Use of SDD, XPIN6 and XPIN13 to Identify Naval Brass in Non-Aluminum Metals/Alloys

This article discusses the results of a comparative study of finding the best detector to determine naval brass CD464 in non-aluminum metals and alloys. The results demonstrate that both PIN detectors and SDD detectors are equally fit in the determination of several common nonaluminum metals and alloys. However, for this application, the lower cost and more perfect operation of a PIN detector make it preferable over an SDD detector.

Comparison of PIN and SDD Detectors

A comparison of typical SDD and PIN detectors is outlined in Table 1. The performance of a typical SDD detector is superior to a PIN detector. Besides having a better ultimate energy resolution, SDD detectors have more detecting area. Moreover, SDD detectors have a lower Fe55 FWHM resolution at a faster peaking time, giving rise to roughly a 3X higher counting rate when compared to the PIN detectors. The resolution is crucial in determining X-ray events from various elements, while counting is vital to obtain better statistics in less time.

Table 1. A brief comparison between typical SDD and PIN detectors.

Detection Area Fe55 Resolution Detector Inner Temp. Upper Input Count Rate (ICR) Limit Price
Typical SDD 10 - 50mm2 120 - 160eV -20 to -40°C ~500kcps More expensive
Typical PIN 5 - 15mm2 150 - 220eV -20 to -40°C ~100kcps Less expensive

A typical PIN detector is less expensive and is often utilized in price sensitive systems such as XRF. PIN detectors are suitable for several XRF applications, including metal and alloy determination, which do not require the performance advantages that the SDD detector provides.

Experimental Setup

Figure 1 shows a standard XRF set-up used in this experiment, which involved the use of Moxtek’s 50kV, 4 Watt Ultra-Lite X-ray source with a tungsten anode and a 250µm beryllium window. The source was set at 15-20µA and at 50kV. The distance between the source and the sample was 25mm and a 70µm copper filter was placed in front of the source. For each detector, the distance between the sample and detector was 25mm.

On the left is a sketch of the XRF setup, outlining the most critical parts. On the right is an image of the set up where all the components including collimators can be seen.

Figure 1. On the left is a sketch of the XRF setup, outlining the most critical parts. On the right is an image of the set up where all the components including collimators can be seen.

Moxtek’s MXDPP-50 processed the detector signal. The X-ray source and the detectors had aluminum sleeved brass collimator over them. The aluminum sleeve was required to prevent the stray XRF signal from the brass in order to ensure that the XRF signal is only from the sample. Most of the X-rays from the source below ~15keV were eliminated by the copper filter, providing a better signal-to-noise ratio in this region. However, the copper does allow one tungsten Lα line through at ~8.3kV.

The excitation from Nickel and the lower Z elements is improved by the tungsten Lα line, which also produces a non-XRF peak. This peak may confuse the inexperienced operator of an XRF algorithm. The XRF spectra from a clean plastic sample are illustrated in Figure 2, showing the Compton scattered tungsten Lα line and Compton scattered bremsstrahlung from the source.

Spectra collected from a plastic sample, which shows the Compton scattered background from a clean XRF sample.

Figure 2. Spectra collected from a plastic sample, which shows the Compton scattered background from a clean XRF sample.

Experimental Procedure

The experiment involved the comparison of SDD, XPIN6 and XPIN13 detectors for their XRF performance to determine non-aluminum alloys and metals. The key technical specifications of each detector used in this experiment are summarized in Table 2.

Table 2. A functional comparison between an SDD, XPIN6 and XPIN13 detector

Detector Area Fe55 FWHM resolution Naval Brass Spectra counts in 30 sec Dead time DPP Peaking time Tube current Detector Temp
SDD 20mm2 150eV 487k 38% 8µsec 20µA -45 °C
XPIN6 6mm2 165eV 148k 21% 20µsec 20µA -35 °C
XPIN13 13mm2 200eV 205k 42% 20µsec 20µA -35 °C

For this experiment, the same setting was applied to the X-ray tube. This leads to different dead times on the detector. As expected, the SDD has better technical performance when compared to the PIN detectors.

Experimental Results

Naval brass is composed of <0.1% Fe, 59-62% Cu, 39% Zn, 0.5-1.0% Sn and <0.2% Pb. An XRF spectrum from a naval brass source was recorded by each detector for 30 seconds. The full spectrum from the SDD, XPIN6, and XPIN13 is depicted in Figure 3, revealing all the major elements. The spectral data focused on the region between 5 and 11keV are shown in Figure 4.

XRF Spectra collected from a naval brass sample, over 30 seconds, with all the major peaks labeled. Y axis of counts is in a log scale.

Figure 3. XRF Spectra collected from a naval brass sample, over 30 seconds, with all the major peaks labeled. Y axis of counts is in a log scale.

The same XRF Spectra collected in Figure 3, with a linear y-scale and energy range from 5 to 11keV.

Figure 4. The same XRF Spectra collected in Figure 3, with a linear y-scale and energy range from 5 to 11keV.

As can be seen, the separation of Kα lines of all the elements is well defined for clear identification. There has been a distinction between the Zn Kα and Cu Kß for the SDD and PIN6. A clear distinction is not observed between these lines in the case of XPIN13 due to its lower resolution. Nevertheless, the estimated elemental concentrations were not affected by this line blurring, showing the proper functioning of the XPIN13.

The spectra acquired from the naval brass CD464 sample were run through an XRF fundamental parameters (FP) routine to convert them into elemental concentrations. Each detector, when properly configured in the FP program, provided almost the same results. The resulting concentrations from each detector are outlined in Table 3. The last column provides the total number of X-ray events identified in each of the spectra.

Table 3. The resulting concentrations from the three compared detectors using the 30 second and 10 second scans from a Naval Brass CD464 sample.

Fe Cu Zn Sn Pb Total counts in spectra
Tabulated Brass CD464 <0.1% 59-62% b (39%) 0.5-1.0% <0.2%
SDD-30 sec 0.18 58.4 40.1 0.78 0.07 487k
PIN6-30 sec 0.20 58.8 39.6 0.99 0.05 148k
PIN13-30 sec 0.16 58.3 40.1 1.10 0.05 205k
SDD-10 sec 0.15 59.1 39.6 0.76 0.04 162k
PIN6-10 sec 0.17 58.6 39.8 1.08 0.05 54k
PIN13-10 sec 0.17 57.7 40.5 1.11 0.11 68k

Each detector was used to take 30 second and 10 second XRF scans, which were then compared. The results showed that all detectors were able to identify in 10 seconds. The higher count rates of the SDD are not actually required for sub-percent level element identification for metals and alloys. Using the FP routine, each detector provided an elemental concentration within 1% or less for each of the elements. This level of accuracy is sufficient for determining naval brass.

Conclusion

The results clearly show that both PIN detectors and SDD detectors are equally effective in determining several common non-aluminum metals and alloys. However, for this application, the lower cost and more perfect operation of a PIN detector make it a better choice over an SDD detector.

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

For more information on this source, please visit Moxtek, Inc.

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