New York City Fire Safety Director (FSD) Practice Test

Ionization detectors create a small, steady current in the sensing chamber by ionizing the air. A sealed radioactive source emits particles (typically alpha radiation) that knock electrons off air molecules, producing positive ions and free electrons. This ionization makes the air conductive between the chamber’s electrodes. When smoke enters the chamber, smoke particles attach to the ions, reducing the number of charge carriers and lowering the current enough to trigger the alarm. The radioactive material is chosen and sealed specifically for safe, consistent ionization—the common source is a tiny amount of radioactive material. The other options don’t provide this ionizing effect: metals like magnesium or copper aren’t radioactive sources for ionizing air, and hydrogen gas isn’t used to create the controlled ionization current needed for triggering the detector.