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Mystery of the First Americans
(My So-Called) Half-Life
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Here's an example of how half-life works: Let's say you have 80 ants, and these ants have a half-life of ten days. In ten days, then, 40 of your ants would still be alive. Ten days after that, 20 ants would still be alive. Ten more days, and ten would be alive. You get the picture.

If you took just one of these ants, though, there would be no way to predict when it would die. It might die right away or it might live for a long time. It's only when you have a lot of them that this half-life thing works.

In real life, you can't use the half-life method to determine ant death rates. You can, however, use the method to determine when radioactive atoms will decay into some other form.

Half-lives vary greatly from one radioactive atom to another. The reason is that a highly unstable atom wants to change quickly, whereas a slightly unstable atom is only slightly uncomfortable with its condition. The half-life of highly unstable radon-222 is less than four days. The half-life of the slightly unstable uranium-238 is 4.5 billion years. Carbon-14 has a half-life of 5,730 years.

Use the graph to the left to see how half-life works. Drag the "Time" pointer up and down to move through time. Watch what happens to the level of carbon-14 as you do so.

Next: Detection Section (what's your deflection?)

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