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Departmental Colloquium


Neutrons, The Gentle Probe That Carries A Wallop

Dr. Ron Rogge
NRC - Chalk River

Time
 
Wed. October 24, 2007     3:30 PM     Stirling A

Abstract
 
We can all agree that the various experimental techniques are complementary, and today experimental techniques are used extensively in tandem with computational methods. One of the distinguishing characteristics of thermal neutrons--the ones used in scattering experiments--is that they gently probe nearly all materials, from hard to soft, but also penetrate deeply in many materials. Another very useful and distinguishing characteristic is isotopic sensitivity, especially for some atomic species. This permits significant changes in scattering properties while having little effect on the structure of the material, and therefore in many cases no impact on function. The high sensitivity to, and varied interactions with hydrogen make neutrons an especially useful tool for studying hydrogenous materials and materials related to the hydrogen economy. In the case of studies of industrial materials and components, the penetration of neutrons is--in most cases--central to their utility. For example, neutron-based studies of industrial problems have changed manufacturing techniques, led to modification in codes, improved the cost effectiveness and competitiveness of industries, and improved safety. The high penetration also means the materials can be studied in realistic conditions giving direct knowledge of their behaviour during manufacture or in-service. In-situ studies are often carried out in conjunction with computational modelling, a powerful combination. The first part of the presentation will give some examples neutrons being used for a variety of examples from basic sciences to engineering. This will include an overview of several neutron techniques available at Chalk River (and some highlights of other labs) with some emphasis of in-situ capability. Thanks to the properties of neutrons, the imagination is the limit. Some of the preceding examples will have demonstrated that existing techniques combined with specimen environments provide unique information. Nonetheless, there remains room to develop new neutron techniques. Recently, we in Chalk River have demonstrated for the first time, thermal neutron holography. This technique takes the original ideas of holography and (as had been done using X-rays) extends the concept to atomic resolution, i.e. the characteristic length scale for neutrons. This permits the direct imaging of atomic structure without the need for model systems. This presents a tremendous opportunity for determining structure, which in turn determines function, for materials that cannot be crystallized. In the case of thermal neutron holography, the strong incoherent scattering of the H atom is what enables this technique. Therefore, neutron holography is anticipated to present a unique opportunity to study biologically relevant materials that are often difficult to crystallize, e.g proteins. The second part of the presentation will describe the theory and development of thermal neutron holography and highlight some developments since the initial demonstration.

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