An Overview of XRF Basics
2. Instrumentation
2.3 Incident Beam Components
2.3.1 The End-window Tube and Generator
The X-ray tube and generator are designed for a permanent output of 4 kW at a maximum high voltage of 60 kV and a maximum tube current of 100 mA or 150 mA. The combination of high voltage and tube current must not exceed 4 kW, e.g. at 4 kW maximum:
- 27 kV, 150 mA = 4.05 kW
- 30 kV, 134 mA = 4.02 kW
- 40 kV, 100 mA = 4.00 kW
- 50 kV, 80 mA = 4.00 kW
- 60 kV, 67 mA = 4.02 kW
The minimum setting is 20 kV, 5 mA for a power of 0.1 kW.
Rhodium is used as the standard anode material. The light elements Be to Cl are effectively excited by the Rh-L beam's high transmission rate through the tube's 75-µm Be window. The characteristic Rh-K radiation excites the elements up to Mo (Ru) (see Table 2). The elements from Rh onwards are excited by the Bremsstrahlung's high-energy "tail."
Four-kW tubes with other anode materials (e.g. Mo, W, Au, Cr) can be used for special applications.
2.3.2 The Primary Beam Filter
The primary beam filter is seated on a changer for 10 positions (including vacant positions) and is equipped with a selection of absorber foils. It is located between the tube and the sample, filters out undesirable or interfering components of the tube radiation for certain applications and improves the signal-to-noise ratio. Al and Cu foils, for example, are used as absorbers in a variety of thicknesses. Primary beam filters can be selected to suit individual requirements when purchasing the instrument.
When measuring Rh K with the Rh tube, the characteristic Rh radiation coming from the tube must be filtered out because it would otherwise be measured as a result of elastic scattering on the sample. By using a 0.2-mm Cu filter, the characteristic Rh tube radiation is largely absorbed prior to reaching the sample. The measurement must be taken with a tube high-voltage of 60 kV as the Rh in the sample is only excited by the high-energy Bremsstrahlung.
Fig. 25 illustrates the tube spectrum acting on the sample without a primary beam filter and tube high-voltage of 60 kV.
Fig. 26 shows the reduction in Rh radiation scattering on a plant sample using different primary beam filters made of Cu or Al.
Fig. 25: Cd and Rh peaks without a copper primary beam filter
Fig. 26: Cd and Rh peaks with a 0.2- or 0.3-mm copper primary-beam filter
Fig. 27: The effect of the aluminum primary beam filter for optimizing the peak/background ratio
When analyzing a sample of very pure graphite, peaks of the elements Cr, Fe, Ni and Cu can occur in the 2θ-spectrum although the sample contains none of these elements. The Cu peak originates from the excitation of the collimator material that mainly consists of copper. The Cr, Fe and Ni peaks are called "spectral impurities" of the tube. If the elements Cr, Fe and Ni are to be measured as traces, it may be advantageous to use the 0.2-mm Al filter to absorb these components.
2.3.3 Sample Cups, the Cup Aperture
In the S4 EXPLORER and S4 PIONEER, the sample to be measured is first transported into the sample changer's pre-vacuum chamber and then into the measuring position where it rotates at up to 30 revolutions per minute, depending on the application, to even out sample inhomogeneity. The collimator mask's optimized screening allows steel apertures to be used in the majority of cases. Other aperture diameters and materials are available upon request.
Depending on the type of sample magazine, samples have to be placed in the cups either manually (magazine with grab) or automatically. Automatic sample magazines include those designed for "bare samples" (magazine with sucker) or those designed for steel rings (magazine with magnet). When using thin filters for measurements, an anti-background scattering cup must be used to eliminate the fixing plate radiation.