What you have is an ionization-type smoke detector. It detects smoke by the fact that smoke interferes with air’s ability to conduct a tiny electric current.
Under ordinary conditions, air doesn’t conduct electricity at all; it’s an excellent insulator. That’s because the nitrogen and oxygen molecules in the air have no electric charge of their own, nor do they contain any loose electrons that could carry charge from one place to another, as metals do.
If that weren’t the case, electricity from those high-tension power lines overhead would zap right through the air to the ground, passing through anything , including us, in its way.
Air molecules, nitrogen, oxygen and a few others, don’t have any net electric charge because the atoms of which they are made contain equal numbers of positive and negative charges that cancel each other out. The positive charges reside in the atoms’ nuclei and the negative charges are in the form of electrons orbiting around the nuclei.
But radioactivity can make air into an electrical conductor by knocking electrons out of the molecules, leaving them with some uncanceled positive charge. These electron-shy, charged molecules are called ions, and we say that the radioactivity has ionized the air. Because ionized air contains electrically charged molecules, it will conduct electricity.
How does radioactivity ionize the air?
The nuclei of radioactive atoms are unstable, and they spontaneously disintegrate by shooting out some of the particles of which they are made at speeds close to the speed of light. The nuclei of americium-241 choose to shoot out alpha particles, which compared with other radioactively emitted particles are as a baseball is to a BB. A hefty alpha particle can do a lot of damage to an atom that it hits, so it is very good at ionizing air molecules.
A tiny amount of americium-241 is packaged inside your smoke detector and its alpha particles keep a small region of air around it continually ionized. The battery provides a very small electric current that flows through that air. But when some smoke particles get into that air, the ions can collide with them and lose their charge. Less charge in the air means that less current can flow. A circuit detects this drop in current and triggers an ear-piercing alarm.
The amount of radioactive americium-241 in a smoke alarm is extremely small: usually nine-tenths of a microcurie, which corresponds to a quarter of a microgram. Even though that quarter of a microgram is emitting more than 30,000 alpha particles every second, they’re nothing to worry about, because alpha particles are such weaklings at penetrating matter that they can be stopped by a sheet of paper. No alpha-particle radiation whatsoever gets out of the smoke alarm box.
Whenever an atom of americium-241 (or any radioactive material) disintegrates, it is no longer the same kind of atom and doesn’t have the same radioactive properties. So as time goes by, the remaining radioactive atoms decrease in number and so, therefore, does the amount of radiation they emit. In the case of americium-241, its number of atoms decreases by half every 433 years. (Techspeak: Its half-life is 433 years.)
So 433 years from now, the americium-241 in your smoke alarm will be emitting only about 15,000 alpha particles per second. But don’t throw it away yet, because after another 433 years it will still be working fairly well while emitting only 7,500 alpha particles per second. I’d advise you to replace it around 433 years after that, however, because by the year 3300 the electric current will be getting pretty weak and the alarm might go off even without any smoke. And those alarms, you know, can make enough noise to wake the dead.
Of course, if by then you’re where I expect to be, smoke alarms aren’t permitted because they’d be going off all the time.