Optical-Model Interpretation of Average Total Neutron Cross Sections in the keV Region

Total neutron cross sections for the elements Nb, Mo, Rh, Ag, Cd, and In in the range of 11 to 101 keV have been measured using the Brookhaven fast chopper at the NRU reactor, Chalk River, Canada. The data, after correction for sample thickness effects, are interpreted in terms of a spherical optical-model potential. The comparison of the data with the model indicates that the potential is more likely to be of the surface-absorption type rather than of the volume-absorption type. There is some evidence of fluctuations in $S$-wave strength functions from nuclide to nuclide, instead of the smooth variation predicted by the optical model. It seems that the parameters of the potential well, chiefly the imaginary part, may be different for $S$- and $P$-wave neutrons.