What is the photoelectric effect in relation to x-rays?

• A. Compton scattering
• B. Pair production
• C. Free electron emission from metal
• D. A photon interacts with an inner shell electron of the atom and removes it from its shell

Answer: (D)

The photoelectric effect happens when an x-ray photon is fully absorbed by an atom and transfers all its energy to a bound electron, ejecting that electron from its shell. In x-ray interactions this typically involves an inner-shell electron, and the photon must have energy at least equal to that shell’s binding energy. Any extra energy becomes the kinetic energy of the ejected electron. The likelihood of this process is higher in heavier (high-Z) atoms and decreases rapidly as photon energy increases, roughly following a Z dependence and an E^−3 trend. Because inner-shell vacancies are created, electrons from higher shells can drop in, emitting characteristic x-rays—this contributes to absorption and image contrast in radiography. Other options describe different interactions: Compton scattering involves a scattered photon and recoil electron, not ejection from a bound shell; pair production requires energies above 1.022 MeV and creates particle–antiparticle pairs; and the described scenario—removing an inner-shell electron with a photon—is precisely the photoelectric effect.