Treating Fluorescence at its Roots: Choice of the X-ray Tube for X-ray Diffraction on Iron-containing Samples

X-ray diffraction experiments on lab instruments are usually performed with Cu-Kα radiation. However, fluorescence leads to a high measurement background and low-intensity signal with this setup for iron-containing samples. Choosing a more suitable X-ray tube for such samples avoids this issue at its origin, leading to high-quality data even with short scan times.

Introduction

For most samples, an X-ray tube with a Cu anode is the best choice for X-ray diffraction (XRD) measurements on laboratory diffractometers, since the wavelength is ideal for diffraction and it provides the highest flux and longest tube lifetime. However, if the sample contains iron or cobalt, fluorescence causes high measurement background and low intensities of the diffraction signal with this standard setup. Additionally, the penetration depth is reduced, which might decrease the representativeness of the results or even lead to graininess issues due to the reduced number of crystallites contributing to the diffraction signal.

Being familiar with this fluorescence issue, XRD measurements in the steel industry are performed with Cr or Co anodes for many decades, since Cr-Kα and Co-Kα radiation cause much less fluorescence with the iron-containing steel samples while still having a suitable wavelength for diffraction experiments1, 2. Nevertheless, using Cu-Kα radiation with iron- or cobalt-containing samples is very common in many other research fields3. Besides many cases, where the fluorescence issue is just ignored and the high background and reduced intensity from diffraction peaks are accepted as unavoidable, two main procedures to improve the quality of the diffractograms collected with a Cu anode in the presence of fluorescence have been established: Firstly, a secondary monochromator may be used to filter out the fluorescence as only the diffracted X-ray can pass through the monochromator. Secondly, an energy-dispersive detector can be used to make the same wavelength selection directly in the detector. Both methods reduce the fluorescence-based background, leading to nice-looking diffractograms. However, they only treat the symptoms of the fluorescence issue – the high measurement background. Since a large amount of the incoming X-rays is converted to fluorescence at the sample, a long measurement time is required for a reasonable signal, which still does not take into account potential problems such as small penetration depth. An additional issue that can arise when doing the energy filtering via the detector is that the uniformly high intensity of the fluorescence background for samples with high iron or cobalt content may cause problems due to the very high count rates observed on the detector.

The Automated Multipurpose Powder X-Ray Diffractometer XRDynamic 500 from Anton Paar, unlike traditional diffraction systems, facilitates the easy and quick exchange of the X-ray tube followed by a fully automatic alignment of the new system setup. Thus, it allows to simply select the most suitable X-ray tube for each sample and to treat the fluorescence issue at its roots. In this application report, iron-containing samples with different levels of iron content are investigated with different X-ray tubes to emphasize the benefits of choosing the correct tube for each sample.

 

References

1. F.S. Gardner, M. Cohen and D.P. Antia, Trans. AIME 154 (1943), 306-317.
2. B.L. Averbach and M. Cohen, Trans. AIME 176 (1948), 401-415.
3. Y.M. Mos et al., Geomicrobiol. J. 35 (2018), 511-517. 

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