An exact closed form solution to the time-integrated master equation of the exciton model is applied to the calculation of the angular distribution for both the preequilibrium and equilibrium decays of the neutron-induced reaction. The distribution probability of two-nucleon collision from g2 to f2' based on the Fermi gas model and the influence of the Fermi motion and the Pauli principle on the shape of the angular distribution are studied in detail. We have concluded that the influence of these effects on the shape of the angular distribution is rather significant for reactions in the energy range of several tens of MeV. As an example to compare with the experiments we have calculated the neutron-induced reaction 93Nb(n,n') at E,=15MeV. It seems that the most significant improvement of the present approach is the rise of the backward direction of the double differential cross section for the higher energy emitted neutrons.
In order to understand angular distribution deviation between the calculation based on conventional exciton model of the preequilibrium and equilibrium decays and the experimental data, especially the underestimate of angular distribution by the theory at backward angles, an exact closed form solution to the time-integrated master equation of the exciton model by including the effects of the Fermi motion, the Pauli principle, the finite nuclear size and the refraction of the incident wave at the nuclear surface is applied to the calculation of the angular distribution. In this paper the neutron inelastic scattering cross sections for 34 elements at 14.6 MeV have been calculated and compared with experimental data measured by Hermsdorf et al. The results show that because the influence of several: physical effects mentioned above has been taken into account the theory can explain the presently available experimental data and is especially suitable for solving the problem of the double differential cross section for emitted neutrons with higher energy at backward angles.
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