Browsing by Author "Freund, AK"
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- ItemKOOKABURRA: the ultra-small-angle neutron scattering instrument at OPAL(Wiley-Blackwell, 2013-12-01) Rehm, C; Brûlé, A; Freund, AK; Kennedy, SJThe new double crystal ultra-small-angle neutron scattering instrument Kookaburra, currently under construction at the ANSTO OPAL reactor, will allow characterization of microstructures covering length scales in the range of 0.1 to 10 microm. Using the 002 and 004 reflections of a doubly curved mosaic highly oriented pyrolytic graphite premonochromator crystal at a fixed Bragg angle of 45degrees in conjunction with two pairs of Si(111) and Si(311) quintuple-reflection channel-cut crystals will allow operation of the instrument at two different wavelengths, thus optimally accommodating weakly and strongly scattering samples in one sample position. The versatility, the estimated neutron fluxes and the low background noise of Kookaburra suggest that this state-of-the-art instrument will have a major impact in the field of large-scale structure determination. © 2013, Wiley-Blackwell.
- ItemOptimisation and fabrication of a composite pyrolytic graphite monochromator for the Pelican instrument at the ANSTO OPAL Reactor(Elsevier, 2011-04-01) Freund, AK; Yu, DHThe triple monochromator for the TOF neutron spectrometer Pelican at ANSTO has been fully optimised in terms of overall performance, including the determination of the thickness of the pyrolytic graphite crystals. A total of 24 composite crystals were designed and fabricated. The calculated optimum thickness of 1.3 mm and the length of 15 cm of the monochromator crystals, that are not available commercially, were obtained by cleaving and soldering with indium. An extensive characterisation of the crystals using X-ray and neutron diffraction was conducted before and after the cleaving and bonding processes. The results proved that no damage was introduced during fabrication and showed that the design goals were fully met. The measured peak reflectivity and rocking curve widths were indeed in an excellent agreement with theory. In addition to the superior efficiency of the triple monochromator achieved by this novel approach, the amount of the crystal material required could be reduced by 1/3. (C) 2010 Elsevier B.V.
- ItemPhase space optimisation of the USANS instrument Kookaburra at the ANSTO OPAL reactor(Elsevier, 2011-04-01) Freund, AK; Rehm, CAn 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 x 10(5) cm(-2) s(-1) is provided for a wavelength of 4.43 angstrom by a doubly curved pyrolytic graphite premonochromator diffracting in a horizontal plane and producing a beam size of about 50 cm(2) at the sample position. The smallest background is achieved for a wavelength of 2.56 angstrom 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 x 10(4) cm(-2) s(-1) is collected into a sample area of 10 cm(2). The bent perfect crystal design is of particular advantage for small samples. (C) 2010 Elsevier B.V.