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Neutron Dark-Field Imaging with Edge Illumination

We report on an Edge Illumination setup enabling neutron dark-field imaging where two amplitude modulators are used to structure and subsequently analyze the neutron beam. The modulator and analyzer are manufactured by laser ablation of readily available thin metal foils. The sample representation in terms of transmission and dark-field contrast is extracted by numerically inverting a convolution model for the intensity modulation function which had a visibility exceeding 80\%. Two test samples are presented to show how dark-field contrast can complement the more conventional neutron radiography, in particular to investigate the micro-structure of materials. Thanks to the simplicity of the setup, the negligible coherence requirements and the robustness of the method, this approach may find application in multi-contrast neutron radiography and tomography.

Neutron imaging techniques produce planar and three-dimensional representations of the interaction of a neutron probe with the sample under investigation.
Neutron radiography is arguably the simplest of such approaches, where image contrast arises from differences in attenuation that the neutrons experience whilst traveling through the sample volume 1 . Neutron scattering is nowadays an established method for investigating samples at short length scales. It typically entails probing a relatively large volume, of several cubic centimetres, over which the measurements are necessarily averaged. Techniques are evolving rapidly and by means of grating interferometers 2,3 it is now possible to investigate the sample's transmission and scattering simultaneously and in a spatially resolved fashion.
This yields images reporting information spanning across a wide range of length scales, from the centimeter down to the micron and below [4][5][6] . We report here on a neutron dark-field imaging approach which we developed building on our previous work with incoherent X-ray radiation 7 . Edge Illumination (EI) is based on the strong structuring of the radiation field by means of an amplitude modulator which is used in conjunction with a harmonically matched amplitude analyzing structure. This setup enables phase sensitivity and the simultaneous measurement of transmission, refraction and scattering properties of a sample. The most common design uses line apertures, in such a way that beam shaping and analysis are one-dimensional, and the same holds for refraction and scattering sensitivity.
EI has been extensively studied with X-ray radiation, both at synchrotrons and in laboratory settings, and has shown promise for diverse applications of planar and three-dimensional imaging including mammography, intraoperative imaging, composite materials studies and security screening. With this study we aim at translating the robustness, sensitivity and achromaticty properties of EI to a setup for application with neutron beams.
The experiment was carried out at the neutron imaging and diffraction facility IMAT 8 , a beamline at the ISIS pulsed neutron spallation source, United Kingdom.
where IF s represents the illumination function in the presence of the sample, However, a transition region is clearly identifiable by looking at the dark-field image, whilst it is much less, if at all, visible in the transmission contrast channel.
We interpret this signal as an indication of the presence of a layer, in the vicinity of the liquid surface, where the packing density of spheres changes rapidly.
Some bright and dark spots are visible in the images in Fig. 2 and Fig. 3, in a way that resembles salt and pepper noise. This is due to the imperfect stability of the retrieval algorithm that does not converge in some pixels and returns largely incorrect values. These localized problems are linked to the presence of a relatively high noise in the raw images which was due to the need to run the camera without cooling, for a temporary problem with its vacuum that occurred during the experiment.
In summary, we reported on an experimental setup for neutron dark-field imaging based on Edge Illumination. The experimental apparatus makes use of an amplitude modulator and an amplitude analyzer, offers high (> 80%) intensity modulation visibility and is completely achromatic. We showed, through experimental results on two test samples, examples of how dark-field contrast can complement standard attenuation contrast, enabling the simultaneous extraction of additional information that can be useful in characterizing the micro-structure of materials. We found these first proof-of-principle results encouraging towards the development of a fully achromatic neutron dark-field imaging system that can efficiently operate across a wide wavelength spectrum.