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Tests against sequoia with known dates from their tree rings showed radiocarbon dating to be reliable and accurate.
He published his theory in 1946, and expanded on it in his monograph in 1955.
He also developed sensitive radiation detectors that could use the technique.
So by measuring carbon 14 levels in an organism that died long ago, researchers can figure out when it died.
The procedure of radiocarbon dating can be used for remains that are up to 50,000 years old.16 feet under: finding a matched set of whitetail sheds can be tough enough when they're lying on top of the ground.
Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.
Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.
To prove the survivability of nuclear war, Libby built a fallout shelter at his house, an event that was widely publicized.
 Libby resigned from the AEC in 1959, he became Professor of Chemistry at University of California, Los Angeles (UCLA), a position he held until his retirement in 1976.
In 1939, Serge Korff had discovered that cosmic rays generated neutrons in the upper atmosphere.