(i) α -particles, β -particles and γ -rays; nature, penetration and range…

The idea

Alpha particles are helium nuclei (2 protons and 2 neutrons), with charge 2⁺ and mass number 4. They are highly ionising but have low penetration, stopped by paper or a few centimetres of air. Beta particles are fast-moving electrons (or positrons), with charge 1⁻ or 1⁺ and negligible mass; they are less ionising than alpha but more penetrating, stopped by a few millimetres of aluminium. Gamma rays are high-energy electromagnetic radiation, with no charge or mass; they are the least ionising and most penetrating, requiring several centimetres of lead to reduce intensity. Range in air: alpha a few centimetres, beta a few metres, gamma effectively unlimited but attenuated. Penetration depends on ionising power and interactions with matter.

What earns the marks

• Alpha particles: helium nuclei, charge 2⁺, mass number 4, highly ionising, stopped by paper, range in air a few centimetres.
• Beta particles: electrons or positrons, charge 1⁻ or 1⁺, negligible mass, moderately ionising, stopped by a few millimetres of aluminium, range in air a few metres.
• Gamma rays: electromagnetic radiation, no charge or mass, weakly ionising, highly penetrating, attenuated by several centimetres of lead.
• Penetration and range are inversely related to ionising power: alpha is most ionising but least penetrating; gamma is least ionising but most penetrating.
• Absorption of beta and gamma is exponential with thickness, while alpha has a definite range.

Common mistakes and how to put them right

• Misunderstanding: thinking that alpha particles can travel several metres in air.

  Put it right: Alpha particles have a range of only a few centimetres in air.

• Misunderstanding: confusing beta particles with gamma rays in terms of charge.

  Put it right: Beta particles have charge 1⁻ or 1⁺, while gamma rays have no charge.

• Misunderstanding: believing that gamma rays are completely absorbed by a few millimetres of lead.

  Put it right: Gamma rays require several centimetres of lead to reduce intensity significantly.

• Misunderstanding: stating that beta particles are stopped by paper.

  Put it right: Beta particles pass through paper but are stopped by a few millimetres of aluminium.

Exam tips

• Memorise the nature, charge, mass and typical penetration for each radiation type.
• Use the correct notation: α, β, γ, and for charges use 2⁺, 1⁻, 1⁺.
• When comparing penetration, relate it to ionising power: more ionising means less penetrating.