1. Smoke Detectors Two types ; Use of Photoelectric Effect Use of Ionising Radiation Amercium -241 in the form of Amercium Oxide which is a decay product of Plutonium 241. Plutonium 241 is formed inside reactors from Uranium 238. Half Life of 432 years and decays by emitting alpha and gamma rays.

2. Smoke Detectors 2 Cost $1500 per gram but this is enough for 3 million smoke detectors.

3. How does it work The alpha particles ionise some of the oxygen and nitrogen atoms in the air. A voltage is applied across the plates and this causes the charged particles to be attracted to either the negative or positive plates. This current keeps the buzzer switched off.

4. How does it work 2 When smoke particles enter the chamber they attach to the ions and neutralise them. Some alpha particles are also absorbed by the smoke particles causing less ionisation of the air particles. The current flowing between the plates drops and the alarm then sounds.