TABLE 1

Summary of X-Ray–γ-Ray Interactions

ProcessInteractionZ, E, ρ effectsComments
Photoelectric absorptionPhoton energy > electron binding energy, photon absorbed, electron ejected from shell with kinetic energy equal to Ephoton − EBEτ ∝ Z3/E3Atom is ionized; high imparted energy; characteristic radiation is be released; generates maximum differential signal
Rayleigh scatteringPhoton interacts with bound atomic electron without ionization; photon is released in different direction without loss of energyσR ∝ 1/E1.2No energy absorption occurs; photons mainly scattered in forward direction
Compton scatteringPhoton interacts with “free” electron, ionizes atom; energy of incident photon shared with scattered photon and recoil electronσ ∝ ρ σ ∝ E0* σ ∝ 1/E†Displaced electron energy is absorbed locally; interaction produces attenuation and partial absorption
Pair productionPhoton energy > 1.02 MeV interacts with nucleus and conversion of energy to e−–e+ charged particles; e+ subsequently annihilates into two 511-keV photonsπ ∝ (E − 1.02 MeV) × ZProbability of interaction increases with increasing energy, unlike other processes

  • ↵* Within diagnostic x-ray energy range of 10–100 keV.

  • ↵† At energies > 100 keV.

  • EBE = electronic binding energy.