Smoke Detectors have an ionization chamber with a 8 particle radiator consisting an Americium-241 foil at its centre and it ionises the air inside the chamber.
Ionization Smoke Detector
An electric voltage is applied to the needle electrode and as a result, a known current flows through the detection chamber when the detector is in its normal state.
In the event of fire, minute aerosol particles (soot particles, combustion gases etc.) attach themselves to the ionized particles.
This increases the resistance in the chamber and consequently reduces the current flow within the detection chamber. This change is registered by Detector’s electronics and a fire signal is sent to the Control Panel.