Neutron Beam Applications

The Neutron Beam Applications group develops novel instrumentation and methods for neutron scattering and imaging. These include the optimization of neutron focusing mirrors, so called Wolter mirrors, and the use of polarized neutron imaging for studying magnetic materials. In addition, we operate a few thermal-neutron beamlines at the MIT Nuclear Reactor; the beamlines are used for tests of neutron optics and detectors, as well as for early tests of new methods and materials.

For inquiries about specific Neutron Beam Applications, please contact Boris Khaykovich.

Neutron scattering is one of the most useful methods for studying the structure and dynamics of matter. Lacking electrical charge, and interacting with atomic nuclei over a short range only, neutrons penetrate deep inside materials. As a result, neutron-scattering measurements can reveal, for example, atomic coordinates in crystal lattices, the molecular conformation of polymers, and structures of complex fluids. Neutrons are especially sensitive to light elements such as hydrogen. Consequently, neutron radiography is used to measure water distribution in roots of growing plants or in very thin (∼10μm) membranes enclosed inside working fuel cells.

The neutron refractive index differs from that of the vacuum by only 10−6. Therefore, lenses have enormous focal lengths for neutron beams, and critical angles for total external reflection are normally no more than a few degrees, depending on the wavelength and the surface coating. The toolbox of neutron optics is thus relatively limited compared to modern optical instruments for visible light and x-rays, which use a variety of focusing devices, including lenses, zone plates, and mirrors. Existing techniques, such as neutron imaging and small-angle neutron scattering, (SANS), use instruments that are traditionally designed as pinhole cameras. Therefore, suitable focusing optics, if available, might bring transformative improvements of the instruments' designs and enable new science by increasing the spatial and temporal resolution of neutron methods by orders of magnitude.

Wolter Optics -- We have pioneered and demonstrated novel neutron focusing optics based on axisymmetric grazing-incidence focusing mirrors (often referred to as Wolter optics) for neutrons, inspired by their successful use in x-ray astronomy. The mirrors have the potential to turn pinhole-camera-like neutron instruments into much more powerful microscopes.

Small-Angle Neutron Scattering (SANS) -- In SANS, the intensity of the beam scattered by a sample at different angles is used to infer details about a material's structure. Traditional SANS instruments require two small apertures to collimate the beam. Our mirrors promise the possibility of increasing the signal by a factor of 50 or more, while improving the resolution and reducing the size and cost of detectors. We built and tested a small prototype Wolter-mirrors-based SANS instrument, which performs in accord with computer simulations.

Neutron Imaging -- Another common technique is neutron imaging, which explores attenuation of a neutron beam by various materials to visualize the internal structure of objects nontransparent to light. It is traditionally performed by illuminating a sample by a collimated beam from a small aperture, while placing a detector right behind the sample to ensure the sharpest possible image. The resolution is thus determined by the aperture size, with a higher resolution requiring a smaller aperture and a smaller incident flux. We have demonstrated a prototype neutron microscope, which uses magnification-4 Wolter mirrors, with samples and a detector aligned at the mirrors' two focal planes. An example of a magnified image is given below. Here, the resolution is determined by the angular resolution of the optics, rather than by the aperture size.