5. Secondary fluorescence

This phenomenon can occur during X-ray emission analysis when a line is emitted very intensely by the sample under the beam. On their way out of the sample, the X-ray photons may in turn interact with the target itself, ionizing new atoms (photo-ionization). The resulting emission is known as secondary fluorescence. Only layers with a critical absorption threshold below the energy of the primary X line can be ionized, which means that only target atoms with an atomic number Z lower than that of the primary emitting atom can induce this phenomenon. Moreover, the probability of photo-ionization is greater the smaller the energy difference between the primary line and the target atom's critical absorption threshold. Secondary fluorescence therefore appears in K-to-K excitation for primary-secondary emitting elements very close in Z, and in L-to-K excitation for heavy primary and light secondary...