2.1 Interaction of Charged Particles
Understanding how charged particles lose energy and travel through matter is crucial for detector design and radiation protection.
The Primary Mechanism: Coulomb Interaction
Charged particles, such as alpha particles and beta particles (electrons or positrons), interact with matter primarily through the **Coulomb force**. As a charged particle travels through a material, its electric field interacts with the electric fields of the atoms in the material.
These interactions are not like a direct collision; instead, they are a series of many small, short-range electromagnetic interactions with the electrons and nuclei of the medium. These interactions transfer energy from the charged particle to the atoms of the material, causing two main effects:
- Excitation: The passing charged particle transfers a small amount of energy to an atomic electron, raising it to a higher energy state.
- Ionization: The passing charged particle transfers enough energy to an atomic electron to completely remove it from the atom, creating an ion pair (a free electron and a positively charged atom). This is the most common and important interaction for radiation detection.
Stopping Power and Range
The rate at which a charged particle loses energy as it travels is called **stopping power** (\(S\)), or linear energy transfer (LET). It is dependent on the charge, mass, and velocity of the particle, as well as the properties of the medium it is traveling through.
The **range** of a charged particle is the average distance it travels in a material before it loses all of its kinetic energy.
- Heavy Charged Particles (e.g., Alpha Particles): Alpha particles are heavy and highly charged, so they lose energy rapidly in a straight path. This gives them a short, well-defined range in matter.
- Light Charged Particles (e.g., Beta Particles): Beta particles are much lighter and have less charge. They lose energy more slowly and undergo significant scattering, leading to a longer, less-defined path and a more varied range in matter.