Powder X-ray Diffraction Basic Course - Second installment: Selection of equipment configuration to obtain high-quality data

It means data that is well suited to the analysis goal. Depending on the job, that may mean high intensity, high angular resolution, high peak-to-background ratio, or high signal-to-noise ratio. There is no single best configuration for every sample. The correct setup depends on what needs to be measured and on the sample type.

A rotating anode is useful when higher intensity is needed, especially to reduce measurement time or improve detection of weak features. In the Si example, the rotating anode produced about 4.6 times the intensity of the sealed tube setup. That kind of gain can make a major difference for throughput or low-signal samples.

The target metal controls the X-ray wavelength, and wavelength affects how well diffraction works for a given sample and application. Cu Kα is suitable for many common materials, but it is not automatically the best choice. If the sample composition leads to strong fluorescence under a given target, background can rise and data quality can suffer. Target choice is tied to both sample composition and analysis purpose, including qualitative work, residual stress, and pair distribution function measurements.

Bragg-Brentano is a strong default for reflection measurements when the sample surface is flat and the goal is high intensity and high resolution. Parallel beam is more flexible in scan geometry and works better for rough surfaces, though intensity can be lower in some cases. Focusing beam optics are used in transmission geometry and can provide both high intensity and good resolution, especially for capillary-style transmission measurements.

They control how the beam spreads, how much of the sample is illuminated, how much scattered radiation reaches the detector, and how much intensity versus resolution is obtained. A wider incident slit increases intensity but can over-illuminate the sample and distort intensity ratios if the beam spills onto the holder. Narrower soller slits reduce vertical divergence and improve peak shape, but they also lower intensity and increase scan time. Receiving slits further affect background rejection and angular resolution.

A Kβ filter is inexpensive and gives relatively high intensity, so it is widely used. The downside is that fluorescence and background may still remain high. A monochromator gives cleaner wavelength selection and can reduce fluorescence and continuous X-rays, which improves peak-to-background ratio, but it also lowers intensity and increases measurement time. A divergence mirror can deliver monochromatic X-rays to the sample and is especially useful when fluorescence from non-Kα radiation is a problem, including some Ni-based, Cu-based, and Fe-based sample situations.

XRF reduction mode uses the detector’s energy discrimination to reject more unwanted fluorescence. That can greatly improve peak-to-background ratio for fluorescence-prone samples, such as Fe-based or Mn-based samples measured with a Cu target. The tradeoff is that diffracted intensity is reduced as well; roughly half of standard mode. So it helps when fluorescence is a real problem, but it is not the best choice for samples that already have low fluorescence background.

Because every improvement comes with a tradeoff. Narrower slits can sharpen peaks but reduce counts. A knife edge can lower low-angle scatter but may block high-angle diffracted X-rays. A thicker Kβ filter suppresses unwanted radiation better, but it also reduces useful Kα intensity. Settings should be optimized as a system, not chosen one by one in isolation.