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Neutron Focusing

neutron focusing lens

Neutron focusing is a technique used to concentrate and direct a cold neutron beam onto a small area. Microcapillary optics have been developed that can be used to focus neutrons. This has been made possible through the fabrication of glass fibers in which an array of microscopic channels are imbedded, each channel a few microns in diameter, and all channels perfectly parallel within each glass fiber. A bundle of such glass fibers make up a neutron lens (see figure), if all the exiting ends of the fibers can be pointed towards a common focus. The fibers must be very gently and accurately bent, so that neutrons can undergo multiple reflections along the interior surfaces of the channels, all the while maintaining an angle of incidence on each reflection that is less than the critical angle for total reflection, typically a fraction of a degree.
Neutron focusing is used to improve spatial resolution and lower detection limits for neutron-based analytical methods, such as Cold Neutron Depth Profiling (CNDP) and Prompt Gamma Activation Analysis (PGAA). One recent application is described in the Technical Highlight, "Investigation of Chromium Diffusion into the Walls of an Industrial Boiler Tank via PGAA with a Focused Neutron Beam."
H. H. Chen-Mayer, D. F. R. Mildner, V. A. Sharov, Q. F. Xiao, Y. T. Cheng, R. M. Lindstrom, R. L. Paul, 1997, Rev. Sci. Instrum. 68 (10) 3744-3750.
David F. R. Mildner, 301-975-6366,
Heather Chen-Mayer, 301-975-3782,

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