Alpha, beta and gamma radiation

Last time

Before we start: last time, in The nuclear atom, nuclides and isotopes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Radiation from the nucleus

Some atoms have unstable nuclei. They throw out particles or energy, all by themselves, and we cannot stop them. This is radioactivity, and there are three kinds you must know: alpha, beta and gamma. Each one behaves differently, and the differences are the heart of this lesson.

Radioactive decay

Radioactive decay is the spontaneous emission of radiation from an unstable nucleus. It is random. We cannot say when a particular nucleus will decay, or predict which one will. It is not affected by temperature, pressure or chemical reactions. It comes from the nucleus.

Alpha particles

An alpha particle is a helium nucleus: two protons and two neutrons. It has a charge of plus two and a mass of four. It is the most strongly ionising radiation, but the least penetrating. A sheet of paper or a few centimetres of air stops it. It is heavy, so it travels only a short distance.

Beta particles

A beta particle is a fast-moving electron from the nucleus. It has a charge of minus one and a tiny mass. It is moderately ionising and moderately penetrating. A few millimetres of aluminium stops it. It can travel several metres in air.

Gamma rays

Gamma rays are electromagnetic waves of very high frequency. They have no mass and no charge. They are weakly ionising, but very penetrating. Only thick lead or several metres of concrete can reduce them. They can travel a long way in air.

Penetrating power

Alpha is stopped by paper. Beta passes through paper, but is stopped by a few millimetres of aluminium. Gamma passes through both, and is only reduced by thick lead. The more strongly a radiation ionises, the less far it can penetrate.

Ionisation

Radiation ionises when it knocks electrons off atoms, making ions. Alpha ionises most, because it is large and has a big charge. Gamma ionises least. It is this ionisation that damages living cells, and that lets us detect radiation.