One type of smoke detector uses Americium-241, an alpha particle source, to detect smoke.
The alpha particles pass between the two charged metal plates, causing air particles to ionise (split into positive and negative ions).
The ions are attracted to the oppositely charged metal plates causing a current to flow.
When smoke enters between the plates, some of the alpha particles are absorbed causing less ionisation to take place. This means a smaller than normal current flows so the alarm sounds.
An alpha source is used because alpha radiation does not penetrate very far.
It is absorbed by a few cm of air.
This means that as long as the detector is high up on a wall, or the ceiling, it is safe for humans to be in the same room.
The source should have a long half-life so that the smoke detector does not have to be replaced too frequently, and so that the count rate remains almost constant each day.
|Advantages of contamination||Disadvantages of contamination|
|Radioactive isotopes can be used as medical and industrial tracers.||Radioactive isotopes may not go where they are wanted.|
|Use of isotopes with a short half-life means exposure can be limited.||It can be difficult to ensure that the contamination is fully removed so small amounts of radioisotope may still be left behind.|
|Imaging processes can replace some invasive surgical procedures.||Exposure to radioactive materials can potentially damage healthy cells.|