Thursday, December 6, 2012

Smoke and Mirrors

This post will also be found in Tehachapi's The Loop Newspaper.
I've just been learning some things about smoke detectors. Were you aware that most actually have a small amount of a radioactive element in them? Any that use ionization will have some kind of radioactive source. (Now there are also optical detectors which use light to detect the smoke, but these are more expensive so you're not going to find them as often.) Now checking out Wikipedia the most common radioactive element used is americium-241 (which denotes the isotope of americium with an atomic weight of 241). This element doesn't occur in nature and was first created in 1944.

Now as a radioactive element americium undergoes radioactive decay. This involves, for this particular element the release of a alpha particle. Now alpha particle are big hulking brutes that are unable to get through even tissue paper. Which makes them particularly good for use in smoke detectors. They cause lots of ionization but can't get through plain old plastic. But the ionization is what is used in the smoke detector. Ionization means that charged particles are created, and these are used to carry a small current in the smoke detector. When smoke gets in the detector this different type of particle (the smoke particle) changes how well the current flows. This change in the current is interpreted as smoke and the alarm goes off.

Pretty remarkable. And it just goes to show how well we really understand the process of radioactive decay. Which was why Bill Nye (the Science Guy) brought up smoke detectors recently on CNN. The topic of the discussion was the age of the Earth (4.5 billion years). And how in the world we know how old it is.

And the answer to that is radioactive decay. Which we understand very well. So well that there are many clocks based on radioactive decay. Not clocks like we use to tell time, not atomic clocks which gives us our standards of time, but clocks used to tell how old something is. 

Now most of us have heard of using carbon-14 to determine how old some fossil or artifact is. In fact, it is most useful for stuff that was recently alive. Well recently as in thousands of years old. Carbon-14 is a naturally occurring radioactive element created by particles from space hitting atoms in our atmosphere. And we measure how long something has been dead by seeing how much of the carbon-14 in it has decayed. (There are all manner of corrections used, but these fine tunings are again due to the fact that we really understand radioactive decay well.)

But this doesn't go back far enough to get us to the time the Earth was formed. For that we have additional “clocks”. There are ways of calculating the passage of time using uranium, thorium, rubidium and other radioactive isotopes. Each has a different amount of time it takes for it to decay. But for most of these there are half-lifes in the order of billions of years. (The half life of an isotope is the amount of time it takes for half of the atoms to decay to another element.)
So because we understand radioactive decay very well and we can tell how much of an element is left in some of the earliest rocks here on Earth we actually have a very good estimate of how old the Earth really is. Four and a half billion years. And if someone tells you differently then they are denying science.

Now hopefully this hasn't struck so much fear into some of my readers they are thinking about getting rid of their smoke detectors. The danger from the radioactivity in them is virtually nonexistent. And it doesn't compare in any way with the very real danger you would be exposing yourself to by their removal.

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