Types of radiation

Three types of ionising radiation are:

Alpha particle

Alpha particle \alpha - is a helium nucleus, two protons and two neutrons. It has a large mass, compared to other ionising radiations, and a strong positive charge.

Beta particle

Beta particle \beta - is a fast moving electron. It has a very small mass and a negative charge.

Gamma ray

Gamma ray \gamma - is a high-energy electromagnetic wave. Gamma rays are caused by changes within the nucleus. They are part of the electromagnetic spectrum and so travel at the speed of light. They have no mass and no charge.

Penetrating power

Each type of radiation has a different ability to penetrate materials. The material is said to have absorbed the radiation.

Alpha radiation stopped by paper, beta radiation passes through paper but stopped by aluminium, gamma radiation passes through paper and aluminium but stopped by lead.

The energy of the three radiations is absorbed by the material through which the radiation passes. The amount of energy which is absorbed depends on the type of radiation and the type of the absorbing material.

  • The range of the alpha radiation in an absorbing material is less than that of beta or gamma. The alpha radiation transfers more energy to an absorber than beta or gamma radiation. Alpha radiation is absorbed by the thickness of the skin or by a few centimetres of air.
  • Beta radiation is more penetrating than alpha radiation. It can pass through the skin, but it is absorbed by a few centimetres of body tissue or a few millimetres of aluminium.
  • Gamma radiation is the most penetrating of the three radiations. It can easily penetrate body tissue. It requires several centimetres of lead or about 1 metre of concrete to absorb it.
The table below shows the properties of each type of ionising radiation
RadiationRange (cm)Ionising powerCan pass through paper?Can pass through 5mm of aluminiumCan pass through 5cm of lead?Electrical field deflection
Alpha3-5Highly ionisingNoNoNoDeflected towards negative plate
Betaabout 15IonisingYesNoNoDeflected towards positive plate
Gammamuch longerWeakly ionisingYesYesNo - although some will still get throughNone

Each type of radiation is caused by a decay (either spontaneous - natural - or induced - a reaction has caused the decay).

The nuclei of some atoms are unstable, and will naturally undergo 'radioactive decay'. This causes the release of ionising radiation which allows the nucleus to become more stable.

Atoms that do this are said to be radioactive.

A comparison of alpha, beta and gamma