Smoke Detectors & Radiation

Smoke detectors and alarms are important home safety devices. Ionization chamber and photoelectric smoke detectors are the two most common types available commercially. Because these pages are most concerned with radiation protection, we will focus mainly on the ionization chamber technology.

Ionization chamber smoke detectors contain a small amount of radioactive material encapsulated in a metal chamber. They take advantage of the ions created by ionizing radiation to develop a low, but steady electrical current.  Smoke particles entering the chamber disrupt the current and trigger the detector's alarm.  Ionization chamber detectors react more quickly to fast flaming fires that give off little smoke.

How much radiation is in smoke detectors?

The radiation source in an ionization chamber detector is a very small disc, about 3 to 5 millimeters in diameter, weighing about 0.5 gram. It is a composite of americium-241 in a gold matrix. The average activity in a smoke detector source is about one microcurie, 1 millionth of a curie.

Americium emits alpha particles and low energy gamma rays. It has a half-life of about 432 years. The long half-life means that americium decays very slowly, emitting very little radiation. At the end of the 10 year useful life of the smoke detector, it retains essentially all its original activity.

How much radiation exposure will I get from a smoke detector?

As long as the radiation source stays in the detector, exposures would be negligible (less than about 1/100 of a millirem per year), since alpha particles cannot travel very far or penetrate even a single sheet of paper, and the gamma rays emitted by americium are relatively weak. If the source were removed, it would be very easy for a small child to swallow, but even then exposures would be very low because the source would pass through the body fairly rapidly (by contrast, the same amount of americium in a loose powdered form would give a significant dose if swallowed or inhaled). Still, its not a good idea to separate the source from the detector apparatus.

Owning and operating a smoke alarm

Regardless of the detection technology used in your smoke alarm, the product label, User's Manual or Warranty should state the expected useful life of the smoke detector. For example, smoke alarms with the UL label have been certified with an expected useful life of 10 years. The product label also will tell you whether this includes the useful life of the battery. If you do not have a lithium long life battery (10 years), fire officials recommend that you change your batteries at the same time you turn your clock back each year for the end of Daylight Saving Time.  It's also important to make sure your smoke alarm is working properly.  You should test the alarm periodically (there should be a button to press).  But be very careful if you use a source of smoke to test the detector.

Smoke alarm and heat detector (which senses the heat from a fire to trigger an alarm or sprinkler system, but does not detect smoke) technologies are all relatively inexpensive for a homeowner. A smoke alarm can usually be purchased for $10 to $25. Many companies make separate products using either photoelectric or ionization technologies, or they combine the technologies in one product. Read the packaging and label material on the product. Smoke and heat detector technologies may also be combined with home break-in alarm equipment to provide a total home security system connected to your local fire and police services. Whether you choose an electrical or battery operated model, you must follow the manufacturer's recommendations for installation, testing and maintenance to get maximum protection.

Ionization sensor smoke alarms contain a small amount of radioactive material, americium embedded in a gold foil matrix within an ionization chamber. The matrix is made by rolling gold and americium oxide ingots together to form a foil approximately one micrometer thick. This thin gold-americium foil is then sandwiched between a thicker (~0.25 millimeter) silver backing and a 2 micron thick palladium laminate. This is thick enough to completely retain the radioactive material, but thin enough to allow the alpha particles to pass.

The ionization chamber is basically two metal plates a small distance apart. One of the plates carries a positive charge, the other a negative charge. Between the two plates, air molecules-made up mostly of oxygen and nitrogen atoms-are ionized when electrons are kicked out of the molecules by alpha particles from the radioactive material (alpha particles are big and heavy compared to electrons). The result is oxygen and nitrogen atoms that are positively charged because they are short one electron; the free electrons are negatively charged.

The diagrams below illustrate how ionization technology works. The positive atoms flow toward the negative plate, as the negative electrons flow toward the positive plate. The movement of the electrons registers as a small but steady flow of current. When smoke enters the ionization chamber, the current is disrupted as the smoke particles attach to the charged ions and restore them to a neutral electrical state. This reduces the flow of electricity between the two plates in the ionization chamber. When the electric current drops below a certain threshold, the alarm is triggered.

Ionization Technology

Alpha particles from the americium source ionize air molecules

In the smoke-free chamber, positive and negative ions create a small current as they migrate to charged plates

Smoke particles and combustion gases interact with the ions generated by the alpha particles, restoring them to their neutral electronic state and decreasing the electrical current passing through the cell.

As fewer ions are available to migrate to the plates, the disrupted current triggers the alarm

Photoelectric technology smoke alarms use a T-shaped chamber fitted with a light-emitting diode (LED) and a photocell. The LED sends a beam of light across the horizontal bar of the chamber. The photocell sits at the bottom of the vertical portion of the chamber. The photo cell will generate a current, when exposed to light.

The diagram below illustrates how the technology works. Under normal, smoke-free conditions, the LED beam moves in a straight line, through the chamber without striking the photo cell. When smoke enters the chamber, smoke particles deflect some of the light rays, scattering them in all directions. Some of it reaches the photocell. When enough light rays hit the photocell, they activate it. The activated photocell generates a current. The current powers the alarm, and the smoke alarm has done its job.

Photoelectric Smoke Alarm Technology