IUCr journals

New apparatus combination creates outstanding performance of Kratky SAXS systems

J. Appl. Cryst. (2000). 33, 869-875 [doi:10.1107/S0021889800000881]

[conventional block collimation]
(a)
[improved block collimation]
(b)
Conventional block collimation (a) and improved block collimation using the parallel beam from a Göbel mirror (b). f = focus, S = slit, B1 and B2 = blocks, R = registration plane.
Small-angle X-ray scattering (SAXS) is a powerful technique to study electron density fluctuations on a nanometer scale. The essential prerequisite for SAXS measurements is an X-ray beam of low divergence and high intensity. For a long time, with conventional X-ray sources, these requirements have been met to some degree only by using long-slit collimation geometry, realized for example with the widely used Kratky camera (Kratky, 1982). Nowadays, technical developments such as rotating-anode X-ray generators, two-dimensional position-sensitive detectors, and in particular, new X-ray optics like parabolically bent, graded multilayers ('Göbel mirrors', Schuster & Göbel, 1995), allow pinhole cameras to be built that meet or even exceed the performance of classical long-slit systems in terms of resolution and primarybeam intensity. Pinhole cameras overcome the most serious drawbacks of long-slit systems, namely the slit-smearing of the signal and the loss of information for anisotropically scattering samples. One might thus even ask why long-slit systems are still of interest today.

In this context, Bergmann, Orthaber, Scherf & Glatter [J. Appl. Cryst. (2000). 33, 869-875] have recently reported on a significant improvement of long-slit systems by taking advantage of the above-mentioned technical developments (Göbel mirrors) for the Kratky camera. Using a parallel, monochromatic X-ray beam from a Göbel mirror, a slight modification of the Kratky block collimation system leads to an intensity gain of a factor of four without loss of resolution. This is simply due to the fact that in contrast to the classical system the whole focus illuminates the registration plane R and that the parallel beam results in a rectangular instead of a triangular beam profile (see figure). Together with the advanced characteristics of the Göbel mirror (15-20 mrad angular acceptance from the source) and the use of a rotating-anode X-ray generator, a total intensity gain of a factor of about 40 in comparison with a conventional Kratky system was reached by the authors. In terms of primarybeam intensity, this would nearly be comparable with some SAXS instruments on second-generation synchrotron radiation sources (bending magnets). This remarkable gain puts Kratky cameras again on a competitive basis for laboratory SAXS instruments, as long as fully isotropic scattering systems are concerned. As quite a significant number of SAXS applications deal with solution scattering (e.g., particles or polymers in solution), the improved Kratky camera can be expected to cover a wide field of applications with up to now unmatched primary-beam intensity.

References

Kratky, O. (1982). Small Angle X-ray Scattering, edited by O. Glatter and O. Kratky. London: Academic Press.

Schuster, M. & Göbel, H. (1995). J. Phys. D Appl. Phys. 28, 270–275.

Oskar Paris, Metal Physics Inst., U. of Leoben
Erich Schmid Inst., Austrian Academy of Sciences, Leoben, Austria