Which interaction's cross-section increases with higher atomic number, making bone more likely to absorb?

Which interaction's cross-section increases with higher atomic number, making bone more likely to absorb?

• A. Photoelectric absorption
• B. Compton scattering
• C. Coherent scattering
• D. Pair production

Answer: (A)

The key idea is how X-ray interaction probabilities depend on atomic number and photon energy. The photoelectric absorption cross-section rises very steeply with atomic number (roughly proportional to Z^4–Z^5) and decreases strongly with increasing photon energy (about E^-3). Because bone contains calcium and phosphorus, its effective atomic number is higher than that of surrounding soft tissue, so photons in diagnostic X-ray energies are much more likely to be absorbed by the photoelectric process in bone than in soft tissue. This makes bone appear more opaque on radiographs. Compton scattering primarily follows electron density and has only a weak dependence on Z, so it doesn’t increase with higher atomic number as strongly. Coherent scattering also grows with Z but is a relatively small contributor at diagnostic energies. Pair production only becomes significant at energies above about 1 MeV and also grows with Z, but it’s not a major factor in typical diagnostic radiology. Thus, the best answer is photoelectric absorption.

The key idea is how X-ray interaction probabilities depend on atomic number and photon energy. The photoelectric absorption cross-section rises very steeply with atomic number (roughly proportional to Z^4–Z^5) and decreases strongly with increasing photon energy (about E^-3). Because bone contains calcium and phosphorus, its effective atomic number is higher than that of surrounding soft tissue, so photons in diagnostic X-ray energies are much more likely to be absorbed by the photoelectric process in bone than in soft tissue. This makes bone appear more opaque on radiographs.

Compton scattering primarily follows electron density and has only a weak dependence on Z, so it doesn’t increase with higher atomic number as strongly. Coherent scattering also grows with Z but is a relatively small contributor at diagnostic energies. Pair production only becomes significant at energies above about 1 MeV and also grows with Z, but it’s not a major factor in typical diagnostic radiology. Thus, the best answer is photoelectric absorption.