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.<\/p>\n
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.<\/p>\n
Scientists at the Paul Scherrer Institute<\/a> 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<\/a>, the work points toward true neutron microscopy and sharper imaging inside complex equipment.<\/p>\n 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.<\/p>\n Design and components of the neutron achromat. (CREDIT: Nature Communications) A beam that refuses to behave<\/p>\n 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.<\/p>\n That problem is called chromatic aberration. Visible-light optics<\/a> 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.<\/p>\n 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.<\/p>\n \u201cThis limits the achievable resolution, as well as the size of the object or sample environment that can be imaged,\u201d said Mano Raj Dhanalakshmi Veeraraj, the study\u2019s first author and a doctoral student in the PSI Center for Photon Science.<\/p>\n 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.<\/p>\n The new lens changes that geometry by forming a magnified image on the detector. The sample no longer has to sit directly beside it.<\/p>\n A sharper view from metres away: This magnified neutron image of a three millimeter PSI logo was acquired using the new achromatic neutron lens, with the logo placed six metres from the detector. Without the lens, comparable resolution would require the object to be placed within centimetres \u2013 or even millimetres \u2013 of the detector. (CREDIT: Paul Scherrer Institute PSI\/Mano Raj Dhanalakshmi Veeraraj) Nickel rings meet diamond curves<\/p>\n 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.<\/p>\n The nickel component contains concentric rings that become progressively narrower. Its smallest outer structures measure 143 nanometers across and rise 5.6 micrometers high.<\/p>\n The diamond component<\/a> 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.<\/p>\n 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.<\/p>\n By pairing them in the correct geometry, the team kept the overall focal length nearly constant across a broad wavelength range.<\/p>\n The nickel structures were fabricated with electron-beam lithography in PSI\u2019s PICO cleanroom. Swiss company SYNOVA S.A. manufactured the diamond component using a Laser MicroJet process.<\/p>\n Demonstration of the achromatic behavior of the neutron achromat and comparison with an equivalent Fresnel zone plate (FZP). (CREDIT: Nature Communications) <\/p>\n \u201cThere are few other places in the world, if any, where this could have happened,\u201d Dhanalakshmi Veeraraj said. \u201cThe 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.\u201d<\/p>\n Sharp images across many wavelengths<\/p>\n The team tested the lens at the NeXT instrument at the Institut Laue-Langevin<\/a> in Grenoble, France. They compared it with a standalone Fresnel zone plate carrying the same focal length.<\/p>\n 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.<\/p>\n Using a Siemens star resolution target, the system achieved 6.7-times magnification and resolved structures smaller than 20 micrometers.<\/p>\n The researchers also imaged a commercial lithium-ion battery<\/a>. The battery sat 5.91 meters from the detector, showing that magnified neutron imaging can work across distances needed for bulky experimental setups.<\/p>\n \u201cThe lack of such a lens has held back neutron imaging for decades,\u201d said Joan Vila-Comamala, a scientist in the PSI Center for Photon Science who led the team. \u201cNow 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.\u201d<\/p>\n Simulation of bandwidth-dependent neutron imaging with a Fresnel zone plate, a compound refractive lens, and an achromat. (CREDIT: Nature Communications) <\/p>\n 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.<\/p>\n Gravity still creates another blur<\/p>\n 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.<\/p>\n Gravity also affects neutrons during long flight paths. Different wavelengths fall by different amounts, creating another source of blur.<\/p>\n