Phase space optimisation of the USANS instrument Kookaburra at the ANSTO OPAL reactor

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 1.7×105 cm−2 s−1 is provided for a wavelength of 4.43 Å by a doubly curved pyrolytic graphite premonochromator diffracting in a horizontal plane and producing a beam size of about 50 cm2 at the sample position. The smallest background is achieved for a wavelength of 2.56 Å 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 6.6×104 cm−2 s−1 is collected into a sample area of 10 cm2. The bent perfect crystal design is of particular advantage for small samples.