Energy-selective neutron imaging for materials science

Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample.

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