|Fig. 1: The interior of an ionization-type smoke detector, showing the ionization chamber (upper left), buzzer (lower left), and battery. Courtesy of Creative Commons.|
Many people are frightened by radioactivity, but the surprising truth is that radioactivity can save lives. An excellent example of this is the household smoke detector.
There are two common types of smoke detector in most countries. Photoelectric-type smoke detectors detect smoke using an optical sensor, whereas ionization-type smoke detectors use an ionization chamber containing radioactive material (see Fig. 1). These two technologies are best used in tandem, since they are sensitive to different-sized smoke particles.  The ionization type is cheaper, at least in the United States, and is particularly common in older buildings. As can be seen from Fig. 1, a typical modern detector contains about 1.0 microcurie of the radioactive element americium, which is equivalent to 37 kilobecquerel (37,000 decays per second), or 0.33 micrograms of americium oxide (AmO2). The average quantity of americium per detector has decreased from 3 microcurie in 1978.  Radium and nickel isotopes were also used in detectors in the past, but most household detectors have used americium.
The isotope of americium used in smoke detectors is americium-241, which decays by α emission to neptunium-237 with a half-life of 432.2 years.  (The isotope americium-243 also decays by α emission and is longer-lived at 7370 years, but it is obtained by irradiating americium-241 with neutrons so it is more expensive.) Neptunium-237 has a half-life of 2.144 million years, so it is mostly stable over the life of a smoke detector, although it eventually decays through a series of longer- and shorter-lived isotopes to the stable isotope thallium-205. The two longest-lived isotopes in this decay chain are uranium-233 (159,200 years) and bismuth-209 (1.9×1019 years). For all practical purposes, the radiation source in the typical smoke detector contains mostly americium and neptunium, with very small quantities of other elements. When the smoke detector is new, it is close to 100% americium. After 30 years, about 4.7% of the americium atoms will have decayed to neptunium.
Americium-241 is made in nuclear reactors by irradiating plutonium-239 with neutrons.  Plutonium-239 atoms tend to fission when they are irradiated with neutrons, but some fraction of the atoms will absorb a neutron instead, forming plutonium-240, then absorb another neutron, forming plutonium-241. The plutonium-241 is then taken out of the reactor, and it finally undergoes beta decay to become americium-241. The half-life of plutonium-241 is about 14 years, so the last step of this process goes quite slowly. This is why large quantities of americium cannot be made quickly. The americium may then be chemically extracted from the other decay products with complex processing. 
Plutonium-239 is itself the primary fissile isotope of plutonium (used in nuclear weapons), and is in turn made from uranium. Therefore, because americium is a byproduct of plutonium production, it is linked to non-proliferation concerns. The U.S. nuclear industry certainly produces more than enough americium for all the world's smoke detectors, but it is interesting to note that any state that can produce americium potentially can also produce a plutonium bomb.
|Fig. 2: Illustration of the principle of an ionization chamber. Courtesy of the European Nuclear Society.|
The ionization chamber in a smoke detector is essentially just two metal plates at different voltages. The ambient air molecules flow between the plates, where they are ionized by the radiation source (see Fig. 2). The negative and positive ions then are attracted to the positive and negative plates, resulting in a measurable constant current.
If the air contains smoke, electrostatic attraction causes the smoke particles to stick to ions in the ionization chamber. The ions do not lose their electric charges when this happens, but since the smoke particles are quite large compared to the ionized air molecules, the average mass of the charged particles in the ionization chamber increases. These particles are still in thermal equilibrium with the surrounding air, so they must have the same average thermal energy. Thermal energy in a gas is proportional to mv2, so if the average mass m of the ions increases, then their average speed v must decrease for the thermal energy to stay the same.  The decrease in average speed shows up as a decrease in the measured current, which is what triggers the smoke detector's alarm.
This way, ionization-type smoke detectors can detect particles that are too small to be seen. This is one of their advantages over photoelectric-type detectors, but it also results in more false alarms. 
Although the radioactive source in a modern detector emits around 37,000 α particles per second, very few of these particles make it out of the detector. Americium-241 emits α particles with energies of about 5.4 MeV, which are stopped by a piece of paper or a few centimeters of air and cannot penetrate the human epidermis.  There is little danger from the α radiation unless the americium is inhaled or ingested. For this reason, it is a bad idea to dismantle or burn a smoke detector, because this could release americium into the environment.
However, americium-241 also emits gamma rays, which are much more penetrating than α particles. The U.S. National Council on Radiation Protection & Measurements estimates that household smoke detector use causes a radiation dose of 9-50 nSv (nanosievert) per year.  To put this in perspective, the dose received from eating a banana is about 100 nSv, the dose of a typical person in the U.S. from natural and manmade sources is about 3.6 mSv per year, and a lethal full-body dose is about 5 Sv. In other words, the radiation from smoke detectors is negligible in comparison to other sources of radiation, so they should be considered safe.
© Eric Eason. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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