Choosing the Material for the Anode of an X-ray Tube

In the specifications of XRF spectrometers from various manufacturers, one can find a parameter such as the material of the anode in the X-ray tube. What is it, how does it affect the analysis, and how can you make the right choice?

Commonly, in X-ray fluorescence (XRF) analysis, tubes with anodes made of silver (Ag), rhodium (Rh), tungsten (W), or gold (Au) are used. The anode material determines the excitation spectrum of the analyzer, which consists of two components - a continuous braking spectrum and the characteristic lines of the anode material (see Fig. 1).

Both components can be used to excite the elements of interest. To optimally analyze a specific element from the periodic table, the excitation spectrum should have high intensity just above the absorption edge of the element, while maintaining a low background in the range of the characteristic lines of that element.

At higher energies, the X-ray tube spectrum consists entirely of bremsstrahlung radiation. The intensity of bremsstrahlung from heavy elements (W, Au) is much higher than from Ag or Rh, making tubes with W or Au anodes better suited for analyzing elements with line energies above 20 keV.

The characteristic lines of the anode material have high intensity and can successfully be used to excite elements with lower energies. This results in a better signal-to-noise ratio in the fluorescence spectrum than when only using the tube's bremsstrahlung radiation. For instance, the excitation using the K-lines of silver or rhodium is excellent for analyzing elements ranging from iron to molybdenum.

However, the characteristic lines of the X-ray tube may overlap with the lines of elements in the analyzed sample, which can hinder their analysis. Notably, lines from tungsten and gold overlap with the characteristic lines of many metals found in most steels and alloys. The silver line in the excitation spectrum prevents the analysis of silver content in the sample.

Here are some general recommendations for choosing the material of the X-ray tube anode:

Tubes with a silver anode are most commonly found in outdated XRF spectrometer models. This is due to the use of software from old spectrometers, which employed the isotope Cd 109 as the excitation source with a similar spectrum. The silver line at 22.1 keV effectively excites elements from chromium to molybdenum, and the L-series of silver around 3 keV can be used to excite light elements from aluminum to sulfur. However, analyzing low concentrations of magnesium with such a tube is impossible due to overlap with one of the silver lines. Analysis of silver in samples is also not possible.

A tube with a rhodium anode has a similar spectrum to a silver anode but lacks the aforementioned drawbacks. However, it cannot be used to analyze the content of rhodium, which is important for some applications, such as analyzing automotive catalysts or jewelry.

A tube with a gold anode provides about twice the intensity of bremsstrahlung compared to rhodium or silver. It is excellent for analyzing elements with line energies higher than those of silver. Additionally, the characteristic lines of gold are low energy and can be easily filtered out to obtain a purely bremsstrahlung excitation spectrum. Unfortunately, such a tube is unsuitable for analyzing light elements.

A tungsten tube is very similar to a gold tube in its characteristics. It also allows for the analysis of gold content in a sample.

If you are unsure which tube would be most suitable for your needs, fill out the form and receive a free recommendation from our specialists.