Neutron diffraction of hydrogenous materials : measuring incoherent and coherent intensities separately - a 40-year-old puzzle solved

Orateur : Laszlo PUSZTAI (WIGNER RESEARCH CENTRE FOR PHYSICS, BUDAPEST)

Accurate determination of the coherent static structure factor of any disordered material containing substantial amounts of proton nuclei has proven to be rather problematic by neutron diffraction, due to the large incoherent cross section of 1H. This problem has continued to set severe obstacles to the reliable determination of liquid structures of hydrogenous materials up to this day, by introducing large uncertainties whenever a sample with a 1H content larger then about 20% had to be measured by neutron diffraction. Huge theoretical efforts over the past 40 years that had been aimed at estimating the ’incoherent background’ of such data have not resulted in any practical solution to the problem. Here we present the first accurate separate measurements, by polarized neutron diffraction, of the coherent and incoherent contributions to the total static structure factor of mixtures of light and heavy water, over a wide momentum transfer range, and over the entire composition range, i.e. for light water contents between 0 and 100%. We show that the measured incoherent scattering can be approximated by a Gaussian function. The separately measured coherent intensities exhibit signs of small inelastic contributions. Out of several possible approaches, we have chosen to subtract a cubic background using the Reverse Monte Carlo algorithm, which has the advantage of requiring an actual physical model (thousands of realistic water molecules at the correct density) behind the corrected data. Finally, coherent static structure factors for 5 different compositions of liquid H2O and D2O mixtures are presented for which the huge incoherent background could actually be measured and separated, instead of being approximated as it has been done every time so far. These experimental results provide a strong hope that determining the structure of hydrogenous materials, including, e.g., protein solutions, may become feasible in the near future.

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