A characteristic photon's energy depends on which of the following factors?

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

Answer: (C)

Characteristic X-ray energies come from the atom’s electron structure. When a high-energy electron ejects a tightly bound inner-shell electron, a higher-shell electron drops to fill the vacancy, and the emitted photon has energy equal to the difference between those two shell binding energies. That energy difference—set by the binding energies of the orbital electrons in the target atom—determines the characteristic photon energy.

So the energy depends on the binding energies of the orbital electrons (and thus on the element’s atomic structure). The incident electron’s energy only needs to be above the inner-shell binding energy to produce the line; once above threshold, increasing it doesn’t change the photon energy. Filtration material’s atomic number affects attenuation, not the emitted photon energy, and neutrons don’t affect electron binding energies.