POLYCAPILLARY OPTICS FOR NEUTRONS AND X RAYS. D.F.R. Mildner, H. H. Chen-Mayer, National Institute of Standards and Technology, Gaithersburg, MD 20899, and R.G. Downing, V.A. Sharov and Q.F. Xiao, X-Ray Optical Systems, Inc, Albany NY 12205

The principle of multiple mirror reflection from smooth surfaces at small grazing angles makes it possible to transport slow neutrons and x rays with high efficiency using narrow tubes. Such a mechanism provides the opportunity to bend and focus the radiation on a scale suitable for laboratory needs. Recent developments in glass polycapillary fibers with thousands of channels having sizes of a few microns have enabled the radiation to be focused to submillimeter spot sizes for neutrons.

A neutron focusing lens can enable an increased spatial resolution and an improvement in the detection limits for individual elements for absorption techniques in analytical and materials research (such as neutron depth profiling and prompt gamma activation analysis). The following parameters have been considered in the design of the focusing lens: the depth of focus, determined by the beam width as a function of distance from the lens, the optimum focal length, obtained by maximizing the current density at the focus for a given size of the focal spot, and a shift in the neutron energy spectrum of the transmitted beam relative to the incident beam, since curved channels preferentially transmit longer wavelength neutrons.

We have constructed a lens using polycapillary glass fibers for focusing a large area neutron beam (50 x 45 mm² corresponding to the exit dimensions of a ⁵⁸Ni guide) to spot of width ~ 0.5 mm at a focal distance of 52 mm with a current density gain of 80. A series of images of the transmitted beam taken at progressively greater distances from the lens exit indicates the convergence of the output of the fibers and their subsequent divergence beyond the focal point. The shape of the profile at the focus has a Gaussian distribution with a full-width at half-maximum of 0.54 mm.

Prompt gamma measurements using the focused beam have been performed to demonstrate the enhanced sensitivity and lateral resolution. The gain in [[gamma]]-ray count rate is about 60, and though the background from unfocused neutrons is increased, the sensitivity of detection is increased by a factor of 20. Recent developments in x-ray focusing will also be discussed.