Ultra Small Angle Neutron Scattering

 
 
 
 

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), 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 at the Institut Laue-Langevin in Grenoble, France. 

The Bonse-Hart camera [1, 2] 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 [3-6].

The following picture gives a schematic view of an USANS instrument:


Recent publications

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
 Dq = 2.899 . 10-6 . rad = 0,596"
Bragg angle
 2q b = 90 °
Wavelength spread
Dl \ l = 2,5 . 10-3
Energy bandwidth
DE \ 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
References
[1] Bonse U., Hart M., Small-angle X-ray scattering by spherical particles of polystyrene and polyvinyltoluene, Appl. Phys. Lett., 7 (1965) 238.
[2] Bonse U., Hart M., In Small-angle X-ray scattering, H. Brumberger, ed., 121 pp. Gordon & Breach, New York.
[3] Gerber Th., Walter G., Schmidt P.W., Use of the sampling theorem for collimation corrections in small angle X-ray scattering, J. Appl. Cryst., 24 (1991) 278.
[4] Gerber Th., Schmidt P.W., The sampling theorem and small angle X-ray scattering, J. Appl. Cryst., 16 (1983) 581.
[5] Lake J.A., An iterative method of slit-correcting small angle X-ray data, Acta Cryst., 23 (1967) 191.
[6] Strobl G.R., A new method for evaluating slit-smeared small angle X-ray scattering data, Acta Cryst., 26 (1970) 367.


Last changes by M. Hainbuchner: 14.1.2000
References and suggestions to  Martin Hainbuchner,
 Mario Villa .