An ionization detector consists of two metal plates with a voltage across them, along with a radiation source of americium. When alpha particles from the americium collide with oxygen and nitrogen atoms in the air, electrons from the atoms are freed. The electron has a negative charge, and the atom with the missing electron, a positive charge. The positive charged atoms are called ions. The ions are attracted to the negative plate, and the free electrons are attracted to the positive plate (opposites attract). The flow of electrons and ions is an electrical current, which is sensed by a detector. Smoke particles attach to the ions and neutralize them (zero charge). This reduces the electrical current, which is sensed by the detected.
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In many cases whether a smoke alarm sounded can be determined by enhanced soot around the horn. Reference: 2017 NFPA 921, Sections 22.214.171.124-126.96.36.199.5.