NEUTRON FOCUSING WITH CAPILLARY OPTICS

It is very difficult to focus neutrons, because they are chargeless particles which interact only weakly with matter. An efficient neutron lens is a device which has long been desired, but, until recently, no viable concept existed for such a device. In the past few years, the situation has changed. Microcapillary optics have been developed which can be used to focus both X-rays and 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, 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 bounce which is less than the critical angle for total reflection, typically a fraction of a degree.

The technology of the glass fiber fabrication originates from the former Soviet Union. NIST scientists, in collaboration with scientists and engineers from Russia and a small U.S.company, are doing the basic science underpinning the development of a viable neutron lens. The lens device is placed in a cold neutron beam at the NCNR, and the image of the focused neutrons is recorded along the exit path by means of a neutron area detector, and a video recording system.

Schematic depiction of the neutron focusing principle and capillary fibers used.	Output of the neutron video recording device. A flux gain of a factor of 80 at the spot, compared with the original beam, and a spot size of less than a millimeter, have been achieved.

 Contacts:
David Mildner, (301)975-6366, david.mildner@nist.gov
Heather Chen, (301)975-3782, heather.chen-mayer@nist.gov
 

• Project Goals

To produce intensified neutron flux on a small sample area.
• Initial Applications Improved spatial resolution and lowered detection limits for neutron-based analytical methods, such as Cold Neutron Depth Profiling (CNDP) and Prompt Gamma Activation Analysis (PGAA).
• Technique Using polycapillary glass fibers as small neutron guides, a neutron lens can be made. The neutron optical properties of both individual fibers and of prototype lenses are being studied.