Carbon 14 Dating History

Willard Frank Libby (1908 -1980), a chemist who had researched uranium as part of the Manhattan Project during World War II, developed the first method for dating material that had a biological origin. This included cloth fibers, bone, bits of wood and charcoal. For his work Libby was awarded the  Nobel Prize in Chemistry in 1960.

Libby had realized that since all living things had a relatively constant carbon 14 to carbon 12 ratio as found in the atmosphere. Carbon 14 is a radioactive isotope (type of atom) and carbon 12 was a stable isotope. The reason was that plants took on carbon dioxide (CO2) through photosynthesis and animals did so through breathing, eating plants and eating other animals. Libby knew that when a plant or animal died it would stop absorbing carbon. As the year went by the carbon 14 isotope would decay into nitrogen 14 according to a very precise schedule while the carbon 12 would remain unchanged. Thus the ratio of carbon 14 to carbon 12 would change and by measuring how much change had taken place it would be possible to know the age of anything that had originated from a live plant or animal.

Libby's work was based on two significant developments:

The method of counting decay used by Libby involved measuring radioactivity in solid carbon using modified Geiger counters. If the radioactivity was lower, more decay had occurred and the carbon was older.

The next development in counting technology was the conversion of sample carbon to CO2 gas for measurement in Gas Proportional counters. Also in the early 1950s, methods were developed for estimating carbon 14 decay using Liquid Scintillation systems.

In 1977, the first Accelerator Mass Spectrometer measurements were conducted. AMS differed from other methods in that the carbon 14 and carbon 12 isotopes were separated in the particle accelerator according to their mass and counted. This was more precise and thus only small milligram-sized samples were required for dating.

Constant advances continue to be made in the precision of Gas Proportional Counting, Liquid Scintillation Counting, and the Accelerator Mass Spectrometry.
 

Advances in Carbon 14 Dating Precision

Libby initially tested the radiocarbon method on samples for which some dates were known, mostly prehistoric Egyptian material. Wood from the tomb Pharaoh Djoser (3rd Dynasty) they knew was about 4700 years old. Since the half-life of carbon 14 was estimated to be about 5568 years he expected to find roughly half of the carbon 14 concentrations as would be found in nature. That was the case.

Other tests were conducted on samples of wood dated using tree-ring dating methods (dendrochronology) indicated that carbon 14 was valid and that the half-life of carbon 14 was about 5568 years.  

However, other measurements produced some unexpected results. Carbon 14 dating produced results that seemed to young. Further study of tree rings suggested that there were, over time, fluctuations in carbon 14 levels. This meant developing methods of fitting carbon 14 dates to known fluctuations.  By testing more and more material with known ages, including tree-rings for wood, improved precision was possible. Such refinement in carbon 14 dating continues.

Refinements in the estimates for the half-life of carbon 14 have also occurred. The current estimate is 5730 years.

Today, carbon 14 dating is considered an accurate method for determining the age of things that have a biological origin.


© 2004 Daniel R. Porter, Bronxville, New York