First-of-its-kind achromatic neutron lens brings the world into sharper focus

Neutrons can pass through dense metal while remaining sensitive to hydrogen, lithium and other light elements. That makes them valuable for looking inside engines, batteries, plants and archaeological objects without cutting them open.

The problem has always been control. Neutrons interact so weakly with matter that they are difficult to bend, focus or collect into a sharp image. Most neutron imaging systems therefore work without a lens.

Scientists at the Paul Scherrer Institute have now demonstrated an achromatic neutron lens that can focus a broad range of neutron wavelengths at nearly the same point. Published in Nature Communications, the work points toward true neutron microscopy and sharper imaging inside complex equipment.

The lens combines a nickel Fresnel zone plate with a diamond compound refractive lens. Together, the elements correct the wavelength-dependent blur that has limited earlier neutron optics.

Neutron beams are usually polychromatic, meaning they contain many wavelengths. Each wavelength bends differently through a conventional lens, so one may focus sharply while others blur.

That problem is called chromatic aberration. Visible-light optics solved it centuries ago by combining lenses made from materials with different dispersions. Neutrons posed a harder challenge because their weak interaction with matter leaves fewer practical options.

Current neutron radiography often uses a pinhole geometry. A small aperture shapes the beam, and the sample sits close to the detector to reduce blur.

“This limits the achievable resolution, as well as the size of the object or sample environment that can be imaged,” said Mano Raj Dhanalakshmi Veeraraj, the study’s first author and a doctoral student in the PSI Center for Photon Science.

That close placement becomes a problem when researchers want to observe samples inside furnaces, cryostats or pressure cells. Such equipment can take up substantial space.

The new lens changes that geometry by forming a magnified image on the detector. The sample no longer has to sit directly beside it.

The achromatic lens uses diffraction and refraction at the same time. The nickel zone plate generates a diffraction pattern, while the diamond structure refracts the neutron beam.

The nickel component contains concentric rings that become progressively narrower. Its smallest outer structures measure 143 nanometers across and rise 5.6 micrometers high.

The diamond component contains four bi-convex parabolic elements machined into a single crystal. The finished achromat measured 1.4 millimeters across and had a focal length of 0.8 meters at a neutron wavelength of 5 angstroms.

The two parts were designed so their wavelength dependencies counteract each other. A diffractive lens changes focal length in proportion to wavelength, while a refractive lens changes more strongly.

By pairing them in the correct geometry, the team kept the overall focal length nearly constant across a broad wavelength range.

The nickel structures were fabricated with electron-beam lithography in PSI’s PICO cleanroom. Swiss company SYNOVA S.A. manufactured the diamond component using a Laser MicroJet process.

“There are few other places in the world, if any, where this could have happened,” Dhanalakshmi Veeraraj said. “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.”

The team tested the lens at the NeXT instrument at the Institut Laue-Langevin in Grenoble, France. They compared it with a standalone Fresnel zone plate carrying the same focal length.

Across wavelengths from 3.5 to 7 angstroms, the achromatic lens kept images focused over a much broader range. The standalone zone plate performed well only near its design wavelength.

Using a Siemens star resolution target, the system achieved 6.7-times magnification and resolved structures smaller than 20 micrometers.

The researchers also imaged a commercial lithium-ion battery. The battery sat 5.91 meters from the detector, showing that magnified neutron imaging can work across distances needed for bulky experimental setups.

“The lack of such a lens has held back neutron imaging for decades,” said Joan Vila-Comamala, a 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.”

Simulations indicated that the achromat retained useful imaging performance at a 40 percent bandwidth. The lens used in the experiment had an estimated throughput of about 32 percent at 5 angstroms.

The experiment remains a proof of principle. Performance is currently limited by alignment between the nickel and diamond elements, neutron shot noise and the detector system.

Gravity also affects neutrons during long flight paths. Different wavelengths fall by different amounts, creating another source of blur.

The achromatic lens corrects wavelength-dependent focusing, but not gravitational dispersion. Future systems could use a gravity-correction prism or adjust images during time-of-flight processing.

Longer beamlines could increase magnification. The main restriction would come from the instrument’s length rather than the lens itself.

“This is just the beginning,” Dhanalakshmi Veeraraj said. “We already see ways to improve the lens. The key point is not simply resolution, but a completely new way of acquiring images.”

An achromatic neutron lens could let researchers examine functioning batteries, engines and other devices while keeping them inside realistic operating environments. It may also improve studies involving pressure, extreme temperatures or magnetic fields.

Magnification could allow facilities to use thicker, more efficient detectors without sacrificing detail. That may ease some trade-offs between resolution, brightness and exposure time.

The design could also support polarized neutron imaging and related methods that probe magnetic structures. New facilities with longer beamlines, including the European Spallation Source, may be better positioned to exploit the approach.

Further improvements in fabrication, alignment and detector performance will determine how far resolution can advance. The lens has a theoretical diffraction limit below one micrometer, although the present experiment remained above that level.

For neutron science, the larger shift is geometric. Researchers now have a practical path from pinhole radiography toward magnified microscopy, creating room for experiments that were previously difficult to arrange.

Research findings are available online in the journal Nature Communications.

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