Not All Natural

When I see a package labeled “All Natural”, I muse about the alternative. What, exactly, is not natural? Not many things in everyday experience cannot be found in nature. For example, plastics are artificially polymerized distillations from found materials. Even that artificial polymerization is just a concentrated effort to do what happens haphazardly in nature.

The same holds true for “artificial” ingredients. Any of them may be found in nature. The artifice comes from concentrating and using them. So what would qualify, to my mind, as unnatural? After all, artifice (the root of artificial) is broadly a work of art, a craft of man. But all the root ingredients he uses are natural, found in nature. Then they are broken down or built up in a manner that is completely consistent with nature.

I could argue that the only thing that would be unnatural is something supernatural. But no supernatural thing has ever been reliably observed. If a supernatural thing could be observed reliably, and its behavior documented, it would become part of the definition of nature, the province of science. But this is not my point.

I believe that I have found an artificial ingredient in my Victorian house. That is, a basic substance that did not exist in any measurable amount on this planet by the time we started looking. Or when God created it, if you follow Young Earth theology. Moreover, I’d bet that my readers have this unnatural thing in their houses, as well.

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I am referring to a chemical element that was not found in nature. The basic elements (beyond hydrogen and helium) were created in stars and supernovae. Basically, the hot quark soup was stirred roughly, and splashed out into a nebula to freeze into the nuclei of elemental atoms. As the quarks congealed, they formed a smear of normally distributed isotopes. Most of those were unstable, and decayed through a series of stages until they arrived at something stable. And most of those decay chains were pretty fast, on the order of milliseconds to a few years.

What we find on our planet (defined as “natural”) is a distribution of isotopes of elements that are either stable, or have very long half-lives. By comparing the relative amounts of each isotope, one can estimate that the nova that produced the planetary nebula from which our solar system formed happened about 6 billion years ago. There are no “natural” elements heavier than Uranium. U-238 (to use the heaviest natural isotope for specific example) has a half life of over five billion years. Most of the “natural” shorter-lived elements (like radon) are here only because they are daughter products of uranium (or protactinium) decay. And some short-lived isotopes, like Carbon-14, are continually produced by cosmic ray impacts.

The object at hand is a smoke detector, wherein lies a modern miracle. The way an ionizing smoke detector works is to have a little open air space that is made conductive by a small radioactive source. A tiny electric current runs through this air space, unless the conductivity of this ionized air is reduced by smoke. Then the alarm goes off. It can detect smoke almost as sensitively as a human nose: A few parts per million.

Therefore when you open a smoke detector, you can see a radiation warning. It is intentionally hard to read, to avoid panicking folks who are afraid of radiation.

There are many radioactive isotopes that could be used, some of them even “natural.” But some are bad because they are chemically toxic. Some are bad because they decay into something dangerous. And some are prohibitively expensive to isolate. So what did they choose?

Americium. This is part of the radioactive waste from nuclear power plants, and also an annoying daughter product that has to be periodically removed from plutonium bomb pits. How radioactive is it? The warning above tells you, if you know how to read it. 1.0 μCi (micro Curies) or 37 kBq (kilo Becquerels) is the answer.

Huh? The latter value is more intuitive. Henri Becquerel shared the Nobel prize with Marie Curie. His unit is the number of decays per second. That is, this smoke detector source shoots off 37,000  alpha particles (helium nuclei) every second. It is an absolute amount, assuming a fresh source, and the correct weight of material.

What I wondered was, “How much material?” This is where the Curie units are more useful. I looked up the Specific Activity of Americium 241, and it is 3.2 Curies per gram.The container says we have one micro-Curie of the stuff, so that gives us 0.31 micrograms. A very small speck. (btw: 1 Ci = 3.7 x 1010 Bq).

If you break the seal, remove the solder, and clip the safeties (don’t try this at home) you can look at the isotope itself. It doesn’t look like much. My sample is over 20 years old and apparently has been splashed with dirty water, possibly during a house fire in 1988. Here is a cool picture of a fresher bit.

So how dangerous is this un-natural bit of nuclear waste in your home? Americium is not chemically toxic, and most of its compounds are relatively insoluble. However, that 37 kBq means it can break 37,000 of your molecules for every second it is in you. So don’t touch it. Because it is an alpha source, it is completely exposed; any protective covering would stop the alpha particles from reaching the air it needs to ionize. Air itself stops alpha particles within a few inches. But each decay also produces a gamma photon. At 37kBq, this gamma source is pretty dark. By comparison, a 100 watt light bulb sends out about 1020 times as many photons per second.

How long will it last? The half life (how long it takes for half of it to decay) is about 433 years. It decays into another alpha source, Neptunium-237 with a half life of two million years (still chemically harmless and 4,619 times less radioactive). So after 20 years, there is still 98% of the original activity in this sample. This is why they prefer Americium-241 to many other isotopes for home safety use. The whole Neptunium series decay chain is listed here, for those who want to know its future.

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1 Response to “Not All Natural”


  1. 1 Dan Klarmann March 3, 2011 at 2:59 pm

    My wife noted that alpha particles are helium nuclei, and asked how long it would take to fill a balloon with helium. I laughed, and then explained.

    The math is simple, if you remember Avogadro. Let’s use a balloon that needs 2.2 liters of helium, which is about 1/10 of a Mole, or 1/10 x 6.02 x 1023 atoms of helium. At 3.7 x 104 atoms/second, it would take 6/3.7 x 1023-4 seconds, = 1.63 x 1019 seconds. That’s about 40 times the over 13 billion year life of the universe. But the original isotope fades by three quarters every millennium, so it will take longer.


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