The “front end” of both types of instruments is the X-ray source. The source consists of an evacuated chamber with a heated cathode, which is usually a tungsten wire, and an anode, which is held at a potential difference of several tens of kilovolts relative to the cathode. Thermal electrons are released from the cathode and accelerated toward the anode. When the electron beam impinges upon the anode, bremsstrahlung radiation as well as X-ray lines characteristic of the anode material are emitted. These photons escape through a beryllium window built into the side of the tube.
There is no one-size-fits-all approach for source selection in XRF. X-ray tube powers may be set up at very different levels, from a fraction of a watt for EDXRF instruments with high detection efficiencies to several kilowatts for WDXRF instruments. In this latter case, the tube must be liquid-cooled since the majority of the power is dissipated as heat. The anode materials must be carefully chosen as well, since the wavelength of their characteristic lines is important for proper excitation of the sample. Some example single-element anode materials are aluminum, chromium, tungsten, palladium, or gold. For detection of light elements, a high intensity of low energy, i.e. 1-10 keV, radiation must be available, while heavy elements require excitation at higher energies up to 50 keV (Jenkins 1995: 43-47, Skoog 1998: 274). It is also important to keep in mind that the primary source of detector background will be the intense primary radiation from the tube, above which the secondary sample radiation must be detected. The use of secondary targets, or filters, can greatly reduce the background and improve sensitivity for specific portions of the spectrum.