In the production of characteristic X-rays, the cascade process refers to which phenomenon?

• A. electron transfer from the outer shell to the inner shell
• B. interaction of the incident electron with the electric field of the nucleus
• C. ejection of an inner shell electron
• D. deflection of the incident electron

Answer: (A)

When a vacancy is created in an inner shell, electrons from higher shells drop into that vacancy, releasing energy as X-ray photons. If the drop to fill one vacancy creates another vacancy in a higher shell, that vacancy can be filled again, producing additional X-ray photons in a chain—the cascade. This is why the phenomenon described is the transfer of an electron from an outer shell to an inner shell to fill the vacancy, with successive transitions possibly emitting more X-rays. The initial event that creates the inner-shell vacancy is not the cascade itself, and other processes like deflecting the incoming electron or interacting with the nucleus are separate aspects of how the beam interacts with matter.