|
|
| Fig. 1: This is what a piece of Am-241 inside a smoke detector looks like. (Source: Wikimedia Commons) |
Many people are unaware of this, but smoke detectors are one of the few household items powered by nuclear energy. Though smoke detectors do produce of a level of γ radiation, there is no need to worry about it. In this note I will further discuss the radiation levels in smoke detectors, and analyze the safety benefits. Residential fires continues to be an issue in the United States as 7,500 and 12,000 lives are lost in fires every year, with 70 percent of these occurring in residential fires. [1] These statistics clarify the importance of having smoke detectors within a residence. Without nuclear energy, approximately 21 million americans would be without nuclear powered smoke detectors. [1]
There are two types of smoke detectors that are used in the United States. The type of detector I am concerned with is the one containing the isotope of Americium Am-241. This can be seen to the right in Fig. 1. The Am-241 is contained in an ICSD (ionization chamber smoke detector). [1] Most of these detectors use approximately 0.25 μg of Am-241 in the form of americium dioxide (AmO2). This small quantity of Am-241 corresponds to 3.0 × 105 Bq of radioactive material. [1] Am-241 emits both α radiation and γ rays. Alpha radiation is absorbed within the detector, while most the γ rays escape into the atmosphere. The γ rays do not travel further than the α particles. [2] In total, the amount of radiation that comes from AM-241 9-50 nSv (nanosievert) per year. [2] Sieverts are units measurement used to quantify the amount of ionizing radiation an object emits. The lethal amount of radiation on the sievert scale is 5 SV. [2] 9-50 nSv is a safe dose for humans to be exposed to. The radiation emitted from Am-241 happens because it is in an unstable state - meaning that it can lower its energy by decaying. Am-241 has a half-life of 432.2 years; every 432.2 years half of the radioactive nuclei originally present have undergone radioactive decay. [2]
 |
| Fig. 1: Illustration of the ionization chamber. (Source: Wikimedia Commons) |
As I mentioned above, the amount of radiation emitted from Am-241 in a smoke detector is not by any means, lethal. [2] In fact, it is less than any natural exposure one would typically see from standard elements like K-40. [3] If this isotope is consumed in any way (inhaled through dust particles, or swallowed residue) it has the potential to be cancerous. [3] Further research has shown that simply swallowing the material will not become an issue since the material is not soluble and would likely go through the digestive tract and be elimnated from the body. However, inhaling this isotope poses a significant danger, since the dwell time in the lung is long, giving a long exposure time to the radiation. [3] According to the NRC fourteen million ICSD's will service about 21 million people. Analysis also shows the risk to the exposed population is about 0.1 fatal cancer. [1]
These ionization detectors use the radiation from the Americium itself to detect smoke. The Americium is kept in a small chamber between two oppositely charged metal plates. [4] Fig. 2 shows the layout of the chamber. When the alpha particles interact with the air that flows through the chamber creating ions. The positive and negatively charged plates each attract the oppositely charged ions, which then creates a steady electric current within the chamber. Once this electric current is blocked by smoke, the electric current is blocked; this immediately sets off the alarm. [4] Without the nuclear power to support this process, we would be left without an affordable way to prevent fatalities caused by household fires.
© Casey Tucker. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
[1] R. Belanger, D. W. Buckley, and J. B. Swenson, "Environmental Assessment of Ionization Chamber Smoke Detectors Containing Am-241," U.S. Nuclear Regulatory Commission, NUREG/CR-1156, November 1979.
[2] E. Eason, "Americium Smoke Detectors," Physics 241, Stanford University, Winter 2011.
[3] N. Cohen, T. L. Sasso, and M. E. Wrenn, "Metabolism of Americium-241 in Man: An Unusual Case of Internal Contamination of a Child and His Father," Science 206, 64 (1979).
[4] C. Wasson, Smoke Detectors and Radioactivity," Earth Isl. J. 5, No. 3, 18 (Summer 1990).
