If you’re reading this at home then it is almost certain, at least I hope so for your sake, that you are sat within a few metres of a substance at least as toxic and far more radioactive than weapons-grade plutonium. What’s more, you probably bought it and put it there. What’s that? You don’t remember applying to the Atomic Energy Authority for a special licence to install equipment that contains highly radioactive material? Luckily, your smoke detector only contains about one fifth of a microgram of americium-241 and therefore doesn’t pose any risk to you or yours.
Make no mistake, though, in large quantities, like the ones you would find in nuclear waste, americium-241 is one of the nastiest nasties out there; indeed americium cannot be found in nature and is, literally, nuclear waste. It has a half life in the region of 240 years and so will hang around for millennia before it decays into neptunium-237, which has a much longer half life of 2.14 million years. In nuclear waste americium is a big problem, it throws out huge amounts of heat and has, until recently, proved extremely difficult to separate out from the other components.
There are, though, different types of radiation; α, β & γ (alpha, beta and gamma); and each of them have their dangers and uses. Americium-241 produces alpha radiation almost exclusively and this is good because, one, alpha radiation has the least penetrative power – you can stop it with the might of such things as paper and air – and, two, alpha radiation is what actually makes it useful in this context.
Your smoke detector works like this: the alpha particles given off by the americium-241 pass between two electrodes, this ionises the air between them and allows a current to flow. When smoke enters the compartment it attaches to the alpha particles and hinders their ionising abilities, this breaks the circuit and the alarm sounds. Simple, elegant and effective.
There are downsides to this type of detector; it isn’t very good at discerning between types of smoke and so you get a lot of false alarms, also, if you were to remove the protective covers and strap a dozen of them to your head then there may be some health implications. Those aside, we can be fairly sure that by allowing a small amount of a deadly substance into our homes we can all sleep a lot safer at night.