Which of the following refers to the interaction when an x-ray photon collides with an atom and causes ionization?

The photoelectric effect refers specifically to the scenario in which an x-ray photon collides with an atom and transfers all its energy to an inner-shell electron, resulting in the ejection of that electron from the atom. This process not only leads to the ionization of the atom but also creates a vacancy that may cause other electrons from higher energy levels to fill that vacancy, subsequently leading to the emission of secondary radiation.

In contrast, Compton scattering involves the interaction of x-ray photons with outer-shell electrons, resulting in partial energy transfer rather than complete absorption, which does not always result in ionization. Characteristic radiation occurs when an atom transitions to a lower energy state after an inner-shell electron is ejected, while Bremsstrahlung radiation is related to the deceleration of electrons in a magnetic field, which does not involve direct photon-atom interactions. Thus, the photoelectric effect is the direct interaction that involves the collision of an x-ray photon causing ionization, making it the correct answer.