Ionization Smoke Detector

By Shawna Miller & Crystal Thomas

Outline

Define Terms Americum-241 Ionizing Radiation Alpha Particles Curie

What it is How it works Look inside an Ionization Chamber Simplify A look into the safety factor Sources

What is americium-241?

Americium (chemical symbol Am) is a man-made radioactive metal, with Atomic Number 95.

A silvery metal; whose isotopes americium-237 through -246 are all radioactive. Americium-241 is formed spontaneously by the beta decay of plutonium-241.

A typical detector contains 0.9 micro-curie of americum-241

What is Ionizing Radiation?

Ionizing radiation is a type of particle radiation in which an individual particle, such as an electron, carries enough energy to ionize an atom or molecule, that is, to completely remove an electron from its orbit.

If the individual particles do not carry this amount of energy, it is essentially impossible for even a large flood of particles to cause ionization.

What are alpha particles?

They are a highly ionizing form of particle radiation which have low penetration. They consist of two protons and two neutrons bound together into a single particle.

What is curie?

A curie is a unit of measure for nuclear material.

If you are holding a curie of something in your hand, you are holding an amount of material that undergoes 37,000,000,000 nuclear transformations per second.

That means 37,000,000,000 atoms are decaying an emitting particles of nuclear radiation per second.

How does this pertain to the smoke detector?

Ionization Smoke Detectors

Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke.

Ionization sensor smoke alarms contain a small amount of radioactive material, americium embedded in a gold foil matrix within an ionization chamber. This thin gold-americium foil is then sandwiched. 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.

More on Ionization Detectors…

The positively charged metal plate attracts the negatively charged ions (electrons) and the negatively charged plate attracts the positively charged ions (mostly oxygen and nitrogen). The steady flow of ions creates a small but steady current.  

These atoms-are ionized when electrons are kicked out of the molecules by alpha particles from the radioactive material.

The current is disrupted when smoke enters the ionization chamber. 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, and when the electric current drops below a certain threshold, the alarm is triggered

How an Ionization Detector Works

The ionization chamber in an ionization detector is an aluminum can containing the ionization source.

There's also an ionization radiation. The ionization radiation uses an Americium-241 and it helps power the detector.

The ionization chamber has two plates separated by a centimeter. When smoke enters the ionization chamber, the particles attach to ions and neutralize them preventing the particles from reaching the plates.

Inside an Ionization Smoke Detector

Inside an Ionization Chamber

Break it Down…

The smoke that permeates smoke detectors is made by carbon.

Americium is what detects the smoke. Smoke particles float around and then reach

the inside of the smoke detector. The smoke gets near the sensor and then

the electric horn is activated. When the horn is activated it's an early

warning telling you to get out of the house before the fire starts all the way.

Other Facts

Does it emit radiation? The very low-level radiation in ionization smoke

detectors is not harmful and when compared, in all cases, is less than the maximum permitted from other consumer products that emit radiation, such as television sets.

Ionization detectors are the safest for flame fires.

Sources

http://en.wikipedia.org/wiki/Main_Page

http://chemistry.about.com/cs/howthingswork

http://home.howstuffworks.com/smoke2.htm

http://www.epa.gov/radiation/sources/smoke_ion.htm

Dr. Gary A Ruff, Glenn Research Center (GRC), NASA's lead center for microgravity combustion research.

Demonstration