You may have heard of certain smoke detectors containing radioactive elements. These alarms are actually really common. Ever wondered how they work?
The smoke detector in your house is almost certainly one of two types: photoelectric or ionization. If you look on the back, and you see a nuclear radiation symbol, then you have an ionization alarm. Don't worry, they're perfectly safe.
Photoelectric smoke alarms work by keeping a light source aimed away from a photodetector. When smoke gets into the internal chamber of the alarm, light reflects off of the dense smoke and into the detector, setting off the alarm. This also can make them sensitive to shower steam, which I have learned the hard way.
Inside of a photoelectric smoke detector
Image credit
Ionization smoke alarms on the other hand use a radiation source, which is naturally why I'm writing about them. Almost all ionization smoke alarms will contain a radiation source made from Americium-241. Americium is a heavy unstable element with 95 protons (past Uranium on the periodic table).
Americium-241 is a specific isotope of Americium. It takes over 400 years for a sample of Am-241 to decay to half of its original decay rate. The most common decay (by far) is for the Am-241 nucleus to release an alpha particle (Helium-4 nucleus) followed by a lower energy gamma ray emission. This process changes the original Americium nucleus into two different elements: Helium (the alpha particle) and Neptunium. The majority of the decay energy goes into the alpha particle's kinetic energy, which comes out to around 5.5 MeV. The gamma ray emitted will have an energy of just under 60 keV, around 100x less than the alpha particle's kinetic energy. A good overview of Am-241 is available here.
Smoke alarms directly utilize this emitted alpha radiation to detect fires.
Most alarms contain about 37 kBq (or 37,000 decays per second) of Am-241. Alpha radiation is extremely easy to absorb, so a little less than half of these decays actually release alpha radiation into the air (the rest are absorbed into the metal backplate of the source).
Alpha radiation only gets a few centimeters in air (on average) before being stopped. Since the kinetic energy of the average Am-241 alpha particle starts at over 5 MeV, any given alpha particle released into the air will be able to ionize many neutral gas atoms in the air before being stopped. The result is that the air a couple centimeters above the Am-241 source used in the alarm has recently ionized atoms in it. And, since ions and the corresponding electrons are free charges, this means that the air above the source can slightly conduct electricity.
The Americium source inside a typical alarm is placed in a metal chamber. This chamber is shown on the below picture as the one with the radiation symbol on it. The other cylinder is a piezoelectric buzzer that makes the actual alarm sound.
This means that when there is no smoke in the air, the air above the source is slightly conductive. The alarm passes current through this gap and the alarm does not sound.
However, when smoke enters the device, this changes. Notice that the metal chamber has vents to allow smoke to pass through. Smoke neutralizes the ions in the chamber and stops/reduces the flow of electrical current across the air gap. Since the radiation source definitely didn't fail (it will decay for hundreds of years no matter what), the drop in current was almost certainly caused by smoke from a fire, and the loud alarm sounds to wake you up.
And there you have it: A reliable way to detect smoke using an artificially produced radioactive element.
Don't worry about the radiation emitted from these alarms. When inside the smoke alarm, essentially none of the alpha radiation can escape, and the gamma emissions are very low due to the low gamma photon energy.
Thanks for reading!
