The most common form of radiometric dating measures the decay of carbon-14 isotopes. This is very useful for dating human artifacts, but it doesn't work for dinosaurs, because all the unstable atoms in the carbon will have decayed after about 50,000 years. So the decay happens much too fast. (If 50,000 years can be called fast!) To date material that is millions of years old, scientists need to look for isotopes with a much longer half-life (which is what gets measured by radiometric dating). Among those used are uranium-238, uranium-235 and potassium-40: all have half-lives of more than a million years. But there's still a problem with dinosaur fossils - they don't contain any of these measurable isotopes.
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