Pattern created by interference of neutron holography. Credit: NIST
Pattern created by interference of neutron holography. Credit: NIST

Neutron Beams Used to Create Holograms That Show Interior of an Object

Neutron beams have been used to create holograms of large solid objects for the first time. Researchers from the National Institute of Standards and Technology (NIST) have shown that these neutron holograms are better than ordinary laser light based visual ones in that they can reveal details about the interior of the object.

Holograms are flat images that give the sense that they are three-dimensional objects as they change depending on the viewer’s perspective. This illusion is created by what is called an interference pattern. Photon and neutron beams can act like rippling waves with valleys and peaks. This ability can be likened to a water wave hitting a gap between the two rocks. When the wave hits the gap, it splits up and then re-combines to create interference patterns that are rich in information.

When an optical hologram is made, a laser is shone at an object. A recording is then made of how the reflected laser light waves interfere with each other, instead of simply photographing the light reflected from the object. The resulting patterns, based on the relative positions of the waves’ peaks and valleys or phase differences, contain more information about an object’s appearance than a photo does. This technique does however not tell us much about an object’s hidden interior.

Animation of neutron scanning data, demonstrating that scientists can use neutron beams to create holograms instead of the usual laser light.

Neutron scientists explore the hidden interiors of objects. Scientists who create a new substance and want to investigate its properties use neutrons beams as these are able to penetrate metals and many other solids. Neutrons do however have limitations, as they are not very good for creating visual images. Neutron experimental data is therefore usually expressed as graphs and the data typically shows how a substance is made in general. This is handy if scientists wants approximate data about an object built from a bunch of repeating structures such as a crystal, but it does not help much if they want to know the details about one specific part of it.

The research team has now found a way in which they can have the best of both worlds.

In previous work performed by the team at the NIST Center for Neutron Research (NCNR), neutrons were passed through a cylinder of aluminum that had a small spiral staircase carved into one of its circular faces. The cylinder’s shape conveyed a twist to the neutron beam. The team did however notice that the beam’s individual neutrons changed phase depending on what section of the cylinder they passed through – the phase shift was bigger with a thicker section. They eventually realized that this was the information they needed to create holograms of the objects’ interior. This method is detailed in their new paper.

Animation of neutron scanning data, demonstrating that scientists can use neutron beams to create holograms instead of the usual laser light.

The discovery adds to the range of techniques scientists have available to explore solid materials. The team has shown that direct visual representations of an object can be created and details about specific points within it revealed by using a beam of neutrons and an interferometer. An interferometer is a detector that measures interference patterns.

Michael Huber of NIST’s Physical Measurement Laboratory notes that although other techniques measure small features as well, they are limited to measuring the surface properties only. He adds that the new method might be a more prudent technique for measuring interfaces buried inside the bulk of the material, as well as small, 10-micron size structures.

Full study has been published in the journal Optics Express.