Interactions of X-rays with Matter

When X-ray photons enter matter, they may pass through without interacting, be absorbed or be scattered. Interactions transfer energy to electrons and can produce ionisation. The type of interaction depends on photon energy and the atomic properties of the material.

Transmission occurs when a photon passes through material without interacting. It retains its original energy and direction. The proportion of transmitted photons depends on the material thickness, composition and photon energy.

Absorption occurs when an incident photon transfers all its energy to matter and disappears. Photoelectric absorption is an important example. The transferred energy can eject electrons, which subsequently deposit energy through ionisation and excitation.

Scattering changes a photon's direction of travel. In Compton scattering, the photon also loses some energy to an electron. Scattered radiation may travel away from the original beam direction and contribute to dose outside the primary beam.

Attenuation is the reduction in the intensity of the primary photon beam as it travels through matter. Photons are removed from the original beam by absorption or scattering. Attenuation depends on photon energy, material composition, density and thickness.

Photoelectric absorption occurs when a photon transfers all its energy to a bound atomic electron, usually from an inner shell. The electron is ejected and the original photon disappears. The atom is left ionised with an electron-shell vacancy.

First, an X-ray photon encounters a bound electron. Second, the photon transfers all its energy and disappears. Third, the electron is ejected as a photoelectron. Finally, an outer electron may fill the vacancy, releasing characteristic radiation or an Auger electron.

The kinetic energy of a photoelectron equals the incident photon energy minus the electron's binding energy. Formula: KE = E_photon - E_binding. Example: a 60 keV photon ejecting an electron with 10 keV binding energy produces a 50 keV photoelectron.

Photoelectric absorption is favoured by lower photon energies and materials with higher atomic numbers. Its probability generally decreases rapidly as photon energy increases, although absorption edges occur at electron-shell binding energies.

Bone contains elements with higher effective atomic numbers than soft tissue, including calcium. At diagnostic kilovoltage energies, photoelectric absorption is more likely in bone than in surrounding soft tissue. This contributes to contrast in X-ray images.

Compton scattering occurs when a photon interacts with an electron and transfers part of its energy to it. The electron is ejected as a recoil electron. The photon continues in a different direction with less energy and a longer wavelength.

First, an incoming photon interacts with an electron. Second, part of the photon energy is transferred to the electron. Third, the recoil electron is ejected. Finally, the scattered photon travels in a new direction with reduced energy.

A recoil electron is an electron ejected during Compton scattering. It carries away some of the incident photon's energy and deposits that energy through further interactions in tissue. This contributes to absorbed radiation dose.

Compton scattering depends mainly on the number of electrons per unit mass and photon energy. Per unit mass, its probability is relatively similar across many soft tissues. It is less strongly dependent on atomic number than photoelectric absorption.

Compton scattering is the predominant photon interaction in soft tissue over much of the megavoltage energy range used in external beam radiotherapy. It produces energetic secondary electrons that transfer energy to tissue, contributing to the absorbed dose.

Photoelectric absorption removes the incident photon completely and ejects a bound electron. Compton scattering transfers only part of the photon's energy to an electron, leaving a lower-energy scattered photon. Photoelectric interactions are strongly influenced by atomic number, whereas Compton interactions depend mainly on electron density.

Pair production occurs when a sufficiently energetic photon interacts with the electric field near a nucleus and is converted into an electron and a positron. The original photon disappears. This process requires a photon energy of at least 1.022 MeV.

The minimum photon energy for pair production near a nucleus is 1.022 MeV because an electron and positron each have a rest-mass energy of 0.511 MeV. Any additional available energy can become kinetic energy of the particles.

After losing kinetic energy, a positron may combine with an electron. Their mass is converted into electromagnetic radiation, usually two photons of 0.511 MeV emitted in approximately opposite directions. This is called annihilation radiation.

Pair production becomes possible above 1.022 MeV and generally becomes more important as photon energy increases. Its probability is greater in materials with higher atomic numbers. It contributes to photon interactions at sufficiently high megavoltage energies.

At relatively low photon energies, photoelectric absorption is often important, especially in high-Z materials. At intermediate and many therapeutic megavoltage energies, Compton scattering is prominent in tissue. Pair production becomes possible above 1.022 MeV and increasingly important at higher energies.

Secondary electrons are energetic electrons released by photon interactions, including photoelectric absorption, Compton scattering and pair production. These charged particles cause ionisation and excitation as they travel through tissue and are responsible for much of the local energy deposition from photon beams.

The build-up region is the area beneath the surface of tissue where absorbed dose initially increases with depth in a megavoltage photon beam. As photons penetrate, they generate secondary electrons, and the number of electrons depositing energy increases until the depth of maximum dose is reached.

dmax is the depth at which absorbed dose reaches its maximum along the central axis of a photon beam under specified conditions. It occurs because secondary-electron fluence builds up beneath the surface. Beyond dmax, dose generally decreases with increasing depth.

Skin sparing occurs because megavoltage photon beams can produce a lower absorbed dose at the skin surface than at dmax. This helps reduce superficial dose when treating deeper targets. Applying bolus can increase surface dose by effectively bringing the build-up region closer to the skin.

Preferential absorption means different materials absorb radiation to different extents. At kilovoltage energies, bone can absorb substantially more radiation than soft tissue because photoelectric interactions are more likely. Understanding these differences helps explain imaging contrast and variations in radiation dose deposition.