Accurate bound coherent neutron scattering lengths, bc, of important isotopes

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Neutron scattering lengths b are the measure of the strength of the interaction between neutrons and the nucleus of an atom. They are fundamental properties of matter, and most neutron scattering techniques rely on accurate b values. Surprisingly, neutron scattering lengths are often only known with moderate or poor accuracy or have not been determined experimentally at all. The aim of this research project is to determine bound coherent scattering lengths for thermal neutrons of important stable isotopes with very high accuracy using neutron diffraction. The isotopes and natural isotope mixtures to be examined are: 6Li, 7Li, natLi, 114Cd, natCd, 123Te, natTe, 141Pr (= natPr), 142Nd, 143Nd, 144Nd, 145Nd, 146Nd, 148Nd, 150Nd, natNd, 147Sm, 147Sm , 152Sm, 154Sm, natSm, 151Eu, 153Eu, natEu, 177Hf, natHf, 185Re, 187Re, natRe, 203Tl, 205Tl, natTl. This will provide the user community with fundamental and important data needed for a plethora of neutron experiments in diverse fields ranging from condensed matter to nuclear physics and biology. First, a robust yet accurate method for determining bound coherent neutron scatter lengths by neutron diffraction is developed. Neutron diffraction has the advantage over other (sometimes more accurate) methods of determining the scattering length that it only relies on relative and not on absolute values and is less prone to systematic errors. It is therefore a robust but fairly accurate (< 1%) method and well suited to redetermining the scattering lengths of important isotopes. Suitable materials like LiF, LiBaF3, CdF2, TeO2, LnN and LnOCl (Ln = Pr, Nd, Sm, Eu), ReO3, TlCl are synthesized from isotopically enriched materials. Neutron powder diffraction experiments on these materials are optimized for accurate intensity measurements and diffraction data are analyzed by Rietveld refinement. Statistical and consistency checks complete the neutron scattering length determination. Most neutron experiments are performed at and in collaboration with the Institut Laue-Langevin in Grenoble, France. Additional neutron diffraction data are collected at MLZ, Garching, Munich, Germany, at PSI, Villigen, Switzerland, and at ANSTO, Lucas Heights, Australia. Alternative methods such as single crystal diffraction and neutron interferometry are used for selected compounds such as LiF, LiBaF3 and ReO3.