Classification of Ionizing Radiation

Directly Ionizing Radiation

Directly ionizing radiation consists of charged particles such as:

• Electrons (e–)
• Protons (p⁺)
• Alpha particles (α)
• Heavy ions (e.g., carbon, oxygen)

These particles deposit energy in the absorber through direct Coulomb interactions with the orbital electrons of atoms. The charged particles interact directly with the electrons, knocking them out of their atomic orbits. This results in ionization and the formation of ions and free radicals, which can lead to tissue damage. Examples of applications of directly ionizing radiation include:

• Alpha radiation in smoke detectors
• Beta radiation in radiotherapy
• Proton therapy for cancer treatment

Indirectly Ionizing Radiation

Indirectly ionizing radiation consists of neutral (uncharged) particles, such as:

• Gamma rays (γ)
• X-rays
• Neutrons (n)

These particles do not directly interact with electrons. Instead, they first produce secondary charged particles within the absorber. For example, gamma rays and X-rays can ionize atoms indirectly by interacting with electrons, leading to the production of energetic secondary particles like electrons (called secondary electrons). These secondary electrons then go on to ionize the medium through Coulomb interactions. Some common uses of indirectly ionizing radiation include:

• X-ray imaging in medicine
• Gamma radiation in cancer therapy
• Neutron radiation in nuclear reactors

Alpha Particles (α)

Alpha particles consist of two protons and two neutrons. Due to their large mass and positive charge, they have a low penetration power and can be stopped by a sheet of paper or human skin. However, they are highly ionizing, depositing a large amount of energy over a short distance. The energy of an alpha particle is typically between 4 and 9 MeV (Mega-electron Volts). For example, if an alpha particle has an energy of 5 MeV, the energy can be calculated as:

E = 5 MeV = 5 × 106 eV 
= 5 × 106 × 1.602 × 10–19 J
 = 8.01 × 10–13 J

Alpha particles are commonly used in radiation therapy and for smoke detection (e.g., Americium-241 in smoke detectors).

Beta Particles (β)

Beta particles are high-energy, high-speed electrons (β^–) or positrons (β⁺) emitted from radioactive nuclei. Beta particles are more penetrating than alpha particles but are still blocked by materials like plastic or glass. They typically have energies in the range of 0.1 to 10 MeV. For example, for a beta particle with an energy of 1 MeV:

E = 1 MeV = 1 × 106 eV
 = 1 × 106 × 1.602 × 10–19 J 
= 1.602 × 10–13 J

Beta radiation is used in medical imaging and radiotherapy, as well as in industrial applications for thickness gauging.

Gamma Rays (γ)

Gamma rays are electromagnetic radiation (photons) emitted from the nucleus of radioactive atoms. They have very high energy and very short wavelengths, typically ranging from 10^–12 m to 10^–10 m. Gamma rays can penetrate deep into materials and are often used in medical imaging, such as in PET (Positron Emission Tomography) scans and cancer treatment. For example, if the energy of a gamma photon is 3 MeV:

E = 3 MeV = 3 × 106 eV 
= 3 × 106 × 1.602 × 10–19 J
 = 4.806 × 10–13 J

Gamma rays are also used in sterilization and food irradiation processes.

Neutrons (n)

Neutron radiation consists of neutral particles (neutrons) that can ionize matter indirectly by transferring energy to atomic nuclei, which then produce secondary charged particles. Neutrons are highly penetrating and are often used in nuclear reactors. The energy of thermal neutrons is typically around 0.025 eV. For fast neutrons, energies can range from 1 MeV to several GeV (giga-electron volts). For a fast neutron with an energy of 10 MeV:

E = 10 MeV = 10 × 106 eV 
= 10 × 106 × 1.602 × 10–19 J 
= 1.602 × 10–12 J

Neutron radiation is used in nuclear reactors and in neutron activation analysis.