NEUTRON FOCUSING OPTICS FOR LOW-RESOLUTION SMALL-ANGLE SCATTERING

Small-angle neutron scattering instruments use large source and sample areas with long flight paths to obtain the necessary resolution. Incr eased count rates may be obtained using collimators that converge to a point on the detector. Further increases may be obtained by convergin g guides in the form of a focusing lens. A low-resolution small-angle scattering instrument that uses converging capillary fibers as a focus ing lens is proposed. Such a device requires the use of a detector tha t has a fine spatial resolution, perhaps less than 0.1 mm. Expressions are derived for the resolution and the intensity optimized for such a n instrument. The relationship is determined between the guide dimensi ons, the focal length and the critical angle of the internal coating o f the individual fiber channels. The critical angle of the focusing le ns dominates the resolution, and such an instrument is useful only for low-resolution measurements. However, the greatly reduced length is o nly valuable if there is a high-resolution detector to match the dimen sions of the guide. Despite its low resolution, such an instrument mig ht eventually be useful for survey or characterization measurements.