X-rays interact with matter mainly by transferring their energy to atoms or electrons, which can cause ionization or excitation. The way they interact depends on their energy and the atomic composition of the material.
Here are the main interaction mechanisms:
- Photoelectric Effect
- An X-ray photon collides with an atom’s inner-shell electron, transferring all its energy to eject the electron.
- More likely at lower X-ray energies and in materials with high atomic numbers (e.g., bone, lead).
- Responsible for high contrast in X-ray images (bones absorb more than soft tissue).
- Compton Scattering
- The X-ray photon collides with an outer-shell electron, loses part of its energy, and changes direction.
- Occurs more at medium energies (typical medical X-rays).
- Causes image blurring and scattered radiation.
- Pair Production (very high energies, >1.022 MeV)
- The X-ray photon’s energy is converted into an electron–positron pair near the nucleus.
- Not relevant in standard medical imaging but important in high-energy physics and radiation therapy.
- Coherent (Rayleigh) Scattering
- The X-ray photon interacts with the atom as a whole and changes direction without losing energy.
- Occurs at low energies; minor contribution in medical imaging.
Absorption vs. Transmission
- Dense materials (bone, lead) → absorb more X-rays.
- Less dense materials (skin, muscles) → allow more X-rays to pass through.