How are characteristic X-rays produced by atoms?

Characteristic X-rays are produced through atomic transitions that occur when an inner-shell electron is removed from an atom. This vacancy can be created by bombarding the atom with high-energy particles (such as electrons or protons) or by irradiating it with a primary X-ray beam.12

Once the inner-shell vacancy is created, the atom is in an unstable, excited state. To reach a more stable configuration, an electron from a higher-energy outer shell quickly transitions to fill the inner-shell vacancy. As this electron moves to a lower energy level, the atom emits a photon with an energy exactly equal to the difference between the two energy states. Because these energy levels are specific to the structure of each element, the resulting X-rays have definite wavelengths that are characteristic of the substance.123

This process is distinct from the continuous bremsstrahlung radiation produced by decelerating electrons in an X-ray tube, and it is widely utilized in X-ray fluorescence to identify the elemental composition of materials.12