Abstract
An optimisation of the USANS instrument Kookaburra has been conducted determining the most efficient type of premonochromator crystal and scattering geometry. The approach has been based on representations in phase space combining direct and reciprocal space diagrams. Results are presented for several scenarios involving flat and curved, perfect and mosaic crystals, horizontal and vertical scattering planes. The highest peak current density of 1.7×105cm−2s−1 is provided for a wavelength of 4.43Å by a doubly curved pyrolytic graphite premonochromator diffracting in a horizontal plane and producing a beam size of about 50cm2 at the sample position. The smallest background is achieved for a wavelength of 2.56Å and a vertical scattering plane using a bent perfect premonochromator crystal and optionally a collimator to decrease the divergence of the beam exiting the cold neutron guide. For this wavelength a peak current density of 6.6×104cm−2s−1 is collected into a sample area of 10cm2. The bent perfect crystal design is of particular advantage for small samples.
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