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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.
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