The White Beam Steady-State Diffractometer: A Next Generation Neutron Diffraction Strain Scanner

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Percival, Aaron
Neutron diffraction , strain diffractometer , NRU , CNBC , white beam diffraction , engineering diffractometer , Neutron scattering
This project proposes a reactor-based instrument, which retains most of the above advantages of spallation source engineering instruments, while maintaining the advantages inherent to steady-state instruments. The main idea is to allow the entire white beam from the reactor onto the sample---no monochromation. Diffraction is then allowed at a fixed angle, and information from two sample directions can be obtained simultaneously. Since a white beam is incident onto the sample, the diffraction condition is satisfied for multiple sample planes in the fixed angle of diffraction. Multiple analyzer/detector pairs are placed downstream from the sample and tuned to scatter only one of the diffracted wavelength bands. Monte Carlo methods were used to create models of both a standard two-axis engineering diffractometer, found on current reactor sources, and the proposed white beam instrument. There models were validated by experiments performed on a standard two-axis instrument, which was also modified to operate in a white beam configuration, in which the position of the sample and monochromator were interchanged. Both the models and the experiments of the white beam instrument showed proof of concept for this design and identified areas of concern that required special attention. Upon a comparison on the results from the standard two-axis instrument to the results from the white beam instrument (both simulation and experimental), it was found that the standard diffractometer showed a better performance in all aspects. However, this project proposes numerous areas where the white beam design can be improved upon in order to enhance its performance as an engineering instrument. The most important of these is the design of an appropriate analyzer/detector system, as the results overwhelmingly show this to be the area of greatest concern. Ideas for a few such designs are also given.
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