Final answer:
Smoke detectors use Americium-241 to maintain a current for detecting smoke, leading to a regular drain on the batteries that necessitates their replacement to ensure the detector's functionality.
Step-by-step explanation:
In residential smoke detectors, secondary batteries must be replaced periodically to ensure proper operation. The reason for this replacement is the presence of Americium-241, a radioactive element that emits alpha particles (a-decay) used to maintain a small constant electric current in the smoke detector. As these alpha particles ionize the air between two metal plates, they enable the flow of ions and thus a current which creates a voltage between the plates. Over time, the constant drain on the battery requires that it be replaced to continue this ionization process. When smoke from a fire enters the ionization chamber, it disrupts this flow, leading to a decrease in the current and triggering the alarm. As stand-alone devices, the maintenance of these devices includes the regular replacement of batteries.