How a Smoke Detector Works

Explanation

If you take a look inside a normal smoke alarm, you'll see a loud speaker, a battery, electronics, and a silver coloured cover. Underneath this cover is about 0.1g of a radioactive element called americium. This is a very heavy element with a nucleus that loses helium atoms, and these high-speed helium atoms are what we call alpha particles. We can detect and count alpha particles with a Geiger counter - very click represents one alpha particle hitting the detector. Our smoke detector (with the cover removed) gave off around 2000 alpha particles every second, which makes it a really quite radioactive source.

But what role does this radiation play inside a smoke detector? The electronics of a smoke detector consists of two metal plates separated by air. This means that an electric current can pass through most of the circuit but is forced to stop when it reaches the gap because air is a good insulator - that is, it can't carry an electric current because electrons can't move through it very easily.

This is where the radioactive americium comes in. The high-speed alpha particles fly into the gap and knock off electrons from air molecules. These free electrons fill in the gap and allow an electric current to flow through it. When the circuit is complete, the alarm does not sound.

In the event of a fire, tiny smoke particles move into the gap and mop up the free electrons, which stops the current flowing and breaks the circuit. The electronics can detect this change and sound the alarm. We can see this by burning something like a leaf near the smoke alarm.

Thankfully most people never experience a real house fire, but setting the alarm off while cooking sausages or burning toast is a much more common occurrence. Although there's no fire in these situations, they still produce lots of small particles, which soak up the electrons in the gap and break the circuit.