The Chemistry of Smoke Detectors 

There are two main types of smoke detectors: ionization detectors and photoelectric detectors. Let’s focus on ionization detectors. 

An ionization smoke detector uses a radioisotope such as americium-241 to produce ionization in air; a difference due to smoke is detected and an alarm is generated. 

The radioactive isotope americium-241 in the smoke detector emits ionizing radiation in the form of Alpha particles into an ionization chamber that is open to the air and a sealed reference chamber. The air molecules in the chamber become ionized and these ions allow the passage of a small electric current between charged electrodes placed in the chamber. If any smoke particles pass into the chamber the ions will attach to the particles and so will be less able to carry the current.