Ultra Small Angle Neutron Scattering ( USANS ) measures the elastic scattering from scattering length density fluctuations in the order of microns in real space, i.e. in the momentum transfer range of 2 x 10-5 - 2 x 10-3 Å-1. The techniques of choice for studying the structure of micron sized particles are electron microscopy, light scattering and atomic force microscopy. There are however a number of cases where none of these techniques is applicable. Examples are in materials of low contrast, opaque materials (for light scattering) or in magnetic structures. In such cases, neutrons are a unique probe when contrast enhancement is necessary. Applications can be found in colloid science (mixtures of particles, strongly correlated colloid crystals, particles of micron size, silicon macropore arrays, opens a file in a new window), materials science (filled polymers, cements, microporous media) and polymer science (constrained systems, emulsion polymerisation).  

For this pupose various double crystal diffractometer instruments (Bonse-Hart camera) are available at different research facilities. We operate the USANS instrument at the 250kW TRIGA-MARK-II reactor in Vienna as well as the USANS option of the S18 instrument, opens a file in a new window at the Institut Laue-Langevin in Grenoble, France.  

[Translate to English:] Sketch of the Bonse-Hart Camera

© Atominstitut, Photo: Harmut Abele

The Bonse-Hart camera is a double crystal (or triple axis) spectrometer with two perfect Silicon channel cut crystals as monochromator and analyser mounted in parallel geometry on a vibration isolated optical bench. The Q-dependence of the scattered intensity is measured by rocking the analyser crystal, the sample being placed in between the crystals. The minimal Q is given by the width of the rocking curve. The drawback of the Bonse-Hart arrangement is the large ratio between horizontal and vertical divergence, typically in the range 100 - 1000. Deriving pinhole desmeared absolute intensities requires a careful deconvolution procedure, but solutions exist to deconvolute the data in routine experiments. The picture gives a schematic view of an USANS instrument.

Some parameters of the USANS - instrument in Vienna

1. Crystal parameters of Silicon

Lattice constant

a =  1,246 Å

bound coherent scattering length

bc = 4,149 fm

Number density of Si

N = 4,99479 . 1028 m-3

2. Technical Data of the double crystal diffractometer in Vienna

Si - channel cut crystals

Reflexion plane

 [ 3 3 1 ]

Structur factor

 F = 4 + 4i
| F | = 321/2

Asymmetry factor

b = -1

Width of the Darwin curve

 Δθ = 2.899 . 10-6 . rad = 0,596"

Bragg angle

 2θ b = 90 °

Wavelength spread

Δl \ l = 2,5 . 10-3

Energy bandwidth

ΔE \ E = 5 . 10-3

Monochromator - Goniometer

Angular resolution of rotation

18,05"

of tilt

18,05"

Analysator - Goniometer

Angular resolution of rotation

0,1513 µrad

of tilt

0,3471 mrad

with a full width at half maximum

3,3 µrad

Other instrument parameters

Cross section of the neutron beam

26 mm x 26 mm
Background

0,05 Neutrons / s
Peakintensity

9,5 cps / cm2

Integral Intensity

285 cps / cm2