Improvement of Angular Differential Elastic Scattering Cross Sections in Nucleon-meson Transport Code NMTC/JAERI97 for Neutron Transmission Experiment Analysis.

The nucleon-meson transport code NMTC/JAERI97 is revised in the treatment of the angular distribution of the neutron elastic scattering for the energy region from 20 to 100 MeV. The approximation of the angular distribution with the first order Bessel function is changed to the random sampling based on the database evaluated by the optical model potential parameters (OMPs). For 43- and 68-MeV quasi-monoenergetic neutrons through iron, concrete and polyethylene shields, neutron transmission experiments are analyzed with the updated NMTC/JAERI97 code. It is shown that the updated NMTC/JAERI97 calculations agree well with the measured transmission neutron energy spectra and give significantly improved C/E values for the attenuation length in comparison with the results of the original version. It is also notable that the updated calculation represents quite well the measured neutron spectra at the positions of 20 and 40 cm off beam axis of the shields. Moreover. it is found that the cross sections evaluated by an OMP gives a better result than those by the systematics for the transmission calculation of neutrons with energies below 40 MeV.