emission angle theta, i.e. fission cross section, prompt neutron number and total kinetic energy are shown to vary with the angle theta as well. Exclusive neutron spectra ($n,xnf)$ at theta equal to 90 degrees are consistent with observed $^{232}$Th$(n,F)$ and $^{232}$Th$(n,xn)$ reaction cross sections within En from 1 to 20 MeV energy range.