Ionizing Photon Interactions
Photons, being uncharged particles, interact indirectly with matter by transferring their energy to nearby particles, most often electrons. Photons in the ionizing energy range interact with matter in 5 main ways:
- Rayleigh scattering
- Photoelectric effect
- Compton scattering
- Pair production
- Photonuclear disintegration
Before describing each interaction in detail, some terms need to be defined for clarity. As mentioned in Introduction to Ionizing Radiation, photon interactions are stochastic, meaning that there is a probability that a photon will not interact with matter and pass through unaffected. These interaction probabilities are quantified using attenuation coefficients, defined as the interaction probability per unit length (e.g. units of cm-1) of matter. Attenuation interactions consist of both absorption interactions and scattering interactions, and are closely related to nuclear physics cross sections (which have units of cm2). We will encounter three main types of attenuation coefficients regarding photons:
- Linear attenuation coefficient: The probability that a photon will be attenuated (absorbed or scattered) by matter
- Energy transfer attenuation coefficient: The probability that photon energy will be transferred to matter; this is often calculated as the linear attenuation coefficient multiplied by the enery transferred per interaction
- Energy absorption attenuation coefficient: The probability that photon energy will be absorbed in matter; not all of the transferred energy will be absorbed, depending on secondary, direct interactions (e.g. electrons)
An individual interaction (e.g. Rayleigh scattering) will have it's own attenuation coefficients, but these attenuation coefficients may also be added together to form total attenuation coefficients, as we will see later. As a final note, attenuation coefficients can also be divided by the material density, and are then referred to as mass attenuation coefficients (with units of cm2/g).
In the following tutorial pages, brief descriptions of all five interactions will be given, along with some quantitative information regarding the kinematics and probabilities of these interactions.
Related Links
- Next Tutorial: Rayleigh Scattering
- Previous Tutorial: Introduction to Ionizing Radiation
- Lab: Compton Scattering Equation Derivation
References and Further Reading
- Introduction to Radiological Physics and Radiation Dosimetry. Chapter 7
- AAPM Summer School 2009. Chapter 2