Reflection measurements are based on the idea that different energies have different penetration depths when samples are measured in reflection geometry.
As such, each peak delivers information over a different depth range in the material. The information derived from diffraction peaks is actually a convolution of the Beer-Lambert law with the property gradient measured (lattice spacing, phase volume, etc.), and can be deconvolved through an inverse Laplace transform.
The depth positions analyzed on a measurement are given by the energies of the peaks collected. Optimization of reflection measurements involve „positioning” diffraction peaks in energies which probe the desired depths. As such, reflection measurement optimization is based on 2th selection.
As an example, the diagram below shows the energy positions for diffraction peaks of a steel ferritic phase as a function of 2th. The colour scale on the background shows the attenuation length as a function of energy and 2th on a low alloy steel. Only the first 15 lower order peaks are shown, higher order peaks cannot typically be discerned from the background. As can be seen, there is a trade-off between near surface data density and maximum penetration depth.
For routine experiments, the attenuation depths available for different materials are shown below. Lesser attenuation depths can be obtained with asymmetric reflection.
| Material | Min penetration depth (µm) | Max penetration depth (µm) |
|---|---|---|
| Al | 75 | 850 |
| Ti | 10 | 450 |
| Fe | 5 | 250 |
| Ni | 3 | 180 |
The first step of any experiment is to optimize the measurement conditions, bearing in mind the science case addressed. The beamline staff generally performs this with the users. When preparing a proposal, please allocate at least half a shift for this optimization.
Measurement times:
The reflection geometry yields high scattering intensity, and as such, most experiments can be done with an exposure time of ~ 1s. The limiting factor for exposure rate is detector saturation rather than incident intensity.
- Longer exposure times are needed when observation of low relative intensity peaks is needed (low structure factor, low phase volume fraction, etc.).
- For in-situ experiments, exposure times of 0.02 s have been successfully used.
Please discuss with beamline staff if an acquisition rate higher than 1 Hz is needed.
Depth resolved stress determination
ψ scans in reflection geometry allow the determination of near surface stress gradients (more information at https://doi.org/10.1016/j.msea.2003.09.073). ψ scans can be done at different φ positions to obtain different stress gradient components. Alternatively, full pole figures can be measured. The beamline has own software for the data analysis using the Modified Multiwavelength Method.
