Radiation and Smoke Detectors

In the late 1930s, a smoker inadvertently made a discovery for detecting smoke. The Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. His device failed: small concentrations of gas had no effect on the sensor’s conductivity. Frustrated, Jaeger lit a cigarette—and noticed that a meter on the instrument registered a drop in the current. Smoke particles had apparently done what poison gas could not. Jaeger’s experiment was one of the advances that paved the way for the modern smoke detector.

Here’s something else surprising: Smoke detectors work because of radiation. They are an example of the beneficial uses of radiation and radioactive materials.

The first significant installations of commercial smoke detectors started in the US around 1969. Since then, the installation of smoke detectors has saved thousands of lives, numerous injuries, and millions of dollars. It has been reported that smoke detectors are installed in 93 percent of US residences. However, it is estimated that more than 30 percent of these alarms don’t work, as users remove the batteries or forget to replace them in a timely manner.

In the US, while smoke detector manufacturers and distributors are subject to NRC regulation, end users of smoke detectors (consumers) are typically not because of the small amount of radioactive material used in each detector.

The most common type of smoke detector consists of an ionization chamber, electronic circuitry, a power source that is usually a battery, an alarm mechanism, and an outer case. The ionization chamber is the main component. It consists of a source of ionizing radiation, usually Americium (Am-241) positioned between two oppositely-charged electrodes. The radiation source is a very small metallic foil disc about 3 to 5 millimeters in diameter.

To give you an idea of the small amount of radiation that is emitted by this disc, a person flying coast-to-coast gets more radiation from cosmic sources in one trip than a person sitting in the close proximity of an ionization smoke detector gets in a whole year.

Here is how the device works: Particles emitted during radioactive decay of the Am-241 interact with neutral air molecules flowing through the chamber and convert them to positive ions by removal of electrons. The removed electrons then form negative ions by attachment to other neutral molecules. The resulting positive and negative ions are attracted toward the electrodes, causing a small, reasonably steady current between the electrodes. The electronic circuitry monitors this current and, if the current drops below a preset level, which it will if the air entering the chamber contains enough smoke, it triggers an audible alarm.

If you are interested in the technical evaluations the NRC has done on smoke detectors and other consumer products containing radioactive material please see NUREG-1717 “Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials” .

Ujagar Bhachu
Mechanical Engineer