Glass fibers to channel neutrons to a focus.

Like flat stones skipping across placid water, neutrons striking a material at very low angles glance off its surface. Taking advantage of this behavior, researchers have now demonstrated that bundles of glass fibers containing microscopically narrow channels can guide, shape and focus beams of neutrons.

Instead of penetrating the glass, neutrons stream down these channels, repeatedly reflected by their smooth, inner walls. By gently curving each of the fibers in a bundle, researchers can manipulate the emerging neutron beams to concentrate them on a spot much smaller than possible without this novel "lens."

"There are many other approaches for focusing neutrons," says R. Gregory Downing of the National Institute of Standards and Technology in Gaithersburg, Md., who is a member of a team studying glass-fiber neutron lenses. "The difficulty has been to do that efficiently--to get intensities of neutrons useful for analytical purposes. By combining a number of technologies that just came together at one time, we've been able to accomplish that."

The idea of using narrow channels fabricated in glass fibers to control and manipulate both X-rays and neutrons originally came from Muradin A. Kumakhov of the I.V. Kurchatov Atomic Energy Institute in Moscow (SN: 4/13/91, p.236). In the June 4 NATURE, Kumakhov and colleague V.A. Sharov describe experiments showing that a lens assembled from 721 bent fibers of lead-oxide silica glass, each threaded by more than 1,000 narrow, parallel channels having inner diameters only 6 micrometers wide, can focus neutrons to a spot slightly smaller than 1 millimeter in diameter. This prototype lens amplifies the neutron intensity by a factor of six or seven.

In an accompanying paper, Downing and his colleagues, along with Kumakhov's group and Walter M. Gibson of the State University of New York at Albany, demonstrate that such bundles of multichannel fibers can also guide a neutron beam. Using glass fibers supplied by the Russian group, the researchers found they could bend a neutron beam through 20[degrees] in a distance of 130 millimeters while losing only half the neutrons.

"These investigations will form the basis for the design of a neutron lens for use in materials research or a beam bender capable of dividing and directing a neutron beam towards several experimental stations," the researchers say.