It's hard to get an X-ray image of low-density material like tissue between bones because X-rays just pass right through like sunlight through a window. Sandia studies myriads of low-density materials, from laminate layers in airplane wings to foams and epoxies that cushion parts. So they borrowed and refined a technique from the medical field, X-ray phase contrast imaging, to look inside the softer side of things. The technique measures not just the number of X-ray photons that get through the sample, as in conventional X-ray imaging, but also the phase of the X-rays after they pass through, offering a complete look at interfaces inside a structure.
Sandia has to be able to spot defects that might cause a high-consequence failure. For example, conventional X-rays can't see a defect called a grafoil in the laminate layers of an airplane wing without removing the copper mesh that protects against lightning strikes. And they can't see the critically important foams and other materials that guard against shock, high voltage breakdown, and thermal stresses in nuclear weapon components. X-ray phase contrast imaging could also be used to inspect micro-fabrication packaging, integrated circuits or micro-electro-mechanical components. It could be used to study ceramics, polymers, chemicals or explosives.
Current techniques aren't sensitive enough to distinguish between materials. If there is a dense material mixed in with a low-density material, and traditional X-rays can't see the low-density material, an observer doesn't know if the gaps are filled with the low-density materials or air.
The principle of operation can be illustrated by considering an orange. A conventional X-ray picture of the orange would be fuzzy, without detail. X-ray phase contrast imaging, on the other hand, would clearly show the differences between the thin layers of zest and pith and how those layers look compared to the thick pulp. When light hits the zest, it bends a little, It hits the pith and it bends a little bit more, then it goes through the pulp, and it bends in another direction. Every interface, every time the material changes within the sample, it bends the light a little bit. Different parts of the sample bend the light differently, giving rise to the phase contrast image.
The technique will be useful in the design stage of a project, for example, when you're trying to understand the distribution of microbeads within an epoxy or how foam mates with the canister it's filling up. It can also be used for quality assurance in the production stage.
Gratings that create interference patterns are critical to the technology. For the next step in the project, the researchers will be studying how to make gratings that operate at higher X-ray energies.