Analytical description of neutron cross-sections and the effect of their energy dependence upon their 2200-m/s values

In the calculation of reaction rates in a reactor, the energy dependence of the cross-sections involved must be known with a precision comparable to that of their absolute values. Many of the measurements of absolute values of nuclear parameters are made in a broad spectrum of neutron energies, and the accuracy with which the 2200-m/s values can be extracted from such measurements depends directly upon the accuracy with which the energy dependences involved are known. With these considerations in mind an effort has been made to obtain precise descriptions of the shape of the low-energy cross-sections of 233U, 235U, 239Pu and 241Pu. Emphasis has been placed on accounting for all available data, including the information given by the measurements of the energy variation of η. The experimental data are fitted by the least-squares technique to obtain the best values of the analytical parameters and their error estimates. The analytical expressions of Fluharty et al. have been used. This formalism has the advantage of simplicity of calculation. It employs a simple polynomial of second order to describe the cross-sections in the energy region below appreciable influence of the lowest observed energy resonance. Above approximately 0.1 eV, resonance terms were added. For each indicated level separate symmetric and asymmetric resonance terms are added, corresponding to the symmetric capture term of non-fission resonances and to the asymmetry introduced by interference in the fission channels, respectively. From the analytical expressions are derived values and error estimates for the Westcott g-values. (author)