5.1 Interaction mechanisms of charged particles

Charged particles interact with matter through ionization, excitation, and bremsstrahlung. These processes transfer energy, creating ions and secondary electrons. Understanding these mechanisms is crucial for grasping how radiation affects materials and biological systems.

The characteristics of charged particles, like stopping power and range, determine their impact on matter. The Bragg peak, a key feature of energy deposition, has important applications in radiation therapy. These concepts form the foundation for studying radiation effects.

Energy Transfer Mechanisms

Ionization and Excitation

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• Ionization occurs when charged particles interact with atoms or molecules, causing the ejection of electrons and the formation of ions
• Requires the particle to have enough energy to overcome the binding energy of the electron
• Excitation is a similar process where the electron is raised to a higher energy state but not completely ejected from the atom or molecule
• Both ionization and excitation are the primary mechanisms for charged particles to deposit energy in matter
• Examples of ionizing radiation include alpha particles, beta particles, and heavy ions

Bremsstrahlung and Coulomb Interactions

• Bremsstrahlung, or "braking radiation," is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an atomic nucleus
• Occurs when a charged particle passes close to an atomic nucleus and is deflected by the Coulomb force, losing kinetic energy in the process
• The lost kinetic energy is converted into a photon, which is emitted as bremsstrahlung radiation
• Coulomb interactions, or electrostatic forces between charged particles, are responsible for the majority of energy loss by charged particles in matter
• These interactions can lead to ionization, excitation, and bremsstrahlung, depending on the energy and proximity of the charged particles involved

Delta Rays

• Delta rays, also known as secondary electrons, are electrons ejected from atoms or molecules with sufficient energy to cause further ionization
• Produced by the primary charged particle as it traverses through matter, creating a track of ionization and excitation events
• Delta rays can have a significant range and create their own tracks of ionization, contributing to the overall energy deposition by the primary particle
• The production of delta rays is more pronounced for high-energy, heavily charged particles such as alpha particles or heavy ions
• The presence of delta rays can lead to a more complex pattern of energy deposition and can affect the biological effectiveness of the radiation

Particle Characteristics

Stopping Power and Linear Energy Transfer (LET)

• Stopping power is a measure of the average energy loss per unit path length of a charged particle as it traverses through matter
• Depends on the particle's charge, velocity, and the properties of the material it is passing through
• Linear Energy Transfer (LET) is a related concept that describes the average energy locally imparted to the medium per unit distance traveled by the particle
• LET is an important factor in determining the biological effectiveness of radiation, as high-LET particles (such as alpha particles) cause more dense ionization tracks and are generally more damaging to biological systems than low-LET particles (such as electrons)

Range and Bragg Peak

• The range of a charged particle is the average distance it travels before coming to a complete stop in a medium
• Depends on the particle's initial energy, charge, and the properties of the material it is passing through
• The Bragg peak is a characteristic feature of the energy deposition profile of a charged particle, particularly for heavy ions
• Occurs near the end of the particle's range, where the energy loss per unit path length reaches a maximum before the particle comes to a stop
• The Bragg peak is the result of the increased stopping power as the particle slows down, leading to a concentrated region of high ionization density
• This property is exploited in radiation therapy, where heavy ions (such as carbon ions) are used to deliver a high dose to a tumor while sparing the surrounding healthy tissue