A new 3D imaging technique using neutrons has been invented by physicists in Germany. The technique, which can visualize magnetic fields inside bulk objects, is an improvement on existing magnetic methods that are limited to surfaces. The method could find use in a range of science and engineering fields and shed more light on various magnetic phenomena in solids, including superconductivity.

Neutrons are subatomic particles that have a net zero charge and can therefore penetrate thick layers of material. They also have a magnetic moment (or spin) and so are sensitive to magnetic fields). When all the spins point in the same direction, a neutron beam is said to be spin-polarized.

Rotating moments

Nikolay Kardjilov of the Hahn-Meitner Institute in Berlin and colleagues used a beam of polarized neutrons from a nuclear reactor to irradiate samples in their experiments (Nature Physics doi: 10.1038/nphys912). As the neutrons travel through a sample, their magnetic moments rotate around the magnetic fields they encounter and the direction of their spin changes. The researchers measure the different spin angles, which depend on the strength of the magnetic fields traversed by the neutrons.

These angles are then converted into intensities by a polarization analyser, located behind the sample. Next, a position-sensitive detector measures these intensities to build up a map of the magnetic fields inside the sample.

“The spin-polarized neutron imaging technique is non-destructive and non-invasive and is better than conventional 2D imaging techniques,” Kardjilov told physicsworld.com. “The method can be employed in a variety of environments—for example, at high or low temperatures — and samples can be investigated from almost any viewing angle. In this way 3D information about the magnetic field distribution is revealed.”

Megapixels in minutes

Moreover, measurements are fast. Each image contains about a million pixels and is taken in just seconds or minutes depending on the sample. “No other method can compete — even in free space,” added Kardjilov.

Our aim is to achieve a spatial resolution of 50 µm in just a few minutes per imageNikolay Kardjilov, Hahn-Meitner Institute

The technique might be used to investigate magnetic flux distribution and pinning in superconducting samples, which could be important for understanding high-temperature superconductivity. Indeed, the researchers have already investigated the trapped magnetic flux inside a polycrystalline lead cylinder, a type-I superconductor. The method could also be used to visualize magnetic domain distributions in bulk ferromagnets in 3D for the first time.

The team, which includes researchers from the Berlin Institute of Technology, Ruprecht Karls University Heidelberg and the University of Applied Sciences Berlin, are now working on improving the spatial and temporal resolution of its technique. “Our aim is to achieve a spatial resolution of 50 µm in just a few minutes per image,” explained Kardjilov. The researchers also hope to increase the sensitivity of the method by further developing an iterative algorithm to quantify results obtained.

'Remarkable technical advance'

Bruce Gaulin , who is Brockhouse Chair in the Physics of Materials at Canada’s McMaster University, described the team’s work as a “remarkable technical advance”. “As a proof-of-principle measurement, it should allow even more detailed mappings of magnetic field distributions in matter”, Gaulin told physicsworld.com.

Kardjilov and colleagues are not the only team developing imaging techniques based on spin-polarized neutrons. “We consider these studies as pilot tests to demonstrate magnetic imaging using polarized neutrons,” commented Eberhard Lehmann from the Paul Scherrer Institute in Switzerland, whose team is working on a similar but “more sophisticated” experiment called “neutron spin phase imaging” (Nucl Instr Meth A 586 15).

Lehmann suggests that dedicated beam lines at suitable intense neutron sources would help advance the field of neutron imaging and so allow for more detailed technical studies.