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.
At LENS these capabilities are also being used in the development of new technologies for neutron spin manipulation (See spin rotation page) particularly through the use of high-temperature superconducting films to control the spatial variation of magnetic fields. LENS has also served a supporting role in a number of fundamental physics experiments ranging from measurements of neutron spin rotation and decay asymmetries to the testing of candidate detectors for dark matter and rare decay searches (See nuclear physics page).
LENS Moderator Development
The LENS source was designed with moderator research specifically in mind. Residual activity near the source is limited through the extensive use of Aluminum alloys and by limiting the proton energy below the 13.4MeV threshold for 7Be production in the target. The water reflector tank was designed to accept moderators significantly thicker than the standard 1.0 cm thick methane moderator normally employed by LENS when it is run for materials research. Typically, the standard LENS moderator may be removed from the reflector after a weekend cool-down, and at the beam powers used for typical moderator experiments it is possible to investigate four different moderator configurations over a 2-week period. A second complete moderator vacuum and cryogenic system has been constructed to facilitate moderator research projects. Moderators up to 10 cm thick and 15x15cm2 in area can be accommodated within the test vacuum can, but most beam lines do not view more than a 12x12cm2 area. Experiments exploiting this unique capability have been performed with researchers from four of the world’s major pulsed neutron scattering facilities. Researchers interested in such experiments should contact Dave Baxter in order to evaluate potential projects for compatibility with LENS systems and safety requirements.
To facilitate moderator research we can measure both moderator spectra and emission time distributions on the SANS instrument by making minor modifications to its sample enclosure area. For decoupled moderators we can measure emission time distributions from 5.4meV to 900 meV, with up to three orders of magnitude in intensity dynamic range being available at 5.4 and 50 meV with counting times on the order of a day or two.
In 2012, at third instrument was constructed at LENS in order to allow one and two dimensional images of the moderator to be collected as a function of wavelength using a 2-D detector. A slit or pin-hole is positioned roughly 2.4 m from the moderator face.
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. At LENS our on-going research in this area is primarily concerned with the study of large-scale structure (on length scales from 1 to a few thousand nanometers), with particular interests in the effects of confinement on the structure of fluids (both simple and complex) and the exploration of materials and systems with hierarchical structures. LENS can perform some of these experiments directly, and in many other cases it provides important sample quality tests for additional experiments to be performed at national user facilities.
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.