1. IP2.30.3 Alpha smoke alarms © Oxford University Press 2011 Alpha smoke alarms

2. IP2.30.3 Alpha smoke alarms © Oxford University Press 2011  Alpha particles are ideal for use in smoke alarms.  Smoke alarms contain a weak source of alpha particles. They ionise the air in a chamber and a small electrical current flows between two electrodes in the chamber.  When smoke enters the chamber it absorbs the alpha particles. This means that current drops and this triggers the alarm.

3. IP2.30.3 Alpha smoke alarms © Oxford University Press 2011  If the half life was too short then the activity of the source would fall too quickly, the current in the ionisation chamber would stop and the alarm would sound.  For a smoke alarm to work properly for a number of years the activity level of the source needs to be fairly constant and therefore it needs to have a long half life. Explain why a source with a long half life is needed in a smoke detector.

4. IP2.30.3 Alpha smoke alarms © Oxford University Press 2011  Other types of radiation are not as ionising and so would not create the current in the detector for it to work properly. Explain why radioactive sources producing beta or gamma radiation aren’t suitable for a smoke detector.