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Americium-241
Dateline: 02/07/00
By Alan Bruzel
What Is Americium-241?
Americium (pronounced am-uh-RISH-ee-um) is a man-made radioactive element. Americium-241 was the first americium isotope isolated (by bombarding plutonium with neutrons in a nuclear reactor). It is isolated nowadays as a decay product from spent plutonium fuel. The decay of americium-241 (half-life of 432 years) yields neptunium-237 plus alpha particles and gamma rays.
Why Use Americium-241 in Smoke Detectors?
There are two kinds of smoke detectors: ionization detectors and photoelectric detectors. A photoelectric detector does not use radioactive elements and senses smoke by its interference with a light beam. In ionization detectors, collisions between americium-241's alpha particles and air molecules yield ions that allow a (small) current to flow between two electrodes. Any reduction of this current by smoke or steam (which combine with charged particles) sounds the alarm. The alarm also sounds when a weak battery causes a reduction in the current.
How Much Americium-241 Is in a Smoke Detector?
About one microcurie, equal to about one five thousandths of a gram. The alpha particles produced are not very energetic and are contained by the shielding in the detector. The more energetic gamma rays are not entirely absorbed by the housing of the detector, but gamma rays from an unshielded smoke detector give a radiation dosage about three thousand times lower than the natural background radiation to which one is exposed every day.
What the Web Has to Say about:
Americium-241
Americium – 50 Years Old
Americium's birthday – the element was first prepared late in 1944 – is
celebrated by The Uranium Institute, London, UK.
Domestic
Smoke Detectors Are Safe
Nevertheless, return old units to the manufacturer. From Radiation Safety Unit,
Department of Human Services, Victoria, Australia.
How
Does a Smoke Alarm Work?
Also, includes answers about some other devices using electric charges. From
Louis A. Bloomfield's How Things Work.
How Does a Smoke
Detector Work?
Article on photoelectric and ionization detectors by Jessen Yu, The Stanford
Daily.
How Much Extra
Radiation Dose Do I Get from a Smoke Detector?
Answers about radiation safety from The Committee For Nuclear Responsibility.
How
Smoke Detectors Work
Principles behind ionization and photoelectric detectors. From Marshall Brain's How
Stuff Works. Also, see his Smoke
Detector Chemical Reaction.
Smoke
Detectors
Recommended disposal of americium-based detectors is to send them back to manufacturer.
From the US Environmental Protection Agency.
Smoke Detectors
Scientific American article by Merton Bunker, Jr. explores historical
background and types of detectors.
Smoke
Detectors and Americium
Nuclear issues briefing paper from Uranium Information Center, Melbourne,
Australia describes synthesis and decay of americium.
Up in
Smoke
Answers from New Scientist readers about ionization smoke detectors.
What
Stops Smoke Detectors from Working?
Replace every five years, not because of losses due to radioactive decay, but because of possibility of
failure of electrical components. From ScienceNet.
Why
Do Smoke Detectors Contain a Radioactive Source?
Radioactive doses from americium detectors. From The MAD Scientist Network.
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