Slow Neutron Cross Sections of Gold, Silver, Indium, Nickel, and Nickel Oxide

The total reflection and selective attenuation properties of mirrors for neutrons were used to obtain essentially higher order free reflections of a beam of reactor neutrons from a quartz crystal. An arrangement of mirror, crystal, and sample, in series was used to measure the total cross section of Au, Ag, In, Ni, and NiO as a function of neutron wave length over intervals in the region between 0.7A and 4.6A. The results on the strong absorbers Au, Ag, and In were analyzed into absorption and scattering contributions. The absorbers exhibited essentially the $\frac{1}{v}$ characteristic expected. The scattering and absorption cross sections of Au, Ag, and In are compared with previously reported values. For the strong scatterers Ni and NiO, the total scattering, ordered scattering, and disordered scattering cross sections, and the Debye temperature were derived from the energy dependence of the total cross section. The ordered scattering cross section per Ni nucleus was found to be 13.1±0.3 barns. The capture and ordered elastic contributions were subtracted from the experimental curve of total cross section vs energy. This residual scattering function of energy was compared to that theoretically expected from a Debye independent-oscillator model and the one-phonon incoherent scattering approximation of Kleinman. In the case of Ni, at the longer wavelengths, both models fit the data well, and at the shorter wavelengths, where multiple phonon processes occur, the independent oscillator model continues to fit the data. For NiO, measurements fit the independent oscillator model at the shorter wavelengths, and lie above the values calculated from either model at the longer wavelengths. Additional measurements of the total cross section of NiO as a function of temperature would be needed to attribute the difference between measured and calculated values at the longer wavelengths definitely to inelastic scattering processes.