Anomalous neutron diffraction studies of disordered materials

Large variations in the bound coherent scattering length for slow neutrons occur close to low energy neutron resonances for a small proportion of nuclei, offering the possibility for determination of partial correlation functions by carrying out diffraction experiments at neutron wavelengths which sample this variation. For continuous reactor sources, experiments involve the choice of a few wavelengths which optimize the sensitivity while giving manageable absorption corrections. When the time of flight diffraction method is employed at a pulsed neutron source with a diffractometer which gives full coverage of scattering angle, each Q-value is examined over a range of wavelengths and hence scattering lengths for the resonant nuclei. Sophisticated methods for absorption correction and data interpretation are essential if the full data set is to be used. The elements which can be studied by this technique are discussed and the calculation of the variation of the complex scattering length with neutron energy (wavelength) is described. One successful experiment (vitreous samarium aluminium germanate) and one unsuccessful (cadmium in a soda lime silicate glass) are described and the possibilities for the future are discussed. (author part.) 8 figs., 15 refs