Neutrons can give unique insights into the structure of materials – but they are hard to manipulate. Like X-rays, neutrons, produced as a beam at research facilities such as the Swiss Spallation Neutron Source SINQ, are used to image inside materials and objects. Unlike X-rays, however, neutrons can penetrate deeply into many metals whilst remaining highly sensitive to light elements such as hydrogen and lithium. In this way, they can be used to observe oil, polymer or lithium distribution inside dense metallic structures such as engines or batteries, reveal water uptake in plants or non-destructively examine priceless archaeological artefacts.
Yet the same weak interaction with matter that makes neutrons such a useful tool also makes them notoriously difficult to deflect or focus – a fact that has limited the development of advanced imaging techniques. Now, PSI scientists have reported in Nature Communications a new type of lens that overcomes this barrier.
A lens for all colours of neutrons
A large part of the challenge lies in the fact that neutron beams typically contain neutrons of many different wavelengths. To achieve a high-resolution image, the lens must bring these to the same focal point. Despite past attempts, no practical neutron imaging lens has been able to focus the broad range of wavelengths in a neutron beam – until now.
Currently, neutron imaging is performed without lenses, but this forces researchers to place samples close to the detector to keep images sharp. "This limits the achievable resolution, as well as the size of the object or sample environment that can be imaged," says Mano Raj Dhanalakshmi Veeraraj, first author of the study and PhD student in the PSI Center for Photon Science.
The new lens – the first of its type in the world – is a so-called achromatic neutron lens, which focuses a broad range of neutron wavelengths to the same point. This enables sharp, magnified imaging with a resolution below twenty micrometers – even for objects that cannot be placed close to the detector.
"The lack of such a lens has held back neutron imaging for decades," says Joan Vila-Comamala, scientist in the PSI Center for Photon Science, who led the team. "Now that we have it, it becomes possible to follow processes inside equipment such as furnaces, cryostats or pressure cells. It also opens the path to neutron microscopy, making it possible to produce magnified images of an object and reveal more detail."
A completely new way of acquiring images
In the study, the researchers tested the lens by imaging a commercial lithium-ion battery. With the battery placed six metres away from the detector, they could magnify the layered structure of the wound electrode assembly by seven times.
In the future, this could make it possible to observe fine internal details of materials and devices while they are functioning in realistic environments, such as detecting structural changes within components of a running engine.
"This is just the beginning," adds Dhanalakshmi Veeraraj. "We already see ways to improve the lens. The key point is not simply resolution, but a completely new way of acquiring images."
Now neutron imaging facilities will have to catch up. To fully exploit the new lenses, some facilities may need longer beamlines. "If you can have a long enough beamline, you can in principle magnify more. It is not limited by the lens, but by the length of the instrument," says Dhanalakshmi Veeraraj. New facilities such as the European Spallation Source, currently under construction in Sweden, are already incorporating these new requirements, paving the way for further growth in neutron imaging and its applications.
Building on X-ray lens success
The technology draws on the team's earlier breakthrough in X-ray optics: the development in 2022 of an achromatic X-ray lens for synchrotron and X-ray free electron laser facilities such as the Swiss Light Source SLS and SwissFEL. The development of the neutron lens combined this X-ray optics expertise from the PSI Center for Photon Science with neutron imaging expertise from the PSI Center for Neutron and Muon Sciences.
The neutron lenses consist of concentric rings made of nickel and precisely shaped diamond structures, arranged in a carefully defined geometry. Unlike conventional visible-light lenses, which rely only on refraction, the neutron lenses also exploit diffraction – the phenomenon that causes waves to spread out or form patterns when passing through gratings or small apertures. The nickel rings generate the diffraction pattern, while the diamond structures refract the neutron beam; together these effects form a magnified image on the detector.
The intricate nickel structures were fabricated using electron-beam lithography in PSI's recently inaugurated PICO cleanroom facilities, while the diamond refractive structures were manufactured by the Swiss company SYNOVA S.A. "The nickel rings get smaller and smaller, with the finest rings measuring well below 200 nanometres," says Vila-Comamala.
After fabrication, the prototypes could be tested rapidly with X-rays at the Swiss Light Source SLS and tested with neutrons at the Swiss Spallation Neutron Source SINQ.
"There are few other places in the world, if any, where this could have happened," says Dhanalakshmi Veeraraj. "The close collaboration between experts in neutron imaging, X-ray optics, and nanofabrication, based within walking distance of one another on the PSI campus, makes technological breakthroughs such as this possible."
Text: Simone Pengue
About PSI
The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of future technologies, energy and climate, health innovation and fundamentals of nature. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2300 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 450 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research).
