A characteristic photon's energy depends on which variable?

• A. Atomic number of filtration material
• B. Energy of the incident electron
• C. Number of neutrons in the nucleus
• D. Binding energy of orbital electrons

Answer: (D)

Characteristic X-ray photons come from electrons dropping between atomic shells. The energy of the emitted photon is the difference in binding energies between the two shells involved in the transition (roughly the inner-shell binding energy minus the energy of the next shell). Because those binding energies are fixed by the atom’s electronic structure, the photon’s energy is a specific property of the element. The energy of the incident electron only affects whether such a vacancy is created and how often, not the actual photon energy. Neutron count and, for the most part, filtration material do not set the discrete photon energy.