## 2 Method  
Monochromatic neutron radiography is based on the analysis of images of a transmitted monochromatic neutron beam which is attenuated by a combination of scattering and absorption. Concerning the effect of scattering induced by a crystalline material, its intensity with respect to the wavelength depends on the crystalline structure of the material. Some pure metal samples with simple cubic structures can be very strong neutron scatterers depending on the nature of the constituent atom, and the coherent scattering effects generate a non-negligible variation of the transmitted intensity as a function of the crystalline state of the analysed sample. In general, when a polycrystalline isotropic sample is analysed, the transmitted beam is attenuated according to a specific model, complementary to the Bragg scattering effect. The wavelength-dependent attenuation spectrum can be measured through the scan of a monochromatic neutron beam tuned at different wavelengths, using, e.g., a double crystal monochromator (DCM). When measuring the attenuation spectrum of an isotropic powder-like sample, it is possible to observe the presence of an attenuation signal following a regular structure, composed by linear intensity variation followed by abrupt discontinuities in correspondence to the backscattering conditions of specific lattice plane families of the metal structure (called Bragg edges) [9].  
The position of Bragg edges measured as a function of neutron wavelength is correlated with the lattice plane families of the crystalline specimen according to the following relation:  
\[\lambda_{\mathrm{hkl}}=2\mathrm{d}_{\mathrm{hkl}}\]  
where \(\lambda_{\mathrm{hkl}}\) is the incident neutron wavelength and \(\mathrm{d}_{\mathrm{hkl}}\) is the distance of the hkl indexed lattice plane family.  
By performing Bragg edge analysis, and verifying how deviant the measured profile is with respect to the ideal powder conditions (large-size, defect-free, random domains) it is possible to evaluate several microstructural features such as the presence of defects, the existence of any anisotropy in the sample and of residual strain along the neutron beam measurement direction in a very similar way as for diffraction analysis. Bragg edge imaging techniques rely on the simultaneous measurement of the attenuation spectrum at each pixel of a detector device, allowing characterization of the microstructural properties of a metal specimen to a spatial resolution comparable with the typical radiographic methods (few hundreds of microns), much higher than typical neutron diffraction gauge volumes.  
In case large size single crystal grains are present within a sample, the wavelength scan analysis of the areas in which they are present shows an intensity attenuation profile, which is affected by the strong scattering effect of a few selected neutron wavelengths interacting with the specifically oriented lattice planes of the single crystal. Since such single crystal areas only interact with specific neutron wavelengths, the transmitted beam exhibits an almost flat profile showing the presence of dips at specific wavelengths corresponding to the Bragg scattering conditions of the lattice planes, according to their 20 orientation with respect to the primary beam direction [10; 11; 12; 13].  
These dips correspond to the interaction of the corresponding neutron wavelength with a specific lattice plane so for low Miller index planes it is, in theory, possible to reconstruct the relative orientation of the planes with respect to the beam. When multiple dips are present, it is further possible to reconstruct the specific orientation of the crystals within the sample. It is uncommon to deal with pure single crystal materials since the production procedure of large size single crystals is quite difficult, thus the samples observed commonly are a mixture of polycrystalline samples containing large size single crystals within. The resulting attenuation spectrum is hence a mixture of Bragg edge profiles combined with Bragg dips.