The energy dependence of the neutron coherent scattering length of europium

General considerations are given for the importance of providing an energy/wavelength-dependent database of neutron coherent scattering lengths, and of the possibility of constructing such a base from the range of neutron cross-section data that are generally available for a wide range of nuclides. The case of europium is considered in detail, partly because it is a good example of an element with strong resonance effects in the wavelength region of importance in crystallographic and condensed-matter studies, and partly because it is an exceptional case from the nuclear point of view, having multi-level interference effects. From the available cross-section data the complex coherent scattering length is constructed as a function of neutron energy up to 0.7 eV, for the isotopes and for the natural element. Some single-level approximations for the scattering-length function of ¹⁵¹Eu are given.