Neutron Spin Manipulation Devices
∗ Supported by a grant from National Science Foundation. (NSF)
Advanced instrumentation using polarized neutrons provides a number of unique opportunities to increase the scientific information obtained from neutron scattering experiments. For example, it permits unambiguous determination of non-collinear magnetic structures and of correlations between magnetism and lattice structure in materials of technological and scientific interest such as those exhibiting Giant Magneto-Resistance or high temperature superconductivity; and it allows scientists to apply the unique advantages of neutron scattering (such as its isotopic contrast) with greater sensitivity and resolution, providing more detailed information about the structures of long-length-scale systems such as polymers, gels, porous media and membranes.
We are working on several different types of neutron spin manipulation device that are required for advanced polarized neutron instrumentation. Although much of the design will be done using computer simulation of magnetic fields, we are constructing several prototypes that will be tested using polarized neutrons at the Low Energy Neutron Source (LENS) at Indiana University. The intent of the project is to develop designs that can be subsequently deployed at U.S. national neutron scattering facilities to enhance the scientific capability of instrumentation at these facilities.
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Spin Echo Scattering Angle Measurement (SESAME)
SESAME is an interferometric method that provides enhanced resolution of neutron scattering angles without the loss of neutron intensity that usually results from the use of better beam collimation. The method works by manipulating neutron spin precession.
In our implementation, SESAME technology uses solenoids to generate magnetic fields that create a phase seperation between "up" and "down" spin components of a neutron beam. When the accumulated precession phase of the two different spin state is the same, neutron spin echo is achieved and the detected neutrons are fully polarized. In the absence of sample scattering, exact phase cancellation is achieved by applying field integral symmetry before and after a central pi flipper. However since the Larmor phase strongly depends on the direction of travel of the neutron, small angle scattering from a sample causes a incomplete phase cancellation which is measured as a depolarization of neutron beam. For bulk samples, the echo polarization maps out the cosine Fourier transform of the usual Debye density auto correlation function.
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