Bilby - 2nd Small-Angle Neutron Scattering Instrument

Bilby Instrument Design Layout

Instrument Scientist

Dr Anna Sokolova
 

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More information on Bilby

Our first small-angle neutron scattering instrument QUOKKA has been so strongly oversubscribed (as at other neutron centres) that we are now building a second one, BILBY. While the two instruments will be very similar, they will not be identical and BILBY will extend the size range of objects both to the large sizes (using VSANS), and to small sizes, by going to larger scattering angles.

In addition, to cover the full range on QUOKKA, one must take two or three measurements with detector at a number of different sample-detector distances between 1m and 20m. If one pulses the neutron beam using one or more mechanical choppers, and measures the neutron wavelength by measuring the time taken to get to the detector (the time-of-flight method), one can take a complete data set in one instrument setting.

This is very useful for the study of kinetic effects, like relaxation after a chemical reaction, or an external impulse like mechanical deformation, or an electric or magnetic field.

Small-angle scattering is a powerful technique for looking at sizes and structures of objects on the nanoscale (1-10nm), like polymer molecules, biological molecules, defect structures in metals and ceramics, pores in rocks, magnetic clusters, magnetic flux lines in type-II superconductors and so on.

ANSTO has both X-ray and neutron small-angle scattering adjacent to each other, and the advantage of neutrons is primarily for soft matter where the contrast-variation method can be used.

In addition, it is useful for magnetic problems and ones in which large samples must be used. In many ways, small-angle scattering is complementary to electron microscopy while direct imaging is the domain of electron microscopy, SAXS and SANS can provide particle sizes, shapes and distributions averaged over a complete macroscopic sample.

Small-angle scattering is rarely able to solve a problem on its own, and is typically used in conjunction with a number of other techniques.

SANS was crucial in showing that polymer molecules are self-avoiding random walks (Flory's prediction Nobel Prize in Chemistry, 1974), and that Type-II superconductors allow magnetic flux to penetrate, forming a lattice of magnetic vortices (Abrikosov's prediction Nobel Prize in Physics, 2003). Other major achievements have been understanding viscosity modifiers in lubricants, solving the coarse structure of the ribosome, understanding the nature and role of particulate additives to tyres, and the porosity of sedimentary rocks in oil and gas reservoirs.

Conventional small angle scattering operates by defining a very well collimated beam using a pair of small well-separated circular apertures, and a roughly similar distance after the sample to a high-resolution detector.

To go to smaller angles, and thereby measure diffraction effects from larger objects (>100nm) one needs even tighter collimation, to the point that there is insufficient intensity to make the measurement using circular apertures.

One can use narrow slits instead, and correct for the averaging along the slit dimension on the assumption that the sample scattering is isotropic this is called VSANS (Very Small Angle Neutron Scattering), and will likely be featured in our new BILBY SANS machine. VSANS can get one up to sizes of order 500nm.

In summary, the BILBY small-angle neutron scattering instrument will look very much like, and operate in a similar way to, the existing QUOKKA instrument. The main additional features will the time-of-flight mode allowing kinetic experiments, and a slit-aperure or VSANS mode, extending its Q-range to smaller Q.

In this sense, it will include features similar to the new VSANS instrument at NIST and D33 at the Institut Laue Langevin in Grenoble, France.

BILBY is funded as part of the Australian Governments Super-Science Initiative, its conceptual design was completed in early 2010, and it will be located on CG2, viewing the cold neutron source.