The Only Man-Made Element You Can Buy at the Market: Americium
Radioactivity is a concept that scares many people. If something is radioactive, we often get the impression of something harmful to be avoided. However, some radioactive elements may be important enough to save our lives. Americium, an artificial element in the 95th box of the periodic table, has such a life-saving feature. A feature that can detect smoke and alert you. Today we’ll take a closer look at americium, the only man-made radioactive element you can buy in grocery stores, and how smoke detectors work!
Americium
Americium was artificially produced in 1944 by Glenn T. Seaborg and his team. Seaborg named this newly produced element Americium, after America. Americium is a radioactive element and is actually extremely dangerous. It is obtained by colliding uranium or plutonium with a neutron, and it is a reliable source of alpha particles and can be used as a smoke detector thanks to this feature.
The isotope of americium often used in smoke detectors is Americium-241. This isotope can be obtained by bombarding plutonium-239 with neutrons in nuclear reactors.
How Smoke Detectors Work
Americium is most commonly used in smoke detectors. However, it is not found as an element in smoke detectors either. Mostly americium dioxide is used as a source of ionization radiation. Americium dioxide emits 5 times more alpha particles than its closest competitor, radium, and causes relatively less harmful gamma radiation.
Inside the smoke detectors are two metal plates with different voltages, called the ionization chamber or chamber. As the air passes between these metal layers, it is exposed to radiations from the Americium. Positive and negative ions charged as a result of radiation move towards (+) or (–) metal plates and create a constant current. The mechanism references this constant current and detects the normal conditions of the environment. Therefore, it does not give any warning signals when exposed to air only.
Image Source: Stanford University
Smoke detectors start to warn when there are different substances in the air than they should normally be. For example, let’s say you have cigarette smoke. Smoke also creates a current, just like the current that air creates as it passes through the ionization chamber. However, since the particles in the smoke are heavier than the air molecule, the measured current difference will decrease. In other words, a different result will occur from the current created by the air itself. Detecting this, the sensor activates the warning mechanism and gives an emergency signal.
How Does Radiation in Smoke Detectors Occur?
The smoke detector contains only 0.29 micrograms of americium. One microgram is equal to one millionth of a gram. Even such a low amount of americium creates the conditions for detecting fumes.
Radioactive elements decay over time and turn into different elements. For example, americium decays into an isotope of neptunium called neptunium-237. This decay time is measured by a different term called the half-life. This term is a concept that shows how long it will take for our substance to decrease by half from its initial amount. The half-life required for americium to convert to neptunium-237 is 432.2 years. This long half-life is long enough to generate the minimum energy needed to detect fumes but short enough not to harm us.
In the previous section, we talked about the working mechanism of smoke detectors. We can combine this part with radioactive decay and repeat it. The smoke coming into the smoke detector absorbs the alpha particles formed during this radioactive decay. In other words, smoke absorbs alpha particles. The two electrodes inside the smoke detectors create an electric current thanks to the absorbed alpha particles. Finally, sensors that detect this current trigger the alarm to make a sound. Alpha particles formed as a result of decay are so sensitive that they can be absorbed by even very small particles.
Are Smoke Detectors Harmful?
As we mentioned, smoke detectors create continuous radiation. During this radiation process, about 37,000 alpha particles are emitted per second. However, only a very small fraction of these particles can escape from the ionization circle. That is, the area of influence of the emitted alpha particle is quite limited. This harmful radiation can be prevented with a small piece of paper. Even if there is no obstacle, it is not strong enough to go beyond a few centimeters of the air layer. Therefore, it cannot pass through human skin. However, it will have dangerous consequences if inhaled or swallowed. Except for these cases, we can say that smoke detectors are not harmful. However, smashing and burning smoke detectors is not a good idea as it will release americium into the environment.
Smoke detectors have another radiation more dangerous than alpha particles: gamma radiation. Gamma radiation is much stronger and much more dangerous than alpha particles. According to the research results of the US National Radiation Protection and Measurement Council, smoke detectors cause 9-50 nSv (nanosievert) radiation per year. Only when we consider that even the amount of radiation we are exposed to when we eat a banana is 100 nSv, we can say that the radiation caused by the smoke detector is negligible and can be considered harmless.
References and Further Readings
Eason, E. (2011, February 16). Americium Smoke Detectors. http://large.stanford.edu/courses/2011/ph241/eason1/.
Environmental Protection Agency. (2019, August 27). Americium in Ionization Smoke Detectors. EPA. https://www.epa.gov/radtown/americium-ionization-smoke-detectors.
Wikimedia Foundation. (2021, June 19). Americium _ Wikipedia, the free encyclopedia.
https://en.wikipedia.org/wiki/Americium.
Illustration of the principle of an ionization chamber. Courtesy of the European Nuclear Society.
Images not cited are used through Canva Pro with a royalty payment.
The proofreading has been done by Asu Pelin Akköse and Mete Esencan.
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