An Overview of XRF Basics
1. Fundamental Principles
1.6 X-ray Detectors
1.6.1 Pulse Height Spectrum
When measuring X-rays, use is made of their ability to ionize atoms and molecules, i.e. to displace electrons from their bonds by energy transference. In suitable detector materials, pulses whose strengths are proportional to the energy of the respective X-ray quanta are produced by the effect of X-rays. The information about the X-ray quanta's energy is contained in the registration of the pulse height. The number of X-ray quanta per unit of time, e.g. pulses per second (cps = counts per second, kcps = kilocounts per second), is called their intensity and contains in a first approximation the information about the concentration of the emitting elements in the sample. Two main types of detectors are used in wavelength dispersive X-ray fluorescence spectrometers: the gas proportional counter and the scintillation counter.
The way these quantum counters function is described below.
1.6.2 Gas Proportional Counter
The gas proportional counter comprises a cylindrical metallic tube in the middle of which a thin wire (counting wire) is mounted. This tube is filled with a suitable gas (e.g. Ar + 10% CH4). A positive high voltage (+U) is applied to the wire. The tube has a lateral aperture or window that is sealed with a material permeable to X-ray quanta (Fig. 8).
Fig. 8: A gas proportional counter
An X-ray quantum penetrates the window into the counter's gas chamber where it is absorbed by ionizing the gas atoms and molecules. The resultant positive ions move to the cathode (the metallic tube), the free electrons to the anode (the wire). The number of electron-ion pairs created is proportional to the energy of the X-ray quantum. To produce an electron-ion pair, approx. 0.03 keV are necessary, i.e. the radiation of the element boron (0.185 keV) produces approx. 6 pairs and the K-alpha radiation of molybdenum (17.5 keV) produces approx. 583 pairs. Due to the cylinder-geometry arrangement, the primary electrons created in this way "see" an increasing electrical field on route to the wire. The high voltage in the counting tube is now set so high that the electrons can obtain enough energy from the electrical field in the vicinity of the wire to ionize additional gas particles. An individual electron can thus create up to 10,000 secondary electron-ion pairs.
The secondary ions moving towards the cathode produce a measurable signal. Without this process of gas amplification, signals from boron, for example, with 6 or molybdenum with 583 pairs of charges would not be able to be measured, as they would not be sufficiently discernible from the electronic "noise." Gas amplification is adjustable via high voltage in the counting tube and is set higher for measuring boron than for measuring molybdenum. The subsequent pulse electronics supply pulses of voltage whose height depends, among other factors, on the energy of the X-ray quanta.
There are two models of gas proportional counters: the flow counter (FC) and the sealed proportional counter (PC). The flow counter is connected to a continuous supply of counting gas (Ar + 10% CH4) and has the advantage of being able to be equiped with a very thin window (< 0.6 µm). The FC is therefore also suitable for measuring the very light elements and is very stable. The proportional counter, on the other hand, has a closed volume and requires a thick window normally made of beryllium. The absorption in this "thick" beryllium window prevents the measurement of the very light elements (Be to Na).
Since innovative, highly transparent organic materials have been in use, there has now been success in developing sealed proportional counters that are just as sensitive to the very light elements (Be to Na) as flow counters are.
1.6.3 Scintillation Counters
The scintillation counter, "SC," used in XRF comprises a sodium iodide crystal in which thallium atoms are homogeneously distributed "NaI(Tl)." The density of the crystal is sufficiently high to absorb all the XRF high-energy quanta. The energy of the pervading X-ray quanta is transferred step by step to the crystal atoms that then radiate light and cumulatively produce a flash. The amount of light in this scintillation flash is proportional to the energy that the X-ray quantum has passed to the crystal. The resulting light strikes a photocathode from which electrons can be detached very easily. These electrons are accelerated in a photomultiplier and, within an arrangement of dynodes, produce secondary electrons giving a measurable signal once they have become a veritable "avalanche" (Fig. 9). The height of the pulse of voltage produced is, as in the case of the gas proportional counter, proportional to the energy of the detected X-ray quantum.
