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How does a Smoke Detector Work?

How is Radioactivity used in a Smoke Detector?

The radioactive source in a smoke detector is americium-241.
Americium-241 is an alpha emitter. The alpha particles
ionise the molecules of the air forming ions and electrons
which are attracted to a pair of charged plates as shown in the
picture below. When the ions and electrons hit the plates they
provide a small amount of electricity
(a small current). This
current stays constant and is monitored by the smoke alarm.

Smoke Detector

What happens when Smoke Enters the Smoke Detector?

When smoke enters the smoke detector, the smoke particles
to the radioactive source absorb many of the alpha particles
before they can ionise the air between the charged plates.
The number of ions and electrons between the charged plates
therefore decreases and so the current which is passed
between the charged plates also decreases.
The smoke
notices the decrease in current and sets off the alarm.

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