Which of the following interactions involves the energy of X-rays being completely absorbed by matter?

• A. Photoelectric effect
• B. Compton effect
• C. Classical scatter
• D. Photodisintegration effect

Answer: (A)

The photoelectric effect occurs when X-rays interact with matter in such a way that their energy is completely absorbed by an inner-shell electron of an atom. This interaction results in the ejection of the electron from its shell, leading to ionization of the atom. The energy of the incident X-ray photon is transferred entirely to the electron, allowing it to overcome the binding energy that holds it in the atom.

This process is significant in bone densitometry because it is the primary mechanism through which X-rays are absorbed in denser tissues, such as bone. Because denser tissues absorb more X-rays, understanding the photoelectric effect is crucial for interpreting bone density measurements accurately.

In contrast, other interactions such as the Compton effect involve partial energy transfer from the X-ray photon to a loosely bound outer-shell electron, resulting in both the scattering of the photon and ionization of the atom, but the photon is not completely absorbed. Classical scatter involves no ionization and results in no energy absorption, merely a change in the direction of the X-ray photon. The photodisintegration effect involves the absorption of high-energy photons by the nucleus of an atom, but this is not common in the diagnostic X-ray range typically used in bone densitometry.