SAXS / WAXS configurations that allow to easily switch between line and point collimation are described. Using point collimation with a 2D detector enables full-pattern 2D SAXS and WAXS measurements, which is essential for anisotropic materials. The 2D SAXS / WAXS performance is demonstrated on some test samples.
On the Empyrean instrument SAXS and WAXS measurements are usually done with a line collimation setup. Here we describe how such a setup can be quickly and easily transformed to a point collimation setup. Examples of full-pattern 2D SAXS and 2D WAXS measurements from polymers, fibers and colloidal crystals will be given.
Traditionally SAXS measurements are done with a line collimation setup, as e.g. in the classical Kratky Kompaktkamera. The X-ray beam is tightly collimated in the equatorial (2theta) direction, but strongly elongated in the axial direction. The line focus of a sealed X-ray tube, especially when combined with a focusing mirror, provides an intense direct beam that is well-suited also for weakly scattering, isotropic samples. A line (or even point) detector can be used. Analysis results are representative for a relatively large sample volume. However, the finite dimensions of the direct beam result in a 'smearing' of the experimental data, which must be taken into account in data analysis. Also low-angle diffraction peaks show significant asymmetry. For the characterization of samples having anisotropic structures, a point collimation setup (resulting in a pencil beam) and an area detector are required. Such setup often gives a less intense direct beam, but experimental data are (almost) free of 'smearing' effects. Also low-angle diffraction peaks are symmetric. A much smaller sample volume is probed, and more localized structural information is obtained.
On the Empyrean instrument SAXS and WAXS measurements are usually done with a line collimation setup. Here we describe how such a setup can be quickly and easily transformed to a point collimation setup. Examples of full-pattern 2D SAXS and 2D WAXS measurements from polymers, fibers and colloidal crystals will be given.
Figure 1. Left: 2D SAXS / WAXS setup with vacuum path (ScatterX78) and GaliPIX3D. Right: 2D WAXS stage and 2D beam stop mounted in front of GaliPIX3D
Figure 2. Left: 2D SAXS / WAXS setup without vacuum path (SAXS / WAXS stage) and PIXcel3D. Right: 2D WAXS stage and 2D beam stop for PIXcel3D being mounted
Figure 3. Holders for different sample types
A SAXS/WAXS setup with a vacuum path (using ScatterX78, see Fig. 1) or with a beam path in air (using the SAXS/WAXS stage, see Fig. 2), an X-ray mirror, and the GaliPIX3D or PIXcel3D detector is used. For switching from line to point collimation the X-ray tube remains in the line focus orientation. Two narrow masks (the 2D SAXS kit) are inserted in the beam path to reduce the axial size of the X-ray beam, resulting in a close-to-quadratic beam cross section. The rectangular beam stop of ScatterX78 is removed from the beam path by a switch at the front of ScatterX78. A 2D WAXS stage and a 2D beam stop are mounted in front of the detector. The detector mode is switched from 1D to 2D. Clamp holders (see Fig. 3) are available for the fixation of different sample types (solids, sheets, fibers, powders).
Figure 4. Dual sample position concept. Left side: sample position for 2D SAXS, right side: sample position for 2D WAXS measurements
In a 2D SAXS experiment the sample is mounted on the stage in the center of the goniometer. For a 2D WAXS measurement the sample is mounted on the 2D WAXS stage, at a short distance in front of the detector. By doing so, the field of view of the detector is increased (see Fig. 4). The accessible 2theta ranges (typical values) are given in the table.
| GaliPIX3D | PIXcel3D | |
| 2θmin (2D SAXS) | ≤ 0.1° | ≤ 0.1° |
| 2θ range (2D SAXS) | -3.4° to +3.4° | -1.5° to +1.5° |
| 2θ range (2D SAXS) | -30° to + 30° | -22° to +22° |
Bulk sample : The isotropic patterns indicate isotropic structures on the different length scales. Stretched sample , arrows indicating the stretching direction t he anisotropic patterns indicate a preferred orientation of the lamellar stack and of the crystal lattice, respectively.
Measured with a setup in air, using PIXcel3D.
SAXS patterns of dried colloidal dispersions, indicating the formation of colloidal crystals
2D SAXS pattern of two crossed fibers of rat tail collagen and 2D WAXS of a polypropylene fiber, pointing to a skin-core morphology
SAXS patterns of dried colloidal dispersions, indicating the formation of colloidal crystals
On the Empyrean platform a line-collimation SAXS/WAXS setup can be easily converted to a point-collimation setup for full-pattern 2D SAXS and 2D WAXS data collection. This is
particularly useful for the analysis of anisotropic structures.
