Effects Following Photon Interactions

1. Effects of Photon Interactions with Absorber

When photons interact with matter, especially in medical diagnostic and therapeutic contexts, several processes occur that lead to secondary radiation effects. These secondary effects play a critical role in how photons interact with tissues and how energy is deposited. The most common interactions in the diagnostic and megavoltage ranges are:

• Photoelectric Effect: Ejection of inner-shell electrons creates vacancies that are filled by electrons from outer shells. This process can result in characteristic X-rays and Auger electrons.
• Compton Effect: Similar to the photoelectric effect, vacancies are produced in the absorber’s atomic shells as a result of the ejection of orbital electrons. The secondary electrons can produce characteristic radiation.
• Triplet Production: This occurs in the context of high-energy photons interacting with matter, where vacancies in electron shells are created and can lead to subsequent radiation effects.

2. Vacancy Creation and Secondary Radiation

When a photon interacts with an atom in the absorber, particularly in the diagnostic and megavoltage energy ranges, the most significant effect is the creation of electron vacancies in the atomic shells. These vacancies can be filled by electrons from higher energy levels, which leads to the emission of secondary radiation. The two key types of secondary radiation are:

• Characteristic Radiation: This type of radiation occurs when an outer-shell electron fills an inner-shell vacancy, releasing energy in the form of an X-ray. The energy of the emitted photon corresponds to the difference in binding energies between the two involved shells.
• Auger Electrons: In cases where the vacancy created by the photon interaction is filled by an electron from an outer shell, the excess energy can be transferred to a third electron, which is ejected from the atom. This ejected electron is called an Auger electron.

3. Pair Production and Annihilation

At photon energies greater than 1.022 MeV, pair production can occur. In this process, the energy of the incident photon is converted into a particle-antiparticle pair—an electron and a positron. After the pair is created, the positron typically undergoes annihilation with an electron, producing two photons (annihilation quanta), each with energy 0.511 MeV. These photons are emitted in opposite directions (at 180°) to conserve energy and momentum.

This phenomenon is significant in high-energy photon interactions, especially in radiation therapy, where positron emission tomography (PET) exploits the annihilation process to provide detailed imaging.

4. Summary of Photon Interactions

The five primary photon interactions of relevance in medical physics are:

• Photoelectric Effect: Predominates at low photon energies and results in the creation of vacancies in atomic shells, leading to characteristic X-rays and Auger electron emission.
• Compton Effect: Occurs at intermediate photon energies, with secondary radiation being produced through ejected recoil electrons. Characteristic X-rays can also be generated depending on the atomic number of the absorber.
• Pair Production: Takes place at high photon energies (above 1.022 MeV), producing electron-positron pairs followed by annihilation and the emission of two 0.511 MeV photons.
• Triplet Production: Similar to pair production but with a third electron involved, creating a triplet of particles.

These interactions form the foundation for many radiation-based diagnostic and therapeutic techniques, such as X-ray imaging, CT scans, and radiation therapy, where understanding the resultant secondary radiation is crucial for both effective treatment and minimizing unwanted radiation exposure.