American Nuclear Society

Home / Public Information / Resources / Special Topices / Smoke

Fire Safety Tip

Startled by the number of deaths resulting from fires, President Calvin Coolidge declared the week of October 9th National Fire Protection Week in 1925. Eighty years later, fire still consumes the lives of thousands of Americans each year. In 2003, over 3,000 people died in fires at their homes.

First patented in 1902, smoke detectors have saved countless lives. Today, the ionization smoke detector is the most commonly used.

Ionization sensor smoke alarms contain a small amount of a radioactive element called americium-241. Americium is a man-made metal and is a source of alpha radiation, which consists of charged particles flying out of the material at high speed.

Americium is mixed with gold and pressed into a very thin foil. This foil is placed between a thick silver plate and a thin palladium foil. The palladium foil holds in the americium, but allows most of the emitted alpha particles to pass through, into the ionization chamber of the smoke detector.

The ionization chamber is an air-filled space between two electrodes. When alpha particles travel through any type of matter, they ionize some of the molecules in the material by colliding with molecule's electrons and knocking them out. In the ionization chamber, air molecules are ionized by the alpha particles emitted by the americium. This ionization of the air allows a weak (and steady) electric current to travel between the electrodes.

When smoke enters the ionization chamber, it interferes with the flow of electricity between the two electrodes, resulting in a measurable drop in the electric current. When the electric current drops below a certain threshold, the alarm is triggered.

This type of smoke alarm is cheaper than other designs and improves the original smoke alarm by measuring more than the heat of a fire. It can detect particles of smoke that are too small to be visible.


National Fire Protection Association
"Fire Statistics."  August 2005.

EPA: Ionization Technology

Last updated July 11, 2012, 9:43am CDT.

Questions or comments about the site? Contact the ANS Webmaster.