Infographics / X-ray Physics

Photoelectric vs Compton dominance

Two panels comparing X-ray photon interactions. Left: photoelectric absorption, where an incident photon is fully absorbed by a tightly bound inner-shell electron, ejecting a photoelectron with kinetic energy hv minus binding energy; probability scales as Z cubed over E cubed and dominates at low energy and high atomic number, leaving no scattered photon so subject contrast is preserved. Right: Compton scatter, where an incident photon scatters off a loosely bound outer electron, loses part of its energy and continues as a lower-energy scattered photon while a recoil electron is ejected; probability scales with electron density and is roughly independent of atomic number, dominates at higher energy, and the scatter degrades image contrast.

Photoelectric absorption versus Compton scatter Two panels comparing X-ray photon interactions. Left: photoelectric absorption, where an incident photon is fully absorbed by a tightly bound inner-shell electron, ejecting a photoelectron with kinetic energy hv minus binding energy; probability scales as Z cubed over E cubed and dominates at low energy and high atomic number, leaving no scattered photon so subject contrast is preserved. Right: Compton scatter, where an incident photon scatters off a loosely bound outer electron, loses part of its energy and continues as a lower-energy scattered photon while a recoil electron is ejected; probability scales with electron density and is roughly independent of atomic number, dominates at higher energy, and the scatter degrades image contrast. Photoelectric absorption complete absorption of the photon e⁻ incident photon photoelectron KE = hν − Eₖ • Photon fully absorbed by a tightly bound inner-shell (K) electron • Probability ∝ Z³ / E³ • Dominant at low energy and high Z (bone, iodine, near K-edges) • No scattered photon, so subject contrast is preserved Compton scatter scatter with partial energy loss e⁻ incident photon scattered photon (reduced energy) recoil electron Δλ = (h/mₑc)(1 − cosθ) • Photon scatters off a loose (≈ free) outer electron, losing part of its energy • Probability ∝ electron density (≈ independent of Z) • Dominant at higher energy (low-Z soft tissue, air) • Scattered photons degrade image contrast
Practise X-ray PhysicsDownload SVG

More in X-ray Physics

Practice the Physics paper

FRCRX is a Part 1 Physics question bank: true/false items with explanations, timed mocks and topic analytics.

Start question practiceSee plans