Which alarm initiating device contains a small amount of radioactive material in its sensing chamber?

• A. Water detector
• B. Ionization detector
• C. Photoelectric detector
• D. Flame detector

Answer: (B)

Ionization detectors use a small sealed amount of radioactive material in the sensing chamber. This material ionizes the air, creating a steady electrical current between two electrodes. When smoke particles enter the chamber, they attach to the ions and reduce the current, which triggers the alarm. This design is specific to ionization-type initiators.

Water detectors respond to moisture; photoelectric detectors use a light beam and a sensor to detect smoke, either by scattering or blocking the light; flame detectors look for the infrared or ultraviolet radiation of flames.