The most useful characteristic x-ray photons are produced when incident electrons interact with which of the following?

• A. Outer-shell electrons
• B. Inner-shell electrons
• C. Atomic nuclei
• D. an entire atom

Answer: (B)

Characteristic x-ray photons come from transitions that occur after a vacancy is created in an inner shell of an atom. When a fast incident electron ejects an inner-shell electron, a vacancy is left in that shell. An electron from a higher shell then drops into the vacancy, and the energy difference between the two shells is emitted as a photon with a specific energy that is unique to the element. This inner-shell ionization and subsequent transition produce the sharp, element-specific lines known as characteristic x-rays. If the interaction only removes outer-shell electrons, the resulting photons are not the same sharp, element-specific lines and may be weaker or different in character, and other processes like bremsstrahlung or Auger emission can dominate. So, inner-shell interactions are what produce the useful characteristic x-ray photons.