An Overview of XRF Basics
2. Instrumentation
2.4 Emitted Beam Components
2.4.1 The Vacuum Seal
When measuring liquids or loose powder samples in a helium atmosphere, the vacuum can be maintained in the spectrometer volume by inserting a thin separating, or sealing, foil between the sample chamber and the spectrometer chamber. This causes the separating foil to absorb less radiation than would be the case if the spectrometer chamber were filled with helium.
For alternate measurements of samples in a vacuum and in helium, the smaller sluicing volume reduces helium consumption and considerably speeds up the change from one operating mode to another.
2.4.2 Collimator Masks
Collimator masks are situated between the sample and collimator and serve the purpose of cutting out the radiation coming from the edge of the cup aperture. The size of the mask is generally adapted to match the cup aperture being used. The SRS 3x00's changer has 4 positions; the SRS 300 and 303 are equipped with a 3-position changer. The S4 EXPLORER, S4 PIONEER and S8 TIGER employ collimator masks that are set very close to the sample and therefore optimally screen the sample cup aperture.
2.4.3 Collimators, the Soller Slit
Collimators consist of a row of parallel slats. They are used to select a parallel beam of X-rays coming from the sample and striking the crystal. The spaces between the slats determine the degree of parallelism and thus the angle resolution of the collimator.
Sequential spectrometers are fitted with a 4-position changer. Besides the standard collimators with aperture angles of 0.15°, 0.23° and 0.46°, two additional collimators can be installed to optimize measurements for a particular application. A 0.77° collimator is available for high-resolution measurements (e.g. with LiF(420)). Collimators with a low resolution (e.g. 1.5° to 2.0°) are advantageous for light elements such as Be, B and C since the XS multilayer crystal's angle resolution is limited. Using a low-resolution collimator increases the intensity significantly. This enables intensity to be increased without a loss in angle resolution when analyzing the light elements (Fig. 28).
Fig. 28: Example of the influence of collimator resolution on the intensity of a light element
2.4.4 The Crystal Changer
The sequential spectrometer's crystal changer can hold up to 8 crystals and can be custom-equipped to suit the requirements of specific applications.
2.4.5 The Flow Counter
The flow counter is located inside the vacuum chamber and has an entrance window made of a thin, coated foil with a thickness of 0.6 µm or 0.3 µm.
The thick entrance window allows optimum measurement of the light elements Be to Na. Fig. 29 illustrates the permeability for a variety of counting tube foils that were used in the SRS 200 and 300. It can be seen from the transmission curve that the permeability of the 1-µm polypropylene foil for Na is around twice that of the 2-µm Makrofol foil.
As plastic foils have a high proportion of carbon, the absorption of nitrogen radiation close to the C absorption edge is very high. This means that even the 1 µm foil only has about 10% permeability for N. For this reason, the measured intensities for N are relatively low. The newer 0.6 µm and especially the 0.3 µm foils are more permeable to N radiation.
Generally, Ar + 10% CH4, also known as "P10 gas," is used as the counting gas. The flow of counting gas is held constant in the instrument as a fluctuating counting-gas density in the counting tube would cause fluctuations in the absorption depth as well as fluctuations in the gas amplification and thus in the position of the peaks in the pulse height spectrum too (refer to Fig. 8).
The high voltage at the counting wire is set higher for light elements (XS Multilayer crystals) than for measuring the K-radiation (LiF crystals) of medium and heavy elements. In sequential spectrometers the detector high voltage is set separately for each element range and thus for each installed crystal (see also Fig. 8).
2.4.6 The Sealed Proportional Counter
The S4 EXPLORER, S4 PIONEER and S8 TIGER utilize a sealed proportional counter, thus enabling even the very light elements (Be - Na) to be determined as effectively as with a flow counter. The basis for this is the Super High Transmission (SHT) window material.
Fig. 29: X-ray transmission for various counting-tube foils
2.4.7 The Scintillation Counter
The scintillation counter is positioned behind the flow counter outside the vacuum chamber. The radiation measured inside it must pass through the flow counter, a 0.1-mm thick vacuum-chamber sealing foil and a 0.2-mm Be entrance window. It therefore makes sense to use the scintillation counter for energies above 4.5 keV (Cr Kα1) as lower energies are absorbed mainly in the flow counter (see Fig. 9).
The scintillation counter's angle range is 4° to 110° (SRS 30x: 4° to 90°).
In the S4 EXPLORER, S4 PIONEER and S8 TIGER, the scintillation counter is directly beside the proportional counter in the spectrometer chamber and can be moved between 0° and 115°.