What does an ionization detector use to detect smoke?

• A. Litmus paper
• B. A photoelectric cell
• C. Radioactive material
• D. Heavy water

Answer: (C)

Ionization detectors rely on a small sealed radioactive source to ionize the air inside the sensing chamber. This ionization creates a steady electrical current between two plates. When smoke enters the chamber, smoke particles attach to the ions and reduce the current. The detector senses this drop and triggers the alarm. This mechanism is different from other types, such as photoelectric detectors that use a light beam and a sensor to detect smoke, or devices that use no radioactive material. The radioactive source used is typically a tiny, sealed amount of americium-241, chosen for its ionizing properties and safety in normal use.