Explain their relative ionising effects with reference to: (a) kinetic energy (b) electric charge

5.2.2 The Three Types of Nuclear Emission ⚛️

In this unit we look at the three main kinds of radiation that a radioactive nucleus can give off: α‑particles, β‑particles and γ‑rays. Each has a different mass, charge and kinetic energy, which determine how strongly they can ionise matter and how far they can travel through the body or a material. Understanding these differences is key for both safety and exam success.

α‑Particles (Helium Nuclei) 💥

An α‑particle consists of 2 protons and 2 neutrons – essentially a tiny helium nucleus. Because it carries a +2 charge, it interacts strongly with electrons in matter, pulling them away and creating ion pairs. Its kinetic energy (≈4–9 MeV) is high, but the heavy mass means it loses energy very quickly. Think of it like a heavy bowling ball that stops after a few steps on a carpet. Consequently, α‑particles have a very short range (a few centimetres of air) and are stopped by a sheet of paper or even the outer skin. However, if inhaled or ingested, they can cause severe damage because they deposit all their energy in a tiny volume.

Ionisation comparison: The ionisation rate for an α‑particle is roughly 10 times that of a β‑particle of the same energy, because the charge is twice as large and the mass is four times larger, leading to a higher stopping power (dE/dx).

β‑Particles (Electrons or Positrons) 📚

β‑particles are high‑speed electrons (β⁻) or positrons (β⁺). They have a single elementary charge (±1) and much lower mass than α‑particles, so they can travel further – a few metres of air for typical energies. Their ionisation power is lower because the charge is smaller and they lose energy more gradually. Imagine a light tennis ball that rolls further across a lawn. β‑particles can penetrate the skin but are stopped by a few millimetres of aluminium or plastic.

γ‑Rays (High‑Energy Photons) 🌌

γ‑rays are electromagnetic radiation with no mass or charge. Their interaction with matter is through the photoelectric effect, Compton scattering and pair production, which are less efficient at ionising per unit path length compared to charged particles. Therefore, γ‑rays have the lowest ionisation power but the greatest penetration – they can travel many metres through air and require dense materials (lead, concrete) for shielding. Think of γ‑rays as a laser beam that can pass through a wall.