A monolithic poly capillary focusing optic for polychromatic neutron diffraction applications

摘要

The relatively low flux from neutron sources means that structural analysis using neutron diffraction requires large crystals that are often not available. We are exploring the possibilities of a polycapillary focusing optic to produce a small intense beam spot of size < ≈ 0.5mm for small crystals. We have conducted measurements at five different thermal neutron wavelengths to determine the transmission characteristics of a tapered monolithic focusing lens with a focal length of 100mm, suitable for time-of-flight diffraction. Both the width of the focused beam and the intensity gain of the optic increase as a function of wavelength. We have performed similar measurements on a polychromatic beam on a pulsed neutron source, where the results are subject to background from short-wavelength neutrons. The use of a beryllium filter shows the increased effective gain for the longer wavelengths at the expense of an increased focused beam width by a factor of two. In a diffraction measurement from an alpha quartz crystal using a 2.1° convergent beam from a pulsed neutron source, we observed six diffraction peaks in the 1.5 A to 4 A wavelength bandwidth transmitted by the optic. These diffraction spots show an intensity gain of 5.8 +- 0.9 compared to a direct beam diffracting from the same sample volume as that illuminated by the convergent beam.