Neutron focusing devices based on segmented thin-film multilayers

Abstract

A neutron converging device has been designed using sections of thin-film multilayer monochromators with the help of ray-tracing calculations (Saxena, Nucl. Instr. and Meth. A 454 (2000) 428, this issue). The device consists of an odd number of sections of multilayers. The central section will have a periodicity and angular orientation with respect to the mean ray that reflects the desired wavelength. The orientation of other sections was adjusted so that the reflected beam from these sections was coincident with that from the middle section. Since a rotation of sections also changes the wavelength of the reflected beam, the d-spacings of the peripheral sections were adjusted to reflect the same wavelength as the middle section. The convergence factor (gain in intensity) will depend on the areas of the primary beam and that of the sample, and also on the number of sections used in the focusing device. For a typical geometry, intensity may be increased by a factor of two or more. Used alone, such a device will both focus and monochromate the neutron beam to give a rather large Δ λ/ λ that is typical of thin-film multilayer monochromators. However, a device of this type may also be used to focus a neutron beam that has already been monochromated by reflection from a crystal monochromator. In this case, the neutron beam will be focused without a significant change of spectrum.