Neutron focusing using microguides

Abstract

Analytical techniques which are dependent solely on the neutron capture reaction rate may be advanced by neutron focusing. We describe a system composed of curved totally reflecting mirrors for the focusing of neutrons by the superposition of intensity transmitted through many thin silicon films, each of which points individually to the desired focus. The acceptance technique shows that the maximum current density is obtained with a large number of microguide sheets of minimum thickness, a short focal length, and a large radius of curvature. This analysis indicates the transmission properties of continuously curved neutron guides as a function of wavelength, and enables the calculation of an estimate of transmission efficiency. Reflection losses increase geometrically with the average number of reflections. The major losses in transmission are caused by misalignment of the layers which diffuse the focus from its theoretical maximum. We propose using 25 μm thin silicon wafers with a 100 nm coating of nickel, loaded into a cassette specially fabricated to ensure overlapping and to enable a small curvature to be maintained. The length of the system should be as short as possible (∼ few cm) to reduce both reflection losses from the nickel coating and scattering losses in the neutron transmitting medium of silicon.