X-ray Fluorescence Analysis (XRA) in a Vacuum

X-ray Fluorescence Analysis (XRA) in a Vacuum

The XUV® 773 is Fischer’s high-end X-ray fluorescence instrument where measurements can be performed in vacuum, in ambient air or with helium purge. This model has many advantages over other vacuum XRA systems on the market in many aspects. Firstly, measurements are consistently linked with video observations. Thus, the measurement spot is indicated in the video image to scale. Moreover, the measurement distance can be adjusted over a wide range. The XUV 773 has a programmable motorised measurement table where the sample can be measured according to a program. The WinFTM® software enables standard-free measurements on both solid and coated structures.

Why use a vacuum for measurements?

For any X-ray without vacuum, the air between sample and detector absorbs radiation. In the case of S-K radiation (2.38keV), only 50% of the signal is suppressed by air. In the case of Al-K (1.5keV), 90% is suppressed. And with Na-K (1.04keV), 99% of the intensity is lost. Thus, an analysis of the elements Na, Mg, Al and Si has been practically impossible without a vacuum chamber – or only possible to a very limited extent.

The other reason is that before the primary radiation interacts with a sample, it interacts with the atmosphere and emits both scattered radiation and also fluorescent radiation from argon in the direction of the detector. This background radiation sometimes distorts readings very significantly (e.g. in the measurement of thin Pd layers).

Figure 1: Tourmaline Analysis
Figure 1: Tourmaline Analysis

Why multiple excitation?

The above three spectra of a tourmaline stone show the special features of a complete analysis of a material containing both light and heavy elements. Evidence of the light elements (Al, Si) can best be obtained with “soft” excitation (low energy X-rays). This is the blue spectrum. The tube voltage is approx. 10 kV. Here we obtain the best signal-to-noise ratio for these light elements. Medium and heavy elements are relatively poorly excited (e.g. Cu), or not excited at all (e.g. Ga), to emit fluorescent radiation. This is only obtained with double the tube voltage (yellow spectrum). However, the background radiation is really high, particularly in the case of the important Ga element.

Furthermore, we find many Bragg peaks in the spectrum, which deteriorate the evaluation of the results. Therefore, it is necessary to work with an even higher tube voltage in combination with a suitable filter (black spectrum). This produces clear conditions in the classification of the peaks – and thus also enables a well-founded Bragg correction to be made. The software provides a quantitative evaluation of these three measurements in one pass.

It is important to use the correct settings for multiple excitation in the WinFTM® measurement task. The optimal definition for a particular class of applications should be elaborated professionally.

Example: Analysis of steel

A steel sample was excited once in a “hard” manner (high energy Xrays) at 50 kV (lilac spectrum) and then in a “soft” manner at 8 kV (orange spectrum). In the evaluation, both sets of information were combined logically to obtain the results shown in the table in Figure 4.

Figure 2: Steel Analysis (50 kV)
Figure 2: Steel Analysis (50 kV)
Figure 3: Steel Analysis (8 kV)
Figure 3: Steel Analysis (8 kV)
Figure 4: Table of Steel Analysis
Figure 4: Table of Steel Analysis

In conclusion, the XUV® 773 is an universally applicable energy dispersive X-ray measuring instrument. It is ideally suited for non-destructive analysis of thin coatings, traces and light elements.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.