Neutron Interferometer Uses Silicon Lenses to Probe Materials

Scientists have demonstrated that a three phase-grating moiré neutron interferometer in a neutron beam could be a robust candidate for large area interferometry applications and for the characterization of materials. The technique would provide neutron interferometry tools with the ability to zoom in and out on details ranging in size from 1 nm to 10 μm — a range that has been difficult to probe with other neutron scanning methods.

The approach, developed by a team from the National Institute of Standards and Technology (NIST), the National Institutes of Health, and the University of Waterloo, equips the interferometer with “lenses” — silicon wafers that act as diffraction gratings. The gratings split the neutron beam and redirect it, causing the waves to bounce off the edges of an object and then collide, creating a visible moiré interference pattern representative of the object.

The neutron interferometer can scan the interior of thick objects, such as this chunk of granite, providing enough detail to show the four types of rock that are mixed within it. Courtesy of Huber & Hanacek, NIST.

The team observed interference fringes with an interferometer length of 4 m and examined the effects of an aluminum 6061 alloy sample on the coherence of the system. It is hoping to use the technique to measure universal gravitational constant.

The researchers say this far-field technique allows for broad wavelength acceptance and relaxed requirements related to fabrication and alignment, thus circumventing the main obstacles associated with perfect crystal neutron interferometry.

Traditionally, crystals that are perfect enough for interferometry also block out most of the neutrons that strike them — thus making the neutron sources weak. For a beam to send enough neutrons through a sample to get an accurate index of refraction would take a long time; other tasks would take even longer.

The new approach sidesteps this issue by using a trio of thin silicon gratings to focus the neutrons instead of a single costly crystal. Under a microscope, the flat surface of each grating looks like a comb with narrow, closely spaced teeth. The gratings allow the entire neutron beam to pass through them, and have the additional advantage of being movable.

Moving these three gratings focuses neutron beams on a sample, allowing them to perceive interior details ranging from 1 nm to 10 μm. Courtesy of Huber & Hanacek, NIST.

“You focus by moving the grating a fraction of a millimeter,” researcher Michael Huber said.

“We can look at structure on lots of different levels and at different scales. It could complement other scanning techniques because its resolution is so good. It has a dramatic ability to focus, and we aren’t limited to looking at thin slices of material as with other methods — we can easily look inside a thick chunk of rock,” Huber said.

The team has already scanned the interior of a block of granite that contains a mixture of four different minerals, and the scan shows the details of where each bit of mineral sits. Huber said the method would be good for noninvasive scans of porous objects like meteorites or manufactured materials, such as gels or foams, which are the basis of many consumer products.

Researchers say only one thing stands in the way of their interferometer becoming a great tool for industry: They need a set of apertures of different widths for the neutron beam to pass through before it hits the interferometer. Right now, they only have a single aperture at their disposal, which limits their vision.

“We can see the full range of 1 nm to 10 μm now, but the image is kind of blurry because we don’t get enough data,” Huber said. “Every different aperture gives us another data point, and with enough points we can start doing quantitative analysis of a material’s microstructure. We’re hoping that we can get a set of maybe 100 made, which would enable us to get detailed quantitative information.”

The research was published in Physical Review Letters (doi:10.1103/PhysRevLett.120.113201).