Friday, January 28, 2011

Practical Friday - Smoke Detectors!

Everybody has seen smoke detectors (if not, maybe you should talk to your landlord...) but not many people know, really, how they work.  Should we be afraid of them?  No, although I wouldn't suggest taking one apart.  Here's why.

Ionizing radiation – how do smoke detectors work?

Most smoke detectors rely on the natural radioactivity of the element, Americium that ejects "alpha" particles (small charged particles with 2 protons and 2 neutrons).  These particles are ejected with sufficient energy that, when they collide with other molecules (like gas molecules - don't forget that we're talking about smoke detectors), they knock electrons out of the other molecule. Radiation like this is called "ionizing" radiation, because the collision leads to the formation of ions.  Chemists and physicists love symbolic notation, so the equation below shows what is going on when Americium-241 decays.  On the left side of the equation is the symbol for Americium-241, the radioactive isotope with 95 protons and 146 neutrons (note that neutrons and protons add up to give 241, which is the mass number of this isotope).  Anyway, the result of the decay even is shown to the right of the arrow, and leads to the formation of Neptunium-237 and an alpha particle (2 protons, 2 neutrons).

O.K., so what?  Is this bad?  Well… yes.

The thing is that the alpha particles, once ejected have enough energy to damage cells (which is why you should never, ever, take apart a smoke detector).

Is there no GOOD news?

1. Alpha particles can’t pass through paper (also, by the way, don’t ever take apart a smoke detector)

2. Sometimes ionizing molecules is good.  Oh Yeah?  How?


Q:   How do they work?  What does this have to do with science?

A:   Inside the detector is a small sample of Americium, a radioactive element that decays by, wait for it… alpha emission.  The alpha particles emitted are constantly colliding with - and ionizing - gas molecules in air (nitrogen, oxygen, etc) and these form ions that are attracted to + and – charged plate in the detector (generating the background current in the device).  When smoke comes along, it soaks up these ions and changes the current, which turns the alarm ON!

Q:  Are they safe? 

A:  Yes. Remember that a particle can’t go through paper.

Q:  Really?

A:  Yes, but don’t take them apart because, after all, there is a little piece of Americium in there…

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