5.1 Alpha particles

An alpha particle is a helium nucleus containing two protons and two neutrons. It has charge +2 and relative mass 4. Alpha particles are emitted from the nucleus at high speed, but because they are relatively massive and doubly charged, they interact strongly with matter.

Alpha radiation has the strongest ionising effect of the three emissions. It loses energy rapidly, travels only a few centimetres in air and is stopped by paper or the outer dead layer of skin. It is particularly hazardous if an alpha-emitting material enters the body.

5.2 Beta-minus particles

In this syllabus, beta radiation means beta-minus. A beta particle is a high-speed electron emitted from the nucleus when a neutron changes into a proton. It has charge −1 and a very small relative mass.

Beta radiation has medium ionising ability and medium penetration. It can travel farther in air than alpha and passes through paper, but a few millimetres of aluminium usually reduces it strongly. Its path is more easily deflected than an alpha path because its mass is much smaller.

5.3 Gamma radiation

Gamma radiation is high-frequency electromagnetic radiation emitted by a nucleus that loses excess energy. It has no charge and no rest mass. Gamma travels at the speed of electromagnetic waves in vacuum.

Gamma has the weakest ionising effect of the three but the greatest penetrating power. It is not completely stopped by a simple thin sheet; thick lead or concrete reduces its intensity. The terms “absorbed” and “reduced” are often more accurate than “stopped” for gamma.

5.4 Ionisation and penetration

Radiation that ionises strongly transfers energy to matter quickly and therefore has a short range. Alpha produces many ion pairs over a short distance. Gamma interacts less frequently, so it may travel much farther before depositing energy.

This inverse relationship helps explain applications and hazards. Alpha is useful in a smoke alarm because its strong ionisation creates a steady current, while gamma is chosen when radiation must pass through packaging or body tissue.

5.5 A comparison for examination answers

Alpha: helium nucleus, +2, mass 4, strongest ionisation, lowest penetration. Beta-minus: electron, −1, tiny mass, medium ionisation and penetration. Gamma: electromagnetic wave, no charge, no mass, weakest ionisation, greatest penetration.

Use comparative words carefully. Gamma is weakly ionising, not non-ionising. Alpha has low external penetration but can be dangerous internally. Penetrating power alone does not determine biological risk.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. Which emission has the greatest ionising effect?
2. Why is beta deflected more strongly than alpha in a field?
3. Which radiation is best able to pass through sealed packaging?