How Much Radioactive Material is in a Smoke Detector?
A smoke detector can give occupants of a building time to escape from a fire. The alarm sounds to alert people of the presence of a fire and its progression.
Ionisation smoke detectors contain a small amount of radioactive material that passes between two electrically charged plates to create an ionization chamber. This ionizes air molecules, which allows current to flow through the device. Smoke particles interrupt this current, causing the electronics to set off the alarm.
The ionization smoke detectors most people own and use are built around an americium source. Americium emits alpha particles, which remove electrons from air molecules to create positively charged ions. These ions attract each other and form an electric current between two electrodes in the detector’s ionization chamber. Smoke interrupts this current, triggering the alarm.
The small amount of radioactivity contained in a smoke detector is sealed inside foil and ceramic, which stops the alpha particles from traveling outside the device. Even if the americium were to escape the detector, it would only travel a few inches and could be stopped by paper or plastic. It would also pose a very low radiation health risk.
The americium in smoke detectors decays through a series of longer-lived isotopes to neptunium-237, which has a half-life of about 2.144 years. This is much longer than the half-life of most other elements, but it is still short enough that ionization smoke detectors are safe to own and use.
Radium 226 is an alpha, beta and gamma emitter that is found in some older smoke detectors. Its gamma emissions can damage the insides of a human body, especially if it reaches high levels. Radium-226 also irradiates bone tissue, causing osteogenic sarcomas.
Smoke detectors that use Radium-226 emit a stream of ions that is monitored by a phototransistor, when smoke enters the chamber it disrupts the ion flow and the sensor activates the alarm. There are two types of smoke detectors, the ionization and the photo electric type.
The amount of Radium in a smoke detector is very small, it only accounts for 0.28 micrograms. If you wanted to make a Hiroshima sized nuclear weapon using this isotope, you would need to buy a lot of smoke detectors. Radium is not a fissile isotope, it takes more energy to split the nucleus than it releases. This is why it is not a weapons material.
The main ingredient of a smoke detector is a synthetic radioactive metal called Americium-241. This isotope decays over time to neptunium-237. Neptunium is fissionable, but unlike uranium has not been used for nuclear weapons.
Smoke detectors rely on radiation to “ionize” air in a small chamber. The alpha particles emitted by Am-241 and nickel-63 remove electrons from neutral air molecules in the detector’s chamber, making them positively charged ions that conduct a small amount of electricity. The electronics detect this current and trigger the alarm when smoke enters the chamber.
As the ionization chamber fills with smoke, the number of positive ions falls. The electronics signal the detector to stop the current and sound the alarm. The total radiation dose to occupants of the house is very low – a few mrem per year, depending on how close you live to your alarm.
Americium is at the heart of most ionization chamber smoke detectors (and the popular Hank Green novel An Absolutely Remarkable Thing). It’s radioactive because it “decays” into particles, which then interact with neutral air molecules, removing electrons from them and creating positive ions. Two electrically charged plates inside the detector then attract these ions and create a small current. When smoke enters the chamber, it interrupts this current and triggers an alarm.
Ionization smoke detectors respond quicker to the fast-moving smoke from flaming fires than photoelectric models do. However, they can falsely detect smoldering fires.
Neither americium nor nickel-63 are considered to pose a health risk in their present form, even if they leaked from the smoke detector. Both emit beta and alpha particles, which can only travel a short distance before they are stopped by the outer layer of the detector’s shell. Additionally, if these particles were to be ingested they would only cause an exposure about six times higher than the desired yearly dose from natural background radiation.