What material is used in an ionization smoke detector to ionize the air in the sensing chamber?

In an ionization smoke detector, a small amount of radioactive material, typically americium-241, is used to ionize the air within the sensing chamber. This ionization process involves the radioactive material emitting alpha particles, which collide with air molecules, forming positive ions and free electrons. The presence of smoke interrupts this ionization process, resulting in a decrease in the electrical current flowing between the detector's electrodes, which then triggers the alarm.

The use of a radioactive material such as americium-241 is crucial because it provides a reliable means to detect fast-flaming fires, where smoke particles might not be as dense. Other materials listed in the options do not have the necessary properties to ionize air or serve the intended function in a smoke detection system. The choice of radioactive material is a key feature that differentiates ionization smoke detectors from photoelectric smoke detectors, which operate based on light scattering principles rather than ionization.