The best samples for the construction of calibration curves are tree rings where individual annual rings and annually laminated sediments are dated by AMS.
Ocean corals and speleothems (cave deposits such as stalactites) dated by another radiometric method—Uranium-Thorium dating—have also helped to extend the calibration curve beyond the age of the most ancient tree-ring chronologies.
Carbon-14 has a half-life of 5,730 years, which may sound like a large number.
But on the scale of existence of the universe, this half-life is quite small and thus a convenient yardstick for researchers.
Carbon-14 dating is especially popular with anthropoligists seeking to date the age of bones. Almost every biology lab will have a phosphate counter.
Physicists have studied tritium decay seeking to understand fusion on the Sun.
These range from fractions of a second to billions of years.
In the two days after that, it drops from 20 to 10 - it halves again - and so on.This is called the half-life of the radioactive isotope.There are two definitions of half-life, but they mean essentially the same thing.The first dated materials included wood from Egyptian tombs, linen wrapping from one of the Dead Sea Scrolls, and heartwood from a California sequoia.Radiocarbon dating is useful for dating organic materials as old as 45,000 to 50,000 years, after which little C atoms in a sample decay.