Dedicated at the University of Chicago on October 10, 2016.In 1946, Willard Libby proposed an innovative method for dating organic materials by measuring their content of carbon-14, a newly discovered radioactive isotope of carbon.
The radioactive atom is absorbed by plants and living matter in the same way as its non-radioactive isotope ; in every thousand billion (ten to the power of twelve) atoms of carbon 12, there will be on average one atom of carbon 14.
This tiny ratio exists in all molecules involving carbon atms, including all living matter.
Based on Korff’s estimation that just two neutrons were produced per second per square centimeter of earth’s surface, each forming a carbon-14 atom, Libby calculated a ratio of just one carbon-14 atom per every 10 carbon atoms on earth.
Libby’s next task was to study the movement of carbon through the carbon cycle.
The nucleus of carbon 14 contains 6 protons and 8 neutrons, as opposed to the 6 and 6 found in ordinary carbon 12.
The imbalance makes carbon 14 a radioisotope with a half-life of 5,700 years, and an emitter of beta particles.Top of page You read statements in books that such and such a society or archeological site is 20,000 years old.We learned rather abruptly that these numbers, these ancient ages, are not known accurately; in fact, it is at about the time of the First Dynasty in Egypt that the first historical date of any real certainty has been established.” —Willard Libby, Nobel Lecture, 12 December 1960 The concept of radiocarbon dating focused on measuring the carbon content of discreet organic objects, but in order to prove the idea Libby would have to understand the earth’s carbon system.In a system where carbon-14 is readily exchanged throughout the cycle, the ratio of carbon-14 to other carbon isotopes should be the same in a living organism as in the atmosphere.However, the rates of movement of carbon throughout the cycle were not then known.Radiocarbon dating would be most successful if two important factors were true: that the concentration of carbon-14 in the atmosphere had been constant for thousands of years, and that carbon-14 moved readily through the atmosphere, biosphere, oceans and other reservoirs—in a process known as the carbon cycle.