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Small Angle Neutron Scattering (SANS) for Nanostructured Materials

B.D. Gaulin, M.C. Rheinstadter, E.D. Cranston, K. Dalnoki-Veress, R.M. Epand, C. Fradin, T.R. Hoare, Y. Mozarivskyj, R. Pelton, H.D.H. Stover.

Principal Investigators

Bruce D. Gaulin, BIMR Director, McMaster University
Maikel C. Rheinstadter, Department of Physics & Astronomy, McMaster University

Introduction

The BIMR organized and led a Canada Foundation for Innovation (CFI) New Initiative Fund proposal entitled “SANS for Nanostructured Materials” which was approved towards the end of 2012.

This $7.5M project proposes to build Canada’s only small angle neutron scattering (SANS) facility at the McMaster Nuclear Reactor, and to use this unique facility to study a broad range of nanostructure in materials on length scales from ~ 0.2 nm to ~ 20 nm. When complete and operating in ~ 2016, “SANS for Nanostrctured Materials” will provide unique advanced characterization of materials for about 25 research groups per year, both at McMaster University and at other Canadian universities. It will run 40-50 experiments a year, and is expected to make a big impact on the scientific productivity of the BIMR.
“SANS for Nanostructured Materials” will itself exploit the unique properties of the neutron as a probe of matter. These properties include the fact that the neutron is uncharged, and can penetrate deeply into matter.

The neutron scatters from the nuclei in matter rather than from the electrons, as is the case with x-rays and electron microscopy. For that reason the neutron “sees” different isotopes of the same element as completely different entities. And the neutron itself carries a magnetic moment. As such the neutron “sees” magnetism in solids, and it can sense magnetic field distributions in matter.

The neutron is very sensitive to the presence of hydrogen in matter. It “sees” hydrogen and deuterium as two completely different entities, even though they are almost identical chemically. Molecular substructure can have their hydrogen replaced with deuterium, with little change in structure or function. However, the “deuterated” substructure possesses a different visibility to neutrons, compared with the hydrogenated substructure it replaced. This approach, known as “contrast variation”, allows complicated molecular substructure to be understood in great detail and with great sophistication. For example, this approach can be used to definitively determine how long a single polymer strand is when configured within a polymer melt. That is, are the polymers intertwined with each other like spaghetti strands, or do they coil up into balls within the melt? It can be used to definitively determine the orientation of molecules such as cholesterol, within a biological membrane, and to understand protein and peptide shapes within aqueous and biologically- relevant environments.

The neutron is electrically neutral, but it carries a magnetic moment. This characteristic means that neutron scattering probes the bulk, as opposed to the surface, of materials, and does so non- destructively. It also means that neutron scattering is an exceptionally powerful probe of magnetism in materials, and of magnetic fields in materials.

For example, certain classes of superconducting materials allow an externally applied magnetic field to penetrate the superconducting material in an inhomogeneous manner, known as a flux lattice. The use of SANS techniques can elucidate this inhomogeneous pattern of magnetic field penetration in the superconductor. Superconductors are routinely used to carry large electric currents in the magnets that run MRI mtic fields – conditions in which these materials display precisely the form of inhomogeneous structure which SANS can inform on best.

SANS (Small Angle Neutron Scattering) refers specifically to the diffraction of neutrons at relatively small angles. This means, by virtue of Bragg’s law which describes diffraction, that the structure which is elucidated in a SANS experiment is large compared with the wavelength of the neutrons used in the SANS experiments. As we will be employing neutrons with de Broglie wavelengths between ~ 2 and 4 A, we will carry out advanced characterization on materials on a length scale of ~ 2 to 200 A. This covers the extremely important nanoscale regime, roughly from 0.2 to 20 nm. Structure and inhomogeniety of materials on this length scale can be very important for determining the physical properties which determine desirable performance, such as rheological properties, as well as mechanical hardness and magnetic anisotropy.

Figure 1:
The layout of “SANS for Nanostructured Materials” at Beamport 4 of the McMaster Nuclear Reactor. In its current design configuration, the maximum sample to detector distance is ~ 10 m.

Acknowledgements

We are very grateful for the support of the Canada Foundation for Innovation, the Ontario Research Fund, and McMaster University.