Matthew Bruers, of Forrestfield Senior High School (Perth), asks :-

How does a radioactive smoke detector work?

John Campbell, a physicist and biographer of Ernest Rutherford, responded.

It works because alpha particles are stopped by the small particles of carbon which make up smoke.

The principles have been known for a hundred years. Ernest Rutherford named the alpha particles and showed that they are nuclei of helium atoms travelling at high speeds. As an alpha particle hurtles through matter it knocks electrons off the atoms it comes near and looses energy. As a consequence an alpha particle travels only about 5cm in air and is stopped by very thin amounts of solid, for example the dead skin on our hands, a sheet of toilet paper or a smoke particle.

A smoke detector consists of a radioactive material (americium-241) which emits alpha particles, a nearby metal plate, some electronics and an alarm. When tiny smoke-particles prevent the alpha particles reaching the plate it loses its electrical charge and the alarm is triggered.

Smoke detectors are incredibly sensitive. If you light a match then blow it out the smoke is sufficient to trigger a nearby alarm. That is why smoke detectors are best not placed near kitchens or showers where burning toast, cooking vapours or water condensation can trigger them.

In one week a few months ago there were four house fires in New Zealand. In three of these people died. The main difference was $20, the price of the smoke detector installed in the one house where the people thereby received warning and escaped unharmed. It is a tiny price to pay for human life.