14 September 2005

A different form of dating

Article found via The Esoteric Science Resource Center.

Scientists at Lawrence Livermore National Laboratories (LLNL) in California have debuted a new technique to determine the age of a tooth, and therefore the age of a body, and therefore narrow down the number of missing people to search through when identifying a corpse in a massive death situation such as New Orleans, the tsunami, or Sept 11.

Between 1955 and 1963 the US government performed atmospheric testing of nuclear bombs. This testing resulted in large amounts of carbon-14 (aka C-14, aka radiocarbon) being present in the atmosphere at the time. (As I understand the process, the fact that this isotope or version of carbon is radioactive is not used in the method.) This C-14 is nicely mixed throughout all the atmosphere of the world due to weather patterns. Much of this C-14 merged with a couple oxygens to form carbon dioxide, CO_2. The CO_2 was "inhaled" by plants and became part of their structure, so the same concentration of C-14 that is in the air can also be found in plants at that time. Additionally, the carbon that is in the plants is eaten by animals and goes into things like their muscles, so at any given time there's the same concentration of C-14 in the air, plants and animals.

Now, the concentration of C-14 in the air has been steadily decreasing, mostly due to the carbon mixing into the oceans. So that means that the amount of C-14 available in the air to go into plants and animals has been steadily decreasing. We know the rate at which it's decreasing pretty well empirically (i.e., from experiments) - you can think of it like we calibrated our C-14 / date conversion scale by testing on dead people's teeth.

And here come the teeth: Your adult teeth are formed before around age 8, and once they're formed they are fixed. You can't add or subtract any C-14.1 So how much C-14 was in the air when you were forming your teeth is how much C-14 will be in your teeth when you die. Forensic scientists can now work this process backwards: they get a set of teeth and take them to their lab and determine how much C-14 is in the teeth. This sets the date of tooth formation at a certain year, and therefore the date of birth at a certain year - plus or minus 1.6 years! That's much better than older methods of tooth ID, which gave plus or minus 10 years.

The only real catch is you have to have been younger than 8 when the atmospheric nuclear testing started, or not yet born. So if someone was born prior to 1943 that's all the test will be able to tell you, not how much earlier. But pretty nifty results for being irradiated with radioactive materials from nuclear weapons testing!

1 Those familar with radiocarbon dating probably took issue with this statement at first. In fact, C-14 does decay down to a steadier element, nitrogen-14, and relatively quickly. It only takes 5,730 years (±40) for half the C-14 in your teeth to turn into N-14. So in a lifetime of roughly 100 years you'll have around one sixtieth of a half life pass, or you'll have decayed away down to 98.85% of your original amount of C-14. Yeah, the passage of time doesn't make that much difference.

Note that the carbon dating in this case, that is the decrease in C-14 in the teeth is NOT due to radioactive decay, but the decreased amount of C-14 in the ecosystem.

Check out this Wikipedia article for a reminder on exponential decay, or I can post my IM conversation with Jeth where we both relearned it from scratch b/c we'd forgotten...

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