Physica B 385–386 (2006) 1180–1182 ‘Quokka’—the small-angle neutron scattering instrument at OPAL Elliot P. Gilbert à , Jamie C. Schulz, Terry J. Noakes Bragg Institute, Australian Nuclear Science and Technology Organisation, PMB 1, Menai NSW 2234, Australia Abstract A small-angle neutron scattering instrument (Quokka) is being built as part of the initial instrument suite for the 20 MW Australian Research Reactor, OPAL. The 40 m long instrument will be located at the end of a curved supermirror neutron guide and will receive neutrons from a large liquid-D 2 cold source. The instrument will have incident beam polarisation and focusing optics using MgF 2 lenses and gravity-correcting prisms in the collimation system. The secondary flight path includes a 1 m 2 area detector with high-speed data acquisition electronics with provision for the inclusion of polarisation analysis at a later date. Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. PACS: 28.20.Cz; 28.41.Rc Keywords: Small angle neutron scattering; SANS; Polarisation; OPAL 1. Introduction A small-angle neutron scattering instrument (‘Quokka’) is being built at Australia’s new research reactor, OPAL, at the Australian Nuclear Science and Technology Organisa- tion (ANSTO), and is to be commissioned in 2006. An international workshop held in December 2001 led to the specification of the requirements for the instrument [1]. Based on the interests of participants and domestic and international research directions, it was determined that the instrument should be in the spirit of D22 at the ILL [2] and the 30 m SANS instruments at NIST [3] and include incident beam polarisation, be capable of conducting time- resolved studies, include a large sample area (ca. 1.2 m diameter) and with provision for future optics (e.g. polarisation analysis). The beam will be defined by a velocity selector and adjustable pinhole collimation system providing source– sample distances up to 20 m with availability of incident beam polarisation and focusing (Fig. 1). Following the sample position, a two-dimensional position sensitive detector will measure neutrons scattered from the sample over a secondary flight path of up to 20 m. The following paper reports aspects of the scientific and engineering design associated with the development of this new instrument. 2. Neutron beam and wavelength selection The OPAL reactor has been designed to deliver an unperturbed thermal neutron flux of the order of 4  10 14 ncm 2 s 1 , operating at 20 MW thermal power [4]. The cold-neutron source is a 20 l vertical liquid-D 2 thermosyphon with re-entrant cavity in the direction of the cold-neutron guides. The 200 mm  50 mm neutron guide is composed of both straight—m ¼ 2 (sides) and m ¼ 3 (top and bottom) and curved sections—m ¼ 3 (top and bottom), m ¼ 2.5 (concave) and m ¼ 2 (convex) coatings. The latter has a radius of curvature of 1.3 km to take the instrument out of direct line-of-sight filtering high energy neutrons and gamma radiation. Monte Carlo simulations indicate that the cold-neutron flux is anticipated to be greater than 5.7  10 9 ncm 2 s 1 before the velocity selector with peak flux at a wavelength of 3.9 A [5]. Quokka will have an end-guide position, receiving neutrons from the top 50 mm  50 mm cross-section of the guide, with the remainder to serve a time-of-flight spectrometer with polarisation analysis. A mechanical velocity selector with wavelength range 4.5–43 A ˚ is located ARTICLE IN PRESS www.elsevier.com/locate/physb 0921-4526/$ - see front matter Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.385 à Corresponding author. Tel.: +61 2 9717 9470; fax: +61 2 9717 3606. E-mail address: elliot.gilbert@ansto.gov.au (E.P. Gilbert).