Smoke Detectors

Americium is an element that, for all practical purposes, did not even exist on Earth before 1944 when it was discovered in some nuclear reactor waste. One may think that this "un-natural" newcomer to the periodic table, having such ignoble origins (and being intensely radioactive to boot!), couldn't possibly have any legitimate use. However, Am-241 has carved an amazing niche for itself. The overwhelming odds are that you gratefully share your home with this radioactive friend.

We are talking about your smoke detector. Am-241 emits alpha particles, which ionize the air in the detector's ion chamber and enable a weak electric current to flow. The alarm circuit senses this current. When smoke-filled air enters the ion chamber and interferes with the current, the alarm is activated and lives are saved. Some people believe that ionization smoke detectors are the trademark of a shadowy conspiracy by the nuclear industry to slow-roast the public with radiation. The obvious truth is that you dramatically increase your risk of death by NOT using a smoke detector. If you don't have americium in your home today...you're probably a fool.

Inquisitive folks and amateur scientists know that there are many other uses for Am-241 besides detecting smoke. The isotope emits gamma radiation of 59.5 keV, making it helpful in calibrating the low energy range of gamma spectrometers. It is a proven favorite alpha source in homebrew spinthariscopes.

Modern detectors for the home contain approximately 1 microcurie of Am-241, while some '70s-vintage units held over 4 microcuries. The sources themselves are small disks approximately 5 mm in diameter, in which the active deposit is sealed under a metal film. Finding exactly where in the smoke alarm the Am-241 source is can be a challenge for the first-timer. Here I will provide a pictorial guideline to source extraction for two of the most common inexpensive ionization smoke detectors. We begin by considering the Kidde 0915 pictured above.

Open the detector package to reveal the circuit. The prominent metal shell on the circuit board is the ion chamber housing the Am-241 source.

A soldering iron is necessary to remove the ion chamber shield. The radioactive source is now visible in the center of the exposed ion chamber innards.

Removing the source from the lower plate of the ion chamber takes some prying / wrenching work. Obviously, don't try to power-drill it out or use any other procedure that might destroy the integrity of the source. (Uncontained Am-241 is an ingestion hazard.) The source is a small button containing about 0.9 microcurie Am-241 beneath a thin foil exposed on one side.

Finally, here are some images of the guts of a Family-Gard FG888D ionization-type smoke alarm ($4.89 at your local Wal-Mart).

Unlike the previous example, this alarm has both the piezo element and the ion chamber on the main circuit board. The ion chamber has the black plastic shield. Removing the Am-241 source entails using a small, sharp flathead screwdriver to pry it out.