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

5.2.2 The Three Types of Nuclear Emission

When an unstable nucleus decays it can emit one of three kinds of radiation:

• Alpha (α) particles
• Beta (β) particles
• Gamma (γ) rays

1. Alpha (α) Emission

Alpha particles are helium‑4 nuclei consisting of two protons and two neutrons.

• Mass ≈ 4 u (≈ 6.6 × 10⁻²⁷ kg)
• Charge = +2 e
• Typical kinetic energy: \$4\text{–}8\ \text{MeV}\$
• Very low penetration – stopped by a sheet of paper or a few centimetres of air.

2. Beta (β) Emission

Beta particles are high‑speed electrons (β⁻) or positrons (β⁺) emitted from the nucleus.

• Mass ≈ 1/1836 u (≈ 9.1 × 10⁻³¹ kg)
• Charge = –1 e (β⁻) or +1 e (β⁺)
• Typical kinetic energy: \$0.1\text{–}2\ \text{MeV}\$ (continuous spectrum)
• Moderate penetration – stopped by a few millimetres of aluminium.

3. Gamma (γ) Emission

Gamma rays are high‑energy photons emitted from the excited nucleus after an α or β transition.

• Mass = 0
• Charge = 0
• Typical energy: \$0.1\text{–}10\ \text{MeV}\$ (discrete lines)
• High penetration – requires several centimetres of lead or several metres of concrete to attenuate.

Relative Ionising Effects

The ability of radiation to ionise atoms depends on two main factors:

1. Kinetic energy – higher energy particles can remove electrons from atoms more readily.
2. Electric charge – charged particles interact via Coulomb forces, producing dense ionisation tracks; neutral photons interact less directly.

Combining these factors gives the following hierarchy of ionising power:

Why the Differences Occur

Kinetic energy: Alpha particles, despite having similar or slightly higher kinetic energies than beta particles, carry much more mass. Their large mass means they lose energy rapidly through collisions, creating a dense ionisation track. Gamma photons, being massless, travel at the speed of light and interact less frequently, so each interaction deposits less energy.

Electric charge: Charged particles (α and β) experience Coulomb forces with electrons in matter, directly stripping electrons and producing ion pairs. The magnitude of the charge influences how strongly they attract or repel electrons; a +2 charge (α) exerts a stronger force than a ±1 charge (β). Gamma rays have no charge, so ionisation occurs only indirectly via secondary processes (photoelectric effect, Compton scattering, pair production), which are less efficient per unit path length.

Practical Implications

• Alpha emitters are hazardous if ingested or inhaled because their high ionising power can damage biological tissue internally, even though they are stopped by skin.
• Beta emitters can cause skin burns and are a concern for external exposure; shielding with thin metal sheets is effective.
• Gamma emitters require dense shielding (lead, concrete) and are the primary concern for external radiation protection.