Final answer:
The device described is an ionization-type smoke detector, which uses Americium-241 to detect smoke by monitoring changes in ionized air conductivity within its chamber, ceasing the alarm once the smoke has dissipated.
Step-by-step explanation:
The item described in the question seems to be an ionization-type smoke detector. This device operates by using Americium-241, an alpha emitter with a half-life of 458 years. Americium-241 is used in tiny amounts in these smoke detectors. Its alpha emissions ionize the air between two electrode plates in the ionizing chamber. When smoke enters the chamber, it impedes this ion movement, which reduces the air conductivity and causes a significant drop in the current. This change triggers an alarm to indicate the presence of smoke. Once smoke is no longer detected, and the air returns to normal ionization levels, thereby cessation of the alarm indicates that all smoke has dissipated.