Explain, in terms of particle behavior why smoke particles cause the detector alarm to sound

Americium-241 is used in a smoke detector because of its decay mechanism. As A-241 decays, it releases energy that temporarily ionizes the oxygen present in the air. This ionization allows the air to conduct an electrical charge from the battery delivery to the alarm between particles, keeping it from sounding. When smoke enters the alarm, the oxygen concentration drops as smoke replaces the molecules in the air. Smoke cannot be ionized, and the current to the alarm drops, causing the alarm to begin ringing.