Innovative Neutron Interferometry Technique for Scanning Innards of Porous Objects

Without lenses that can focus, samples cannot be viewed clearly; whether those lenses are in the eye or a microscope that used to investigate. An inventive method for focusing neutron beams might enable researchers to investigate the interiors of opaque objects at a size range they could not view earlier, thus enabling them to analyze the inner sides of objects such as advanced manufactured materials and meteorites, without any harm to them.

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. (Image credit: Huber & Hanacek/NIST)

The technique has been reported in the Physical Review Letters journal on March 12, 2018, and can transform a support tool conventionally used for neutron science into a well-developed scanning method. It can uncover details varying in size from 1 nm up to 10 mm within larger objects. The strategy offers the tool, called neutron interferometry, and its first movable “lenses”, with the potential to zoom in and out on details in this size range, which has been challenging to investigate, even by adopting other neutron scanning techniques.

In precise terms, these “lenses” are silicon wafers that function as diffraction gratings, which make the most of the wave-like characteristics of neutrons. The gratings divide and redirect a neutron beam such that the waves are reflected off the edges of an object and then bump into each other, developing a visible moiré interference pattern characteristic to the object that can be easily understood by experts.

The technique was devised by a group of scientists from the National Institute of Standards and Technology (NIST), the National Institutes of Health (NIH), and Canada’s University of Waterloo. As stated by Michael Huber from NIST, the strategy can make neutron interferometry to be one among the foremost exploratory tools to be included in the kit of a material scientist.

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.

Michael Huber, Physicist at NIST’s Physical Measurement Laboratory

Interferometry is an area of specialization in the realm of neutron science. Prior to investigation of the innards of an object by using a neutron beam, researchers have to initially have a few basic details related to the way the neutrons will get reflected off the atomic structure of the object. One such detail is the index of refraction of a substance, which indicates the extent to which the substance will bend the beam from the direction in which it is traveling (light is bent by water bends in a related manner, the reason behind one’s arm looking bent away when it is dipped into a swimming pool). Neutron interferometry is the most optimal method for acquiring that critical evaluation.

Neutron interferometry can be applied to different applications in basic physics, for example, precise evaluation of the gravitational constant. The technique is adequately sensitive to monitor the way in which the gravitational force of an object can deflect neutrons, similar to the way Earth attracts a flying ball and vice versa. However, the disadvantage of the neutron technique is that it works very gradually. To focus neutrons on a material sample, an interferometer mandates a crystal chiseled to accurate dimensions from a single, huge block of high-cost, top-quality silicon (other neutron methods can be performed by using crystals of far lower quality).

However, crystals that are superior enough for interferometry also inhibit the passage of a majority of the neutrons striking them, indicating that considerable amount of time is needed for a beam to send adequate neutrons through a sample to ensure a precise index of refraction. Other efforts would need considerably more time.

The neutron sources are already very weak. It would take a hundred years to get a good answer to fundamental questions such as the value of the gravitational constant.

Dmitry Pushin, Waterloo

The innovative strategy overcomes these difficulties by using three thin silicon gratings to focus the neutrons in the place of only a single expensive crystal. When viewed under a microscope, the flat surface of each grating resembles a comb that has narrow, closely spaced teeth. Apart from allowing the whole of the neutron beam to move through them—instead of allowing only a portion of neutrons to pass through the crystal—the gratings also have the crucial benefit of being transferable.

You focus by moving the grating a fraction of a millimeter,” stated Huber. “It’s slight but not difficult.”

The team’s strategy was exhibited at the NIST Center for Neutron Research and is the advancement over a finding initially made at NIH, where researchers were investigating the application of the gratings to X-ray beams and observed the formation of moiré pattern on their visual imager.

The idea was first developed by our lab to capture the image of materials where X-rays travel at slightly different speeds than in the air, such as the human body itself. Central to this idea is X-ray gratings, which were made with the highly specialized tools at the NIST Nanofab facility.

Han Wen, Senior Investigator at NIH’s National Heart, Lung, & Blood Institute

Coincidentally, the NIST and Waterloo researchers had a chance to meet the NIH group members at a conference and initiate a partnership, anticipating that the gratings would function well for neutrons similar to the way they worked for X-rays. The NIH group took the gratings back to NIST and assembled them into the neutron interferometer.

After correspondingly better outcomes at the NCNR, Huber stated that there is only one aspect that hinders their interferometer from transforming into an outstanding tool for the industry: They require a set of apertures with varying widths through which the neutron beam will pass through before hitting the interferometer. At present, there is only a single aperture, and it restricts their vision.

We can see the full range of 1 nanometers to 10 micrometers now, but the image is kind of blurry because we don’t get enough data,” he stated. “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 a hundred made, which would enable us to get detailed quantitative information.”

The group has already scanned the innards of a granite block that is formed of a blend of four disparate minerals, and the scan reveals the details related to the positioning of each bit. Huber stated that the technique would hold good for non-invasive scans of porous objects such as meteorites or manufactured materials (foams or gels), which are the fundamental materials in various consumer products.

We’re also hoping we can finally do that gravitational constant measurement,” he stated. “We could put a big block of some heavy metal like tungsten nearby and see how it bends the beam. It would improve our understanding of the universe and wouldn’t take longer than our lifetimes.”

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