Wolter Mirrors Focus Neutrons in a Compact SANS Instrument
by Tim Palucka
Materials Research Society | Published: 16 October 2013
Neutron scattering and imaging have been valuable materials science tools since the 1950's, but they remain flux-limited. Now, borrowing an idea from x-ray astronomers, researchers at MIT and NASA have built a small-angle neutron scattering (SANS) instrument that uses axisymmetric mirrors to focus neutrons over a small focal length with high throughput. If the mirrors were optimized, they could significantly improve the performance of neutron-scattering and imaging facilities, acting as an image-forming lens of a microscope. Neutrons can penetrate deep into bulk of materials, and are especially sensitive to hydrogen, oxygen, and other light elements that would be invaluable to scientists investigating biomaterials and other soft materials.
"NASA and the x-ray astronomy community have invested amazing amounts of effort and money for many years to make x-ray mirrors and telescopes that would work," says physicist Boris Khaykovich of MIT, the project leader of the research described in a paper that appeared recently in Nature Communications. Hans Wolter proposed in 1952 that the double reflection from a combination of ellipsoid and hyperboloid mirrors could be used to focus x-rays; Wolter's ideas have been widely adopted for x-ray telescopes. Khaykovich points out that it was not obvious at first that optical surfaces could be polished to a degree that would be useful for x-rays, which have wavelengths between 1 and 10 angstroms, because that is the same size as some typical unit cells of materials. Diffraction from the mirror surface could easily produce enough noise to blur the images beyond usefulness. But once the astronomers demonstrated that the optics worked with x-rays, that opened the door to the possibility that similar optics could work for neutrons, which have roughly the same wavelength as x-rays. After much research, "what happened is a lucky result that Wolter optics look very suitable for neutrons," Khaykovich says.
"Scientists have long thought about ways to focus neutrons," says Dazhi Liu of MIT, the paper's lead author. Neutrons have been used in a suite of characterization techniques, including SANS, reflectometry, inelastic neutron scattering, and others. Because of the difficulty of focusing neutrons, these techniques have tended to rely on delivering a collimated beam of neutrons through a pinhole to the sample, losing a large fraction of the neutrons in the process. In a few cases, refractive lenses have been used to focus neutrons, but the large focal length (up to several tens of meters) of the system has prevented widespread adoption of suchlenses. Furthermore, these lenses can only be used with a monochromatic neutron beam, further limiting their applications.
In this latest research Liu, Khaykovich and colleagues used a polychromatic neutron beamline at Oak Ridge National Laboratory with nested co-axial confocal nickel mirrors made by NASA to prove their concept. The mirrors, which are axisymmetric around the beam, were made from cut pieces of an ellipsoid directly attached to a cut piece of a hyperboloid, resulting in the two reflections necessary for high fidelity imaging of the sample. Liu describes the mirror's shape as similar to a paper cup with the bottom cut out; this shape is necessary because neutrons can reflect from surfaces at glancing-incidence angles only. The neutron source aperture and the detector were placed at the two foci of the mirror, with the sample between the mirror and the detector. The detector recorded the position of each scattered neutron, and the time-of-flight- of each neutron was noted. "The scattered pattern represents the position of particles in the sample, and it also can measure the dynamics of the particles," Liu says.
Using a standard porous silica sample (Porasil B) and silver behenate powder to test their SANS instrument, the researchers obtained a low-momentum transfer (Q) limit of about 0.02 A-1, and a Q resolution of approximately 14%, or approximately 0.9 λ-1, for neutron wavelengths between 6.4 and 9.2 angstroms. The authors conclude the paper by stating that their prototype SANS instrument has the potential to improve the performance of SANS facilities by orders of magnitude in terms of signal rate, while simultaneously improving the resolution. All of this could possibly be done in a more compact instrument with simpler detectors than are currently used in SANS devices, they conclude.
"The authors have creatively used technology from the x-ray telescope community and drawn on somewhat similar optical techniques in use in x-ray scattering facilities to successfully attack the technically more difficult neutron case," says W. Michael Snow, an experimental nuclear physicist at Indiana University who was not involved in this research. Another independent expert, Robert Dalgliesh at the Rutherford Appleton Laboratory of the Science and Technology Facilities Council in the UK, agrees with Snow's assessment: "The initial results are extremely encouraging and should enable significant gains to be made in small angle neutron scattering for numerous experimental configurations." He cautions that "the annular beam shape and inherent larger sample footprint does lead to the need to manufacture larger volumes of sample to extract maximum benefit from the optics," but acknowledges that this "will not prevent significant gains being made in many areas."
"The goal here is to improve on the state of the art by making qualitatively different instruments-a different way of doing neutron scattering and neutron imaging," says Khaykovich. "What we did is a lot of preliminary work and demonstrations. Whether the neutron scattering community will pick this up is not a done deal, but they are clearly very interested."
Read the abstract from Nature Communications here.