3.1.2.1 Direct excitation

The excitation using x-ray tube emission implies using a polychromatic beam. The probability distribution of the x-ray tube emission varies with the applied high voltage. The larger the applied high voltage, the higher is the maximum energy of the photons and more intense is the x-ray production. The number of emitted photons can be proportionally enlarged if the tube anode current is increased. If the tube is operated at lower high voltage to excite low energy characteristic emission, the current can be significantly increased and the sensitivity improved.

Each tube has a maximum electrical power of operation and the anode current must be chosen as not to exceed such value. Another important precaution to extend the time of use of the x-ray tube is to lower the current to the minimum possible value during pauses in operation.

All the x-ray photons in the excitation beam with energies larger than that of the absorption edge of the element which concentration needs to be determined do contribute to its excitation. On the contrary, the photons with energies lower than that of the absorption edge only suffer scatter in the sample, thus contributing to the increase the continuum background (main noise) in the x-ray fluorescence measured spectrum. To improve the signal to noise ratio different strategies are followed to modify the excitation spectrum.