MadSci Network: Physics

Re: smoke alarm dust leakage?

Date: Mon Jul 16 23:43:57 2007
Posted By: Michael Kay, President and Consultant AMBRY, Inc., and
Area of science: Physics
ID: 1184441241.Ph

Dear Frank,

There is a very high probability that the dust contains Americium-241. Do not disturb the dust. I notice the past tense of the verb in “I had an old smoke detector”; what did you do with it? If you still have it, contact a health physicist or radiation expert through your city’s household hazardous waste program, the fire department’s hazardous materials experts, or the radiation safety officer at a college.

Now for what happened:

Your old smoke detector contained 10 times the amount of Americium-241 used in current models (5 microCuries vs 0.5 microCuries as you will shortly see). The useful lifetime of an Am-241 source is 10 years.

All radioactive sources of alpha emitters suffer from spreading contamination. How? The alpha recoil reaction. One of the laws that must be obeyed in radioactive decay is the conservation of momentum. When the alpha particle is emitted with several million electron volts (MeV) of energy, the remaining nucleus must receive enough energy to fulfill the conservation of momentum. Think of a rifle—the bullet is very light and travels a long distance. The recoil of the rifle makes it travel a short distance because the rifle is so much heavier than the bullet.

Because of the large mass difference between the alpha-emitting nucleus and the helium atom, almost all of the energy is carried away with the alpha-particle. Although the {recoil MK} kinetic energy of the daughter nucleus is small in comparison with that of the alpha-particle, it is large (72 000 electron Volts) in comparison with chemical binding energies (less than 5 eV). Thus the recoiling daughter easily breaks all chemical bonds by which it is bound to other atoms. In 1904 it was observed by H. Brooks that measurements on Po-218 obtained from radon, led to a contamination of the detection chamber by Pb-214 and Bi-214. This was explained by Rutherford as being due to daughter recoil in the alpha-decay of Po-218. The recoil may affect such chemical properties as the solubility or dissolution rate of compounds. For example the dissolution of uranium from uranium rich minerals is considerably higher than one would expect from laboratory solubility data because alpha and U-atom recoil have moved U-atoms away from their normal sites in the mineral.

Alpha Recoil Reaction:

The recoiling daughter atom has enough energy to break further bonds in the material, releasing very small amounts of the source material. Each alpha daughter has the ability of break source material bonds. If the decay proceeds through a gaseous daughter product (a radioisotope of Radon), the daughter can become airborne. Then, when that daughter decays, it can adsorb on the nearest solid surface. Am-241 and its daughters emit 8 alpha particles (plus several beta particles) in their decay chain to end up at stable Bi-209. It is the alpha recoil reaction that can destroy the thin coating over the source and allow source material to recoil out of the source.

Although all I wanted in the following reference was the table of the properties of Am compounds, I have included a bit more for your information. From the:

Public Health Service
Agency for Toxic Substances and Disease Registry
April 2004
http://www.atsdr.cdc .gov/toxprofiles/tp156.pdf

The predominant commercial use of 241Am is in ionization-type smoke detectors. This application relies on the alpha particles that are produced when the isotope decays as an ionization source. A typical household smoke detector contains 0.9 microcuries (ěCi; a ěCi is one millionth of a curie) or 33,000 Bq of 241Am, and 1 g of americium dioxide is sufficient to make 5,000 smoke detectors.

241Am can be released to the environment from nuclear reactors, nuclear explosions, and accidents, as well as from manufacturing products containing americium (such as smoke detectors). Americium is a byproduct of plutonium production. 241Am released to the atmosphere will be associated with particles {such as dust MK} and will settle to earth or be washed from the air in precipitation (e.g., rain, snow).

You may be exposed to 241Am by breathing air, drinking water, or eating food containing 241Am; however, the levels of 241Am in air, water, soil, and food are generally very low, and of little health consequence. People who produce or handle 241Am in smoke detectors or other devices may be exposed to higher levels.

Ionization smoke detectors that contain americium are safe when installed; however, you should not allow your children to play with these detectors. If the detectors are damaged or are no longer being used, they should be promptly returned to the manufacturer for disposal.

Table 4-3. Properties of Some Americium Ions

Am3+Pink or yellowStable; difficult to oxidize
Am4+unknownOccurs in solution only as complex fluoride and carbonate ions
AmO2+YellowDisproportionates in strong acid; reduces fairly rapidly under action of own alpha emissions (241Am) at low acidification
AmO22+Rum coloredEasily reduced; reduces fairly rapidly under the action of its own alpha radiation (241Am)

Source: Seaborg 1991 *Seaborg, GT. 1991. Actinides and transactinides. Fourth ed., Vol. 1. Encyclopedia of chemical technology, 412-445

The most common application of americium is in ionization smoke detectors, and most of the several kilograms of americium produced each year are used for this purpose. Smoke detectors today typically contain approximately 1 ěCi (37 kBq) of 241Am (EPA 2004b). {see below for the current sources used in commercial smoke detectors MK} Note from the table that Am ions are at the yellow end of the color spectrum. I haven’t been able to find the color of the oxides of Am. Given the age and strength of the source, I strongly suspect that there is Am and its daughters in the dust.

I found this site which makes mounted sources for modern ionization smoke detectors:

Descriptions of modern smoke detector sources manufactured by AEA Technology, UK, US, GER, and Hong Kong. These sources have a palladium laminate that keeps the Americium layer in place letting the alpha particles go through to be the ionization source. This is an attempt to slow the dispersion of the Americium Oxide through the recoil reaction. Their recommendation for the working life of their sources in their source-holders is 10 years. All the listed sources are 0.5 microCuries.

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2006. All rights reserved.