Powder XRD Detectors: Need for Speed – or Sensitivity?

While the powder X-ray diffraction (PXRD) industry tends to favor linear strip detectors (1D) over point detectors (0D) due to their speed, there are many situations in which users would obtain greater benefit by using a point detector.

Both point and linear strip detectors have advantages and disadvantages that must be considered when configuring an XRD instrument.

Point detectors, such as the Proto SPD (Figure 1), convert diffracted X-ray photons directly into an electronic signal. Their electronic circuitry can be made to have much lower noise than strip detectors, and as such, these detectors are extremely sensitive and allow for the detection of the low-intensity peaks that would be generated when analyzing trace or small samples. This makes point detectors advantageous for use in Rietveld analysis of complex samples comprised of multiple phases when low-concentration phases are present.

Figure 1. Proto’s SPD (Silicon Point Detector). Image Credit: Proto

In contrast, linear strip detectors (see Figure 2) are composed of multiple detector elements arranged in a linear array. The detector collects a swath of diffracted X-rays as it moves through 2θ space, while the software computes the resulting peak for each point.

The numerous detector elements allow for reduced collection times when compared to point detectors. This makes them advantageous for applications such as time-resolved XRD, where phase changes can occur at a faster rate than the point detector can optimally measure due to its requirement for a longer collection time. However, the electronic noise in strip detectors is significantly higher than in point detectors, meaning point detectors offer increased sensitivity.

Figure 2. Dectris’s MYTHEN2 R 1D strip detector. Image Credit: Dectris

Due to their long history of use, point detectors have been well optimized and are widely accepted for most PXRD applications; in fact, point detectors have undergone significant improvements in recent years, making them faster and more effective than before.

Of late, questions have been raised as to whether linear strip detectors perform effectively with X-ray sources other than the standard Cu tubes. Below is a comparison of a geological material analyzed with both types of detectors to highlight the difference in data quality.

The same material was loaded into a sample cup and placed in a Proto AXRD Benchtop X-ray diffractometer equipped with a 143-mm-radius goniometer and Co X-Ray tube (λ=1.7889 Å). Data was collected from 5–80° 2θ using Proto’s SPD point detector (0D) and a MYTHEN2 R 1D linear strip detector in both 1D and 0D settings.

For the SPD and MYTHEN 0D data, a step size of 0.02 degrees was used with a collecting time of 2 seconds per step. Experimental scan times were 18 minutes for the MYTHEN 1D data and 167 minutes each for the MYTHEN 0D and SPD data.

The data were then imported into MDI JADE and plotted together (Figure 3). The data were not altered, and no stacking was performed.

Figure 3. Overlayed diffractograms of the same geological material collected using the SPD (black), linear strip in 0D mode (green), and linear strip in 1D mode (red). Image Credit: Proto

An initial cursory analysis of the overlay in Figure 3 highlights the improved intensity and detectability obtained with the SPD detector. The data shows a maximum intensity of over 6000 counts for the SPD compared to approximately 2000 and 750 counts with the MYTHEN operating in 1D and 0D modes, respectively.

Once the background levels are considered, this equates to approximate maximum peak-to-background ratios of 60:1 for the SPD, 5:1 for the MYTHEN in 0D mode, and 3:1 for the MYTHEN in 1D mode. This difference is further highlighted by examining the filmstrip output for each scan (Figure 4). The stacked filmstrips clearly demonstrate a significant difference in peak-to-background ratios between the SPD and the linear strip detector.

Figure 4. Stacked filmstrip output of the overlayed scans in Figure 3 of the SPD (top), MYTHEN 0D (middle), and MYTHEN 1D (bottom) highlighting the high peak-to-background ratio of the SPD detector. Image Credit: Proto

Further analysis of the overlays (see Figure 5) highlights the variation in both the background and noise levels in the low, mid, and high 2θ ranges. The high background of the linear strip detector masked the low-intensity minor-phase peaks that were detected by the SPD.

In addition, peak broadening becomes more noticeable with the linear strip detector compared to the point detector. This is especially true in the high range, as shown in Figure 5C, where the splits between Kα1 and Kα2 become more apparent. This region of 2θ is highly influential in the determination of unit cell parameters, with fine refinements relying on good-quality high-angle data.

Figure 5. Clipped sections of the overlay highlighting A) low, B) mid, and C) high 2θ data. The peak-to-background and noise levels are illustrated to a higher degree in the low and high ranges. Image Credit: Proto

Both linear strip and point detectors are commonly used in various powder XRD applications, and choosing the right detector depends on the specific goals of the application. Strip detectors are highly advantageous for time-resolved studies or use cases in which many samples are analyzed each day and only major phases need to be identified. However, if time is not a constraint, point detectors significantly improve the resolution and quality of powder XRD data.

This information has been sourced, reviewed and adapted from materials provided by Proto.

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