Angular distribution calculation based on the exciton model taking account of the influence of the Fermi motion and the Pauli principle

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 fromΩ toΩ′ 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 reaction⁹³Nb(n,n′) atE _n =15 MeV. 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.