ar X iv : n uc l - th / 9 41 00 30 v 1 1 9 O ct 1 99 4 Total Cross Sections for Neutron Scattering

Measurements of neutron total cross-sections are both extensive and extremely accurate. Although they place a strong constraint on theoretically constructed models, there are relatively few comparisons of predictions with experiment. The total cross-sections for neutron scattering from 16 O and 40 Ca are calculated as a function of energy from 50 − 700 MeV laboratory energy with a microscopic first order optical potential derived within the framework of the Watson expansion. Although these results are already in qualitative agreement with the data, the inclusion of medium corrections to the propagator is essential to correctly predict the energy dependence given by the experiment. In the region between 100 and 200 MeV, where off-shell tρ calculations for both 16 O and 40 Ca overpredict the experiment, the modification due to the nuclear medium reduces the calculated values. Above 300 MeV these corrections are very small and depending on the employed nuclear mean field tend to compensate for the underprediction of the off-shell tρ results. 2 Accurate measurements of total cross-sections for the scattering of neutrons from a variety of nuclei provide stringent constraints on theoretical models. In this sense one would like to incorporate a realistic physical description of the nucleon-nucleon [NN] interaction , which quantitatively reproduces the NN observables, including the total neutron cross-section, into a theoretical model of neutron-nucleus scattering. Such a description would ideally result from multiple scattering theory and the use the NN interaction together with nuclear wave functions as input to obtain nucleon-nucleus observables. In a recent publication [1] we presented a microscopic treatment of the modification of the NN propa-gator due to the nuclear medium, formulated to be consistent with the first order spectator expansion [2,3]. Calculations of proton and neutron elastic scattering from 40 Ca in the first order spectator expansion were presented, in which the effect of the nuclear medium was taken into account. These results were very encouraging. In particular, the neutron total cross-section for scattering from 40 Ca was shown as a function of energy. It was gratifying to observe [1] that the effect of the nuclear medium, although not large, lowered (relative to those for which a free NN t-matrix was employed) the calculated values in the energy regime between 100 and 300 MeV and brought the theoretical predictions into closer agreement with the measured values. Furthermore, above ∼400 MeV, where the 'free' predictions fell below the experimental data, the medium …