Neutron diffraction intensities from arrays of isotopically substituted particles in an invisible matrix

Neutron scattering and diffraction experiments employing selective isotopic substitution allow one to render a component of the sample effectively 'invisible" by matching the scattering-length densities of the solvent and sample. Ultimately, the resolution of the structural details of the visible portions of such samples are limited by the presence of scattering-length fluctuations. Here it is shown that for arrays of visible particles, which are small compared to the particle(s) in which they are embedded, their diffraction intensities can be obtained to a higher resolution than is attainable by conventional density matching. By appropriately combining measurements on samples in which the array particles have three different levels of isotopic substitution, it is possible to generate a diffraction pattern which is free of intensity contributions and fluctuation distortions from the matrix. This technique may be applied to ordered arrays and to solutions of such arrays. It will be particularly useful for biological systems where the deuteration level of nucleic acid or protein can be varied.