Ionisation Smoke Detector: Principle and Limitations

Ionisation smoke detector is a smoke detection device that uses a small radioactive source to ionise air inside a sensing chamber and detects smoke by the reduction in current that occurs when smoke particles attach to the ionised air molecules.

The radioactive source is typically a tiny quantity of Americium-241 mounted between two electrodes. In clean air the ionised molecules carry a steady predictable current. When smoke particles enter the chamber they attach themselves to the ions, the combined particles move more slowly than free ions, and the chamber current drops. The drop is measured against a threshold and the device alarms.

Ionisation detectors respond more quickly than optical detectors to fast, hot, clean-burning fires that produce mainly small particles, and more slowly to smouldering fires whose larger particles are detected first by optical scattering. Because most fatal building fires begin as smouldering events, the industry consensus has moved decisively toward optical detection as the default smoke-sensing technology. Ionisation devices also carry end-of-life regulatory cost as low-level radioactive waste in most jurisdictions, accelerating the shift.

The technology retains a small role in specific industrial scenarios where fast clean burns are the credible threat, but for almost every modern non-domestic project the appropriate choice is optical or multi-sensor. For the comparison, see ionisation vs optical smoke detection. For the optical alternative, see photoelectric smoke detector.