The Science of Smoke Detectors

A smoke detector is an important invention that has saved many lives. It is so important that many fire departments will offer them free to people in their area if they can’t afford to purchase one. Each year we are asked to check to see that our smoke detector is working properly. This is not often the same done at the same time that we must change our clocks as we change to or from “daylight savings time”. This gives us two test dates per year. It is a good idea to have one in each room of the house. The one in or near the kitchen may be activated when we cook. You may have to ventilate the kitchen for a little while if it goes off accidentally while cooking.

Inside a typical smoke detector there is a very small amount (0.1 g) of the radioactive element “Americium”. This element loses helium atoms as alpha particles. This amount of Americium will give off about 2000 alpha particles per second. Inside the smoke detector there are two metal plates separated by a small distance and of course air. When the air is clear (no smoke) then the electric current can pass between the plates. When the electric flows the alarm does not sound. If there are particles of smoke in the space between the plates they block the path of the radiation and stop the current from flowing which then sounds the alarm.

You test the smoke detector by pressing a button on the outside and when pressed you should hear the alarm. If it does not sound you should replace the battery that is in the circuit and re-test the detector. Because this is a radioactive substance care should be taken when you dispose of an old smoke detector. Even though the amount of radiation is low it could still pose a hazard. If it were somehow swallowed it could lodge in a part of the body and remain there causing radiation damage to cells right near the radioactive source. Cell damage from radiation has been linked to cancer.

Usually smoke detectors once installed are very high on walls or ceilings and are not likely to be a danger to children or pets, but they can fall and break open or children may be curious and want to see what’s inside. Alpha particles do not travel far from the source and have little penetration power, but you still would not want them hitting the same area of cells for long periods of time.

You can use a Geiger counter to hear the alpha particles being given off from the radioactive material and you can get a good idea of the distance from the source and how it affects detection. Other types of radiation are significantly more dangerous.

Remember smoke detectors usually do not check for carbon monoxide although some models now will also check for this dangerous gas. Carbon monoxide can also kill and this often happens in houses that are not properly ventilated.