Use of SDD, XPIN6 and XPIN13 to Identify Nitronic 60 in Aluminum Metals/Alloys

PINs and SDDs are the two main types of energy dispersive detectors. This article presents the results of a comparative study of SDD and PIN detectors to find the best detector for identification of aluminum metal and alloys. The results demonstrate that although the XPIN6 and XPIN13 detectors are capable of identifying aluminum alloys, there are some additional advantages for using an SDD detector. The material studied in this experiment was Nitronic 60.

Comparison of Detectors

A brief comparison between a typical SDD detector and a PIN detector 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 a larger detecting area. Moreover, SDD detectors have a lower Fe55 FWHM resolution at a faster peaking time, resulting in better energy resolution and roughly a 2.5X 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 125 - 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 offers.

Experimental Setup

Figure 1 depicts a standard XRF set-up used in this experiment, which involved the use of Moxtek’s TUB00083 X-ray source with a silver anode and a 125µm thick beryllium window. The source was set at an emission current of 15 or 30µA and at 12kV. The distance between the source and the sample was 25mm. For each detector, the distance between the sample and the detector was 8mm and there was no filter in front of the source.

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.

Both XPIN detectors were equipped with a 25µm thick beryllium window, while the SDD had a 12µm beryllium window. An aluminum sleeved brass collimator was mounted on the X-ray source. A silver aperture was positioned on the end of the collimator to eliminate the stray aluminum XRF signal from this collimator.

Collimators were not mounted on the detector in order to position the detector as close as possible to the sample. This reduces air absorption, a crucial condition for successful aluminum alloy identification. In order to eliminate stray XRF signals, this analysis relied exclusively on the internal aperture of the detectors.

An XRF background spectra from a plastic sample were illustrated in Figure 2, showing the Compton scattered bremsstrahlung and the Compton scattered silver Lá lines from the source. The excitation from aluminum, silicon, phosphrous, sulfur, and chlorine is proved by the silver Lá lines. An inexperienced operator of an XRF algorithm may confuse the Compton scattered silver Lá lines as an XRF signal.

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.

With Cl and S in the plastic, the XRF signal was not completely clean, with an Al signal probably from the tube collimator and a slight Si peak from an unknown source. The background Al signal was considerably lowered to the level depicted in Figure 2 using the silver aperture mounted on the collimator of the tube.

Experimental Procedure

The experiment involved the comparison of SDD, XPIN6 and XPIN13 detectors for their basic XRF performance. The key technical specifications of each detector used in this experiment are summarized in Table 2. Each detector was run at a 50% dead time by modulating the tube emission current to compare the performance of a quazi-normalized XRF. The SDD has better technical performance when compared to the PIN detectors.

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

Detector Area Fe55 FWHM resolution Nitronic 60 Spectra counts in 30 sec Dead time DPP Peaking time Tube current Detector Temp
SDD 20mm2 150eV 594k 67% 8µsec 5µA -45 °C
XPIN6 6mm2 165eV 229k 52% 20µsec 5µA -35 °C
XPIN13 13mm2 200eV 231k 53% 20µsec 3µA -35 °C

Experimental Results

Nitronic 60 is composed of 4% Si, 17% Cr, 8% Mn, a balance (~62%) of Fe, 8.5% Ni and 0.75% Mo. An XRF spectrum from the same Nitronic 60 sample was recorded by each detector. The entire spectrum from the SDD, XPIN6, and XPIN13 is shown in Figure 3, revealing all the key elements. The separation of the Ká lines of all the elements is adequate for clear identification.

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.

Some blurring of the K lines can be observed in the case of XPIN13, which has lower resolution. Nevertheless, this line blurring did not influence the estimated elemental concentrations, indicating the proper functioning of the XPIN13.

The spectra acquired from the Nitronic 60 sample were run through an XRF fundamental parameters (FP) routine to convert the spectra 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. This table shows the resulting concentrations from three detectors using the 30 second and 10 second scans from an Al 6061 sample. Mg was excluded due to computational issues.

Si Cl Cr Mn Fe Ni Cu Mo Total counts in spectra
Tabulated Nitronic 60 4% --- 17% 8% Bal. 8.5% --- 0.75
SDD-30 sec 4.0 0.18 14.7 8.2 63.9 8.6 0.20 0.22 454k
PIN6-30 sec 3.7 0.34 14.7 8.1 64.1 8.2 0.31 0.52 229k
PIN13-30 sec 4.0 0.10 14.5 7.8 64.9 8.5 0.10 0.16 231k
SDD-10 sec 4.0 0.29 14.6 8.0 64.3 8.3 0.31 0.30 198k
PIN6-10 sec 3.9 0.20 14.8 8.2 63.4 7.9 0.80 0.70 76k
PIN13-10 sec 4.0 0.39 14.6 7.2 64.9 7.8 0.83 0.34 76k

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.

Conclusion

Both PIN and SDD detectors can be used to determine Nitronic 60, with 4% silicon being a major element for determination. The elemental accuracy is sufficient to identify Nitronic 60 with the help of the FP routine. From the results, a PIN detector is equally good for identification of aluminum alloys, but has cost advantage over an SDD detector. Nonetheless, SDD detectors may be a better option depending on the specific requirement.

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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