Alpha particles in smoke detectors

Radioactive isotopes which emit alpha particles can be used in smoke detectors. The animation below shows a smoke detector with the radioisotope Americium 241 as the alpha source.


Smoke detectors make use of the ionising properties of alpha particles. They contain an ionisation chamber which consists of a positive and negative electrode along with a very small amount of the radioisotope Americium-241. Amercium-241 has a half life of 432 years and is a good source of alpha particles. The long half life is useful as it ensures a continuous source of alpha particles meaning the detector is very reliable and will not stop functioning. It also means the alarm does not require regular replacement; however the battery does require regular replacing and this is indicated by a warning beep or light on the alarm.

The ionisation chamber consists of open channels allowing the air form the room to flow through it. The alpha particles emitted from the Americium-241 collide with the oxygen and nitrogen molecules in the air causing them to ionise. To ionise means to knock off electron/electrons from an atom. Thus the alpha particles knock off an electron from the oxygen and nitrogen molecules resulting in negatively charged electrons and positively charged atoms. The negatively charge electrons are attracted to the positive electrode and the positively charged atoms to the negative electrode. A very small current is generated which is detected by the electrical circuit in the smoke detector.

When smoke enters the ionisation chamber the alpha particles collide with the smoke particles instead of the air particles. The collisions with smoke particles does not result in ionisation so the current drops. The electric circuit registers the drop in electric current and triggers the alarm to beep.