Which type of X-ray is produced when outer shell electrons drop to fill a lower shell vacancy?

Characteristic X-rays are produced when an outer shell electron transitions to fill a vacancy in a lower energy shell, releasing energy in the form of X-rays. This process occurs after inner shell electrons are ejected from an atom, often due to high-energy interactions such as those from incoming radiation. When the outer shell electron moves in to take the place of the ejected electron, it loses energy equivalent to the difference in binding energy between the orbits, and this energy is emitted as an X-ray photon.

The key point here is that characteristic X-rays are specific to each element, as the energy levels of the electron shells are unique to the atomic structure of that element. This leads to the production of X-rays with specific energies that can be identified and used for elemental analysis.

In contrast, secondary X-rays typically refer to X-rays produced from materials that have been irradiated and may not necessarily correspond to the characteristic transitions within a specific atom. Bremstrahlung X-rays (or Bremsstrahlung) are produced when charged particles, such as electrons, are decelerated in the vicinity of atomic nuclei, which is a different mechanism of X-ray production. Continuum X-rays refer to the broad spectrum of X-rays that can be produced from any energetic processes