In diagnostic radiology, radiation is classified based on its ability to ionize matter. The most relevant types of radiation include particulate radiation (such as electrons) and electromagnetic radiation (X-rays and gamma rays). Here we will focus on the concept of particulate radiation, as well as ionizing and non-ionizing radiation.
Particulate Radiation
The only type of particulate radiation considered in diagnostic radiology is the electron. Electrons are subatomic particles with the following properties:
- Rest mass of electron: \( 9.109 \times 10^{-31} \, \text{kg} \)
- Rest energy of electron: \( 511 \, \text{keV} \) (kiloelectronvolts)
Electrons, due to their charge, play an important role in ionizing radiation interactions when accelerated in X-ray tubes or other radiology equipment.
Ionizing vs Non-Ionizing Radiation
Radiation is classified based on its ability to ionize atoms, which refers to its capacity to remove electrons from atoms. There are two main categories:
Non-Ionizing Radiation
Non-ionizing radiation cannot ionize matter, meaning it cannot remove electrons from atoms. Examples include:
- Visible light
- Infrared radiation
- Radiofrequency radiation
These types of radiation typically interact with atoms without causing them to become ionized.
Ionizing Radiation
Ionizing radiation has enough energy to remove electrons from atoms. This type of radiation can be further classified into two categories based on the nature of the interaction:
Directly Ionizing Radiation
Directly ionizing radiation consists of fast charged particles, such as electrons or protons, that deposit their energy directly in matter through numerous small Coulomb (electrostatic) interactions with orbital electrons along their tracks. Examples include:
- Alpha particles
- Beta particles (electrons)
Indirectly Ionizing Radiation
Indirectly ionizing radiation consists of photons, such as X-rays or gamma rays, and particles like neutrons. These types of radiation do not ionize atoms directly, but instead transfer their energy to fast charged particles, which then ionize atoms indirectly. Examples include:
- X-rays
- Gamma rays
- Neutrons
Ionization Potential
The minimum energy required to ionize an atom, or to remove an electron from its orbit, is known as the ionization potential. The magnitude of the ionization potential varies depending on the element or molecule. For example:
- Alkali metals: A few electronvolts (eV)
- Helium: 24.5 eV
- Water: 12.6 eV
This value is crucial because it determines the energy threshold that radiation must exceed to cause ionization in different materials, including biological tissue in the context of radiology.
Electromagnetic Radiation and Ionization
Electromagnetic radiation is classified based on its frequency and energy. Radiation with energy higher than the near-ultraviolet region of the electromagnetic spectrum is considered ionizing. This includes:
- X-rays
- Gamma rays
On the other hand, radiation with energy below the far-ultraviolet region is non-ionizing. Examples include:
- Visible light
- Infrared radiation
- Radiofrequency radiation