Rietveld Texture Analysis by Neutron Diffraction of .highly Absorbing Materials

We discuss the impact of strong absorption for thermal neutrons on data analysis and compare absorption corrections in the GSAS and MAUD Rietveld codes for texture and structural parameter refinement. Diffraction data were collected on the neutron powder diffractometer HIPPO at LANSCE from dysprosium and erbium, which are moderate to strong absorbers for thermal neutrons with absorption cross-sections of 159 barns for Er and 994 barns for Dy at = 1.8 Å. Both elements have hexagonal close-packed (hcp) crystal structures, and the samples were various thicknesses of rolled foils. The orientation distribution functions (ODF) were fit to the same neutron time-of-flight data sets using two very different full pattern Rietveld analysis procedures. Spherical harmonics functions were fit to the textured data using GSAS. These data were also analyzed by the modified direct method E-WIMV using MAUD. The resulting pole figures from the ODFs determined by both Rietveld analysis packages are qualitatively similar, and the textures were confirmed by X-ray diffraction. Additionally, data from orthorhombic dysprosium and erbium fluoride powders show that atomic positions are not sensitive to absorption. We address inconsistencies and methodologies in data analysis when strong absorption is present. BACKGROUND AND EXPERIMENT For most materials, neutrons penetrate deeply into the sample, making surface preparation or absorption effects negligible. As a result, neutrons generally offer a non-destructive probe for characterization of materials properties. Neutron time-of-flight (TOF) measurements, even in the presence of multiple phases, allow simultaneous refinement of crystal structure and texture [1,2]. Because neutron scattering has advantages over other techniques, it is desirable to develop methods to look at all materials, even those with large absorption cross-sections. Therefore, we wish to understand the relationship (and correlation) between absorption and other parameters to prevent misinterpretation of data, which will allow strong neutron absorbers to be more readily studied with neutron characterization methods. For example, Pu is an isotope with a high absorption cross-section that would be of interest to study with neutron diffraction. In this study, we examine absorption corrections in two Rietveld refinement programs on data collected from surrogate rare earth elements with moderate and strong absorption for thermal neutrons. Dysprosium and erbium are both hcp materials (space group P63/mmc). The lattice constants for dysprosium are a = 3.590 Å, c = 5.648 Å (c/a = 1.573), and for erbium are a = 3.559 Å, c = 5.587 Å (c/a = 1.570) [3]. These rare earths were chosen as surrogate materials to study plutonium239, which has a thermal absorption cross-section of 1017 barns. At = 1.8 Å, the absorption 156 Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002