ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2023 21 as a map in Fig. 4d. The wavelength distribution is relatively random. Wavelength variations are likely based on the neutron source and the fitting accuracy. The authors chose to map the fitted wavelengths rather than the strain since this is an annealed powder sample (i.e., no strain). However, based on the fitted position of each Bragg edge, one can calculate the elastic lattice strain εhkl given by where d0 hkl is the strain-free lattice spacing. At epithermal energies, i.e., neutron energies above ~1 eV, the neutron transmission spectrum exhibits attenuation dips called resonances that are element/isotope specific, as illustrated in Fig. 5a, which displays a resonance pattern from a modeled sample made of 50 µm of tantalum (Ta) and 50 µm of tungsten (W). Figure 5b shows resonances for 3 of W isotopes, namely 182W, 183W, and 184W. As illustrated, resonance imaging is sensitive to elements and isotopes. Since this technique can be performed in 3D, elemental/isotopic volumetric maps can be visualized in θ increases as a function of the wavelength until it reaches π/2, at which point λBragg = 2dhkl. Past λBragg, a sharp drop in transmission called the Bragg edge is observed. Hence, the measurement of the λBragg is a direct measurement of the lattice spacing, dhkl. Using thermal and/or cold neutrons, Bragg edge imaging maps crystalline lattice planes that satisfy Bragg’s law in transmission, phases, and lattice strain. An example of a full Bragg edge pattern (measured at the SNS SNAP beamline) of a 5-mm thick face- centered cubic (FCC) nickel power (Ni) is displayed in Fig. 4. Figure 4a depicts a typical radiograph acquired using a broad wavelength range of neutrons. Figure 4b shows the selected binning (16 x 16 pixels, with a pixel length of 55 µm). Figure 4c shows the sample- averaged Bragg edge pattern. To quantify materials properties like strain, the <200> Bragg edge was arbitrarily chosen to fit each binned area. Fitting was performed using an in-house software iBeatles, which utilizes an analytical method[30] to fit the edges. The resulting <200> Bragg-edge wavelength fitted position for each binned pixel is displayed a sample. Fitting of the resonances allow for quantitative analysis, as demonstrated by Tremsin et al.[31] SUMMARY AND FUTURE PROSPECTS Neutron imaging capabilities at pulsed neutron sources such as the ORNL SNS facility offer unique capabilities for materials science and engineering applications. The SNS VENUS beamline is optimized for two main hyperspectral capabilities: Bragg edge and resonance imaging. Bragg edge radiography and tomography offer a unique approach to measuring crystalline properties such as lattice planes, preferred grain orientation, crystalline phases, and strain. At the SNS, software development is ongoing to improve the automated Bragg edge fitting software, iBeatles, which provides strain maps of an object. While Bragg edge tomography has been demonstrated in a few cases with simplified sample geometries, it is a difficult problem to solve since each voxel contains information about a tensor with six unknowns (rather than a linear attenuation scalar value). The ORNL Fig. 3 – Depiction of the workflow on a hyperspectral neutron radiography experiment. Neutrons that transmit through a sample are measured on a 2D pixelated detector. Each pixel records a time-of-flight (or wavelength-dependent) spectrum. When using thermal and/or cold neutrons, the transmission spectrum as a function of wavelength reveals Bragg edges. At higher energies (shorter wavelengths), the transmission plot as a function of energy through a crystalline or amorphous material exhibits attenuation dips that are unique to an element or isotope.
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