Which of the following results in the production of characteristic x-ray photons in an x-ray tube?

Characteristic x-ray photons are produced when electrons from the electron beam strike the target material within the x-ray tube and cause inner-shell electrons of the target atoms to be ejected. When an inner-shell electron is removed, the atom becomes ionized and unstable. To regain stability, an electron from a higher energy level (outer shell) falls into the vacancy left by the ejected electron. This transition releases energy in the form of x-ray photons. The energy released corresponds to the difference in binding energy between the two electron shells involved.

The concept of released binding energy is critical here because it directly relates to why characteristic radiation is produced: the energy difference between the higher (outer shell) and lower (inner shell) energy states results in the emission of photons. The exact energy of the emitted photons is characteristic of the target material used in the x-ray tube, hence the name "characteristic x-rays."

In contrast, other options do not directly lead to characteristic photon production. For instance, while decelerating electrons and excitation of outer-shell electrons can produce x-rays or other types of emissions, they do not specifically result in characteristic x-ray production in the way the release of binding energy does. Nuclear radioactivity is a different process altogether and does not