COMPOUND REFRACTIVE OPTICS FOR THE IMAGING AND FOCUSING OF LOW-ENERGYNEUTRONS

Low-energy neutrons are essential for the analysis and characterizatio n of materials and magnetic structures. However, both continuous (reac tor-based) and pulsed (spallation-based) sources of such neutrons suff er from low fluence. Steering and lensing devices could improve this s ituation dramatically, so increasing spatial resolution, detectable sa mple volume limits and even perhaps opening the way for the constructi on of a neutron microscope. Neutron optics have to date exploited eith er Bragg diffraction(1,2), such as bent crystals, or reflection, as in mirror(3) guides or a Kumakhov lens(4,5). Refractive optics remain an attractive alternative as they would permit full use of the beam cros s-section, allow a compact and linear installation and, because of sim ilarity to conventional optics, enable the use of commercial design an d simulation tools. These advantages notwithstanding, single-element r efractive optics have previously been considered impractical as they a re too weakly focusing, too absorptive and too dispersive. Inspired by the recent demonstration(6) of a compound refractive lens (CRL) for h igh-energy X-rays, we have designed, built and tested a prototype CRL for 9-20-Angstrom neutrons by using readily available optical componen ts: our CRL has gains greater than 15 and focal lengths of 1-6 m, well matched to small-angle neutron scattering.