The nuclear radiation emitted may be:

The idea

When an unstable nucleus decays, it emits nuclear radiation. The three main types are alpha, beta and gamma, and they differ in nature, charge, penetrating power and ionising ability. An alpha particle is a helium nucleus, 2 protons and 2 neutrons, with charge 2⁺; it is highly ionising but stopped by a few centimetres of air or a sheet of paper. A beta particle is a fast-moving electron emitted when a neutron changes into a proton, with charge 1⁻; it is moderately ionising and stopped by a few millimetres of aluminium. Gamma radiation is a high-energy electromagnetic wave with no charge and no mass; it is weakly ionising and absorbed by thick lead or concrete. A neutron is also emitted in some decays. Knowing these properties lets students identify the radiation from absorption data and balance nuclear equations.

What earns the marks

• Alpha radiation is a helium nucleus, 2 protons and 2 neutrons, with charge 2⁺.
• Beta radiation is a fast-moving electron emitted from the nucleus, with charge 1⁻.
• Gamma radiation is a high-energy electromagnetic wave with no charge and no mass.
• Alpha is the most ionising and least penetrating; gamma is the least ionising and most penetrating.
• Alpha is stopped by paper or a few centimetres of air, beta by a few millimetres of aluminium, and gamma is reduced by thick lead or concrete.
• A neutron may also be emitted in some nuclear decays.
• The type of radiation emitted can be identified from how its count-rate changes as different absorbers are placed between source and detector.

Common mistakes and how to put them right

• Saying beta is a proton.

  Put it right by describing the electron and the nuclear change.

• Describing gamma as a particle.

  Put it right by calling it a wave or photon.

• Mixing up penetrating power and ionising power.

  Put it right by learning the pair together.

• Writing charges as 2+ or 1-.

  Put it right: Use Unicode superscripts, 2⁺ and 1⁻, and include the sign.

Exam tips

• Learn the nature, charge, penetration and ionisation for alpha, beta and gamma as a table.
• When identifying an unknown source, compare count-rate with and without each absorber.
• In nuclear equations, check that mass numbers and atomic numbers balance after emission.
• Use precise wording: alpha is a helium nucleus, beta is a fast-moving electron, gamma is an electromagnetic wave.
• Remember that gamma has no charge, so it is not deflected by electric or magnetic fields.