Neutrons are a unique probe of the structure and dynamics for a great variety of systems studied today in many scientific disciplines. With the construction of the Low Energy Neutron Source (LENS) at CEEM, potential users in non-traditional fields such as chemistry and biology and industrial researchers can be made familiar with the power of neutron scattering techniques and acquire the experience needed to perform critical experiments of relevance to their fields. Major obstacles to expanding the neutron community in this country are the existing general unfamiliarity of US researchers with neutron techniques, a diminishing number of national and local facilities where novice users may be introduced to neutron scattering techniques, and the lack of flexible facilities for engineering and technical studies where new ideas for neutron science and engineering can be pursued. The variable pulse width offered by our source, its low cost, and its greatly reduced level of background radiation, put it in a unique position to have an impact on education and emergent technologies such as biomolecular engineering, chemistry, moderator development and instrumentation design.
LENS Moderator Development
The Low Energy Neutron Source (LENS) produces cold neutron beams for fundamental and applied research using relatively low energy (7-13 MeV). The facility is capable of characterizing any moderator prototype that can fit within the existing 50cm diameter/50 cm tall cylindrical water reflector volume. The slab geometry, tight coupling, and cold temperature utilized in the LENS moderator will make the facility an important testing ground for a number of technologies that may be used in future high-power neutron scattering facilities. Furthermore, the low proton energy used will result in sufficiently low activity in the target/moderator system for materials to be handled without extensive remote handling facilities which is important for its use as an educational resource.
Dr. Chen-Yu Liu is pursuing ultra-cold neutron (UCN) production in solid oxygen. The physics of UCN production in solid oxygen involving magnon (spin wave) exchanges is fundamentally different from the well-known phonon mechanism in solid deuterium. In solid deuterium sources, the finite concentration of para-deuterium (spin 1) can retain unquenched rotational energy even at low temperatures. This effect results in excessive upscattering of neutrons out of the ultracold state. The greatly enhanced solid oxygen UCN flux will enable a comprehensive set of studies on UCN production physics.
Materials Research & Condensed Matter
Materials research is an interdisciplinary field which involves study of the synthesis, properties and structure of a wide range of materials, many of practical or technological importance. The field draws contributions from condensed matter physics, chemistry and engineering and, more recently, from biology. Condensed matter physics studies the "condensed" phases that appear whenever the number of constituents in a system is extremely large and the interactions between them are strong. Condensed phases range from normal solids and to the Bose-Einstein condensate found in certain atomic systems at very low temperatures. Also included are the superconducting phase exhibited by conduction electrons in certain materials, or the ferromagnetic and antiferromagnetic phases of electron spins on atomic lattices. Solid-state physics is now considered to be one of the main sub-disciplines of condensed matter physics.
Complex fluids include systems such as colloids, foams, slurries, emulsions, membranes, and polymer melts. Researchers at CEEM are studying such questions as how inter-particle interactions influence the rheological properties of such fluids, and the dynamics and lateral structure of model biological membranes.
Low Energy Neutron Source (LENS)
|Inelastic neutron scattering in Xerogel.|
The Low Energy Neutron Source (LENS) is a major asset for conducting condensed matter research at CEEM. When neutrons are scattered from matter, the resulting angular and velocity distributions of the scattered neutrons can be interpreted to determine where atoms are located and how they move. Neutrons interact with matter in a unique manner that allows them to identify hydrogen and other light atoms among heavy atoms, making them very useful for the study of biological macromolecules and man-made polymers, both of which contain substantial amounts of hydrogen. This feature also made it possible for neutrons to make the first determination of the crystal structure of yttrium-barium-copper oxide (YBCO), the first of the so-called "high-temperature" superconducting ceramics". YBCO wires may someday be used to increase the energy efficiency of electric motors, generators, transmission lines, transformers, and magnet-containing devices, such as particle accelerators for research, medical diagnostic machines, and levitated, high-speed trains. Most of what we know about the atomic level magnetic structure of materials has also been obtained using neutron scattering.
At LENS, neutron scattering will be used primarily to study large-scale (1 -1000nm) structure of materials. Paul Sokol uses neutron scattering in several studies including the collective excitations in confined quantum liquids, the momentum distribution of hydrogen on surfaces, the microscopic structure of confined solids, wetting on nonstructural surfaces, the dynamics of hydrogen in reduced dimensionality, and the properties of hydrogen storage materials.
Because of their unique sensitivity to hydrogen atoms, neutrons can be used to precisely locate hydrogen atoms. Large biological molecules contain numerous hydrogen atoms many of which are crucial for functions such as the enzymatic activity of proteins. Because hydrogen and deuterium (a heavy isotope of hydrogen) scatter neutrons differently, the best way to examine a particular part of a biological macromolecule with neutron scattering is through isotope substitution, i.e. replacing hydrogen with heavy hydrogen (deuterium) atoms.
Thus, in a technique called contrast variation, scientists can highlight different types of molecules, such as a nucleic acid or a protein in a chromosome, by substituting deuterium for hydrogen at the interesting sites. This allows them to glean independent structural information on each component within the macromolecular complex. The structures of complex fluids and biomolecular systems both in bulk and as films and membranes, are being studied by Roger Pynn using these techniques.
Thin and multi-layered films represent prototypical examples of nano-structured materials whose structure and properties can be easily controlled through layer-by-layer growth through vacuum, chemical, or electro-deposition processes. The growth can be used to tune the composition of the material at length scales comparable to the fundamental physical lengths that determine a material's properties. For instance, in the Giant Magnetoresistance (GMR) effect, it is possible to make a magnetic field sensor by separating two magnetic materials by a non-magnetic layer that is thin enough for electrons traveling between the two magnetic layers to maintain memory of their quantum mechanical phase and momentum. The GMR effect has provided a rich variety of phenomena to unravel, and the magnetic recording industry is now completely dominated by GMR technology for its read heads. Researchers at CEEM are interested in such questions as how you can quantify the disorder at interfaces in these materials (and how that disorder is correlated from one interface to the next), as well as how that disorder influences the properties of the material.