Ionization Smoke Detectors Function by Detecting the Alpha Decay of Americium

Some smoke detectors contain a small amount of americium-241 which is a radioactive element that undergoes alpha decay into neptunium-237. This is a really good example of how nuclear decay can be very useful and seen in a common household device. These detectors work because when the alpha particles from the americium-241 are released they ionize the air inside the chamber of the detector. This creates a constant flow of electrons. When smoke gets into the chamber, this flow of electrons is interrupted which triggers the alarm to sound.

Driving Question

• How do smoke detectors work?

Probing Questions

• What are some benefits that can be associated with radiation/radioactive decay? Drawbacks?
• How can we engineer safer smoke detectors?
• What considerations do engineers need to make when they design materials that are going to be used in a home as opposed to an industrial setting?

Classroom Suggestions

Students could:

• Research different elements and try to determine why we use americium-241 and not another radioactive element. This could lead them into some engineering principles and allow them to do a cost-benefit analysis for the situation.
• Analyze the workings of a smoke detector and discuss whether or not they think it is safe to have a radioactive element in a common piece of household device.

Relevant Related Resources

Alpha Decay | PhET Simulation: This simulation shows the alpha decay of Polonium-211. Students can take out multiple atoms of Po-211 and watch as they take varying amounts of time to decay. Students can relate these times to a graphic that shows the half-life of this element.
Radioactive Smoke Detectors | YouTube: As students are discussing the cost-benefit analysis of different materials that could be used in smoke detectors, you could have them watch all or part of this video that explains the inner workings of a smoke detector so that they have some valuable background information.