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

An ionization detector contains a small amount of radioactive material, which is key to its function. Inside the sensing chamber of the device, this radioactive material emits alpha particles that ionize the air, creating a small electrical current between two electrodes. When smoke enters the chamber, it disrupts this ionization process by attaching to the ionized air particles, decreasing the current flow. This drop in current is detected by the alarm system, triggering it to alert occupants to the presence of smoke or fire.

The other options do not contain radioactive materials or do not function in the same manner. Water detectors are designed to sense the presence of water, typically through conductivity. Photoelectric detectors utilize a light source and sensor to detect smoke based on light scattering or blocking. Flame detectors sense the specific wavelengths of light emitted by flames, often using infrared or ultraviolet sensors. Each of these devices operates on different principles that do not involve the use of radioactive materials.