A diagram showing the three types of radiation.

An alpha particle is one type of radiation released from an unstable nucleus. An alpha particle is a helium nucleus, made up of two protons and two neutrons.

Properties of alpha particles[edit | edit source]

Alpha particles are a very slow form of radiation and can only travel a short distance. They are stopped by a sheet of paper and can travel only a few centimetres. However, this is because they have a very strong ionising effect. This means that they knock out electrons from air molecules, meaning they become positively charged.

The discovery of the atomic model[edit | edit source]

Alpha particles were used to work out what the structure of atoms was. Positively charged alpha particles were fired into an atom. They were only deflected in a very small area. This was the positively charged nucleus.

Use of alpha radiation[edit | edit source]

A common use of alpha radiation is in smoke detectors. The device consists of a source of alpha particles and a detector. As the alpha particles travel through the air between the source and detector they ionise it and only a limited number of alpha particles reach the detector. If there is a fire the alpha particles will pass through a mixture of smoke and air and a higher level of ionisation will occur. This will result in fewer alpha particles reaching the detector and the count rate will fall. The detector is callibrated so that if the count rate falls below a critical value a warning sound is produced.

An alpha source with a long half life is used so that it does not have to be replaced. Alpha particles are used because they are blocked by a few centimetres of air, so the smoke detector can be conveniently sized and safe to use in the home.

See Atomic model for more

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