Ionization Smoke Detectors An Ionization Smoke detector generally contains two chambers. The first is used as a reference to compensate for changes in ambient temperature, humidity, or pressure. The second chamber contains a radioactive source, usually alpha particle, which ionizes the air passing through the chamber where a current flows between two electrodes. When smoke enters the chamber the current flow decreases. The drop in current flow is used to initiate an alarm. Watch the video and an