Characteristic X-ray energies are determined by which of the following?

• A. Sum of binding energies
• B. Energy difference between two shells
• C. Nuclear binding energy
• D. Kinetic energy of incident electron

Answer: (B)

The energy of characteristic X-rays comes from a transition between bound electron shells. When an inner-shell electron is removed, an electron from a higher shell drops into that vacancy, and the emitted photon has energy equal to the difference between the binding energies of the two shells involved. This energy difference is specific to the atom, so the characteristic X-ray lines are unique to each element.

The sum of binding energies isn’t what’s emitted in a single transition, and the nuclear binding energy governs the nucleus, not these electronic transitions. The incident electron’s kinetic energy must be high enough to eject the inner shell, but the photon energy produced is determined by the shell energies themselves, not by the incident electron’s energy.