Characteristic X-ray energies originate from the energy difference between the shells involved in the transition.

• A. Energy difference between the K and L shells
• B. Nuclear transition energy
• C. Energy difference between the shells involved in the transition
• D. Kinetic energy of the incident electron

Answer: (C)

Characteristic X-ray energies come from the energy difference between the two atomic shells involved in the electron transition. When an inner-shell vacancy is created, an electron from a higher shell drops into the lower, and the energy released equals the binding energy of the higher shell minus that of the lower one. This energy appears as a photon with a discrete, characteristic energy. The exact energy depends on which shells participate (for example, a transition from the K shell to L, or from K to M, and so on). It is not set by a nuclear transition (that would produce gamma rays) and it isn’t simply the kinetic energy of the incident electron, though the incident energy must be sufficient to ionize the atom in the first place.