An Overview of XRF Basics
1. Fundamental Principles
1.3 Generating the Characteristic Radiation
The purpose of X-ray fluorescence is to determine chemical elements both qualitatively and quantitatively by measuring their characteristic radiation. To do this, the chemical elements in a sample must be caused emit X-rays. As characteristic X-rays only arise in the transition of atomic shell electrons to lower, vacant energy levels of the atom, a method must be applied that is suitable for releasing electrons from the innermost shell of an atom. This involves adding to the inner electrons amounts of energy that are higher than the energy bonding them to the atom.
This can be done in a number ways:
- Irradiation using elementary particles of sufficient energy (electrons, protons, α-particles, etc.) that transfer the energy necessary for release to the atomic shell electrons during collision processes
- Irradiation using X- or gamma rays from radionuclides
- Irradiation using X-rays from an X-ray tube
Using an X-ray tube here proves to be the technically most straightforward and, from the point of view of radiation protection, the safest solution (an X-ray tube can be switched off, a radionuclide cannot).
In an X-ray tube, electrons are accelerated in an electrical field and shot against a target material where they are decelerated. The technical means of achieving this is to apply high voltage between a heated cathode (e.g. a filament) and a suitable anode material. Electrons emanate from the heated cathode material and are accelerated towards the anode by the applied high voltage. There they strike the anode material and lose their energy through deceleration. Only a small proportion of their energy loss (approx. 1-2%, depending on the anode material) is radiated in the form of X-rays. The greatest amount of energy contributes to heating up the anode material. Consequently the anode has to be cooled which is achieved by connection to a water-cooling system.
The proportion of the electron energy loss emitted in the form of an X-ray can be between zero and the maximum energy that the electron has acquired as a result of the acceleration in the electrical field. If 30 kV (kilovolt) are applied between the anode and cathode, the electrons acquire 30 keV from passing through this voltage (kiloelectron volts) (Definition: 1 eV = the energy that an electron acquires when passing through a potential of 1 Volt).
A maximum X-ray energy of 30 keV can be acquired from deceleration in the anode material, i.e. the distribution of the energies of numerous X-rays is between zero and the maximum energy. If the intensity of this type of X-ray is applied depending on the energy, the result is the Bremsspektrum (= continuum) of the tube.
Fig. 3: A Bremsspektrum (= continuum) with characteristic radiation of the anode material
In addition to the Bremsspektrum, an X-ray tube of course emits the characteristic radiation of the anode material as well, which is of major importance for X-ray fluorescence analysis (Fig. 3).
All X-ray tubes work on the same principle: accelerating electrons in an electrical field and decelerating them in a suitable anode material. The region of the electron beam in which this takes place must be evacuated in order to prevent collisions with gas molecules. Hence there is a vacuum within the housing. The X-rays escape from the housing at a special point that is particularly transparent with a thin beryllium window.
The main differences between tube types are in the polarity of the anode and cathode and the arrangement of the exit window. The two most significant types are the end-window tubes and the side-window tubes.
In side-window tubes, a negative high voltage is applied to the cathode. The electrons emanate from the heated cathode and are accelerated in the direction of the anode. The anode is set on zero voltage and thus has no difference in potential to the surrounding housing material and the laterally mounted beryllium exit window (Fig. 4).
Fig. 4: The principle of the side-window tube
For physical reasons, a proportion of the electrons are always scattered on the surface of the anode. The extent to which these backscattering electrons arise depends, among other factors, on the anode material and can be as much as 40%. In the side-window tube, these backscattering electrons contribute to the heating up of the surrounding material, especially the exit window. As a consequence, the exit window must withstand high levels of thermal stress and cannot be selected with just any thickness. The minimum usable thickness of a beryllium window for side-window tubes is 300 µm. This causes an excessively high absorption of the low-energy characteristic L radiation of the anode material in the exit window and thus a restriction of the excitation of lighter elements in a sample.
The distinguishing feature of the end-window tubes is that the anode has a positive high voltage and the beryllium exit window is located on the front end of the housing (Fig. 5).
Fig. 5: The principle of the end-window tube
The cathode is set around the anode in a ring (annular cathode) and is set at zero voltage. The electrons emanate from the heated cathode and are accelerated towards the electrical field lines on the anode. Due to the fact that there is a difference in potential between the positively charged anode and the surrounding material, including the beryllium window, the backscattering electrons are guided back to the anode and thus do not contribute to the rise in the exit window's temperature. The beryllium window remains "cold" and can therefore be thinner than in side-window tubes. Windows are used with a thickness of 125 µm and 75 µm. This provides a prerequisite for exciting light elements with the characteristic L radiation of the anode material (e.g. rhodium).
Due to the high voltage applied, non-conductive, deionized water must be used for cooling. Instruments with end-window tubes are therefore equipped with a closed, internal circulation system containing deionized water that cools the tube head as well.
End-window tubes have been implemented by all renowned manufacturers of wavelength dispersive X-ray fluorescence spectrometers since the early 1980's.
Current and high voltage for the X-ray tubes as well as the heating current for the cathode are produced in a so-called X-ray generator. The generators available today supply a maximum tube current of 170 mA and a maximum high voltage of 60 kV at a maximum output of 4 kW, i.e. current and voltage must be selected in such a way that 4 kW is not exceeded. The architecture of modern control electronics and software ensures that damage to the tube resulting from maladjustment is impossible. The reason for restricting the maximum excitation power to 1 kW is so that cooling with external coolant can be eliminated, which simplifies installation requirements.